forked from OSchip/llvm-project
6573976f57
This was reverted because it would miscompile code where the cmp had multiple uses. That was due to a deficiency in the existing code, which was fixed in r291630 (see the PR for details). This re-commit includes an extra test for the kind of code that got miscompiled: @test_sub_1_setcc_jcc. llvm-svn: 291640 |
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.. | ||
AsmParser | ||
Disassembler | ||
InstPrinter | ||
MCTargetDesc | ||
TargetInfo | ||
Utils | ||
CMakeLists.txt | ||
LLVMBuild.txt | ||
README-FPStack.txt | ||
README-MMX.txt | ||
README-SSE.txt | ||
README-UNIMPLEMENTED.txt | ||
README-X86-64.txt | ||
README.txt | ||
X86.h | ||
X86.td | ||
X86AsmPrinter.cpp | ||
X86AsmPrinter.h | ||
X86CallFrameOptimization.cpp | ||
X86CallLowering.cpp | ||
X86CallLowering.h | ||
X86CallingConv.cpp | ||
X86CallingConv.h | ||
X86CallingConv.td | ||
X86EvexToVex.cpp | ||
X86ExpandPseudo.cpp | ||
X86FastISel.cpp | ||
X86FixupBWInsts.cpp | ||
X86FixupLEAs.cpp | ||
X86FixupSetCC.cpp | ||
X86FloatingPoint.cpp | ||
X86FrameLowering.cpp | ||
X86FrameLowering.h | ||
X86ISelDAGToDAG.cpp | ||
X86ISelLowering.cpp | ||
X86ISelLowering.h | ||
X86Instr3DNow.td | ||
X86InstrAVX512.td | ||
X86InstrArithmetic.td | ||
X86InstrBuilder.h | ||
X86InstrCMovSetCC.td | ||
X86InstrCompiler.td | ||
X86InstrControl.td | ||
X86InstrExtension.td | ||
X86InstrFMA.td | ||
X86InstrFMA3Info.cpp | ||
X86InstrFMA3Info.h | ||
X86InstrFPStack.td | ||
X86InstrFormats.td | ||
X86InstrFragmentsSIMD.td | ||
X86InstrInfo.cpp | ||
X86InstrInfo.h | ||
X86InstrInfo.td | ||
X86InstrMMX.td | ||
X86InstrMPX.td | ||
X86InstrSGX.td | ||
X86InstrSSE.td | ||
X86InstrSVM.td | ||
X86InstrShiftRotate.td | ||
X86InstrSystem.td | ||
X86InstrTSX.td | ||
X86InstrTablesInfo.h | ||
X86InstrVMX.td | ||
X86InstrXOP.td | ||
X86InterleavedAccess.cpp | ||
X86IntrinsicsInfo.h | ||
X86MCInstLower.cpp | ||
X86MachineFunctionInfo.cpp | ||
X86MachineFunctionInfo.h | ||
X86OptimizeLEAs.cpp | ||
X86PadShortFunction.cpp | ||
X86RegisterInfo.cpp | ||
X86RegisterInfo.h | ||
X86RegisterInfo.td | ||
X86SchedHaswell.td | ||
X86SchedSandyBridge.td | ||
X86Schedule.td | ||
X86ScheduleAtom.td | ||
X86ScheduleBtVer2.td | ||
X86ScheduleSLM.td | ||
X86SelectionDAGInfo.cpp | ||
X86SelectionDAGInfo.h | ||
X86ShuffleDecodeConstantPool.cpp | ||
X86ShuffleDecodeConstantPool.h | ||
X86Subtarget.cpp | ||
X86Subtarget.h | ||
X86TargetMachine.cpp | ||
X86TargetMachine.h | ||
X86TargetObjectFile.cpp | ||
X86TargetObjectFile.h | ||
X86TargetTransformInfo.cpp | ||
X86TargetTransformInfo.h | ||
X86VZeroUpper.cpp | ||
X86WinAllocaExpander.cpp | ||
X86WinEHState.cpp |
README.txt
//===---------------------------------------------------------------------===// // Random ideas for the X86 backend. //===---------------------------------------------------------------------===// Improvements to the multiply -> shift/add algorithm: http://gcc.gnu.org/ml/gcc-patches/2004-08/msg01590.html //===---------------------------------------------------------------------===// Improve code like this (occurs fairly frequently, e.g. in LLVM): long long foo(int x) { return 1LL << x; } http://gcc.gnu.org/ml/gcc-patches/2004-09/msg01109.html http://gcc.gnu.org/ml/gcc-patches/2004-09/msg01128.html http://gcc.gnu.org/ml/gcc-patches/2004-09/msg01136.html Another useful one would be ~0ULL >> X and ~0ULL << X. One better solution for 1LL << x is: xorl %eax, %eax xorl %edx, %edx testb $32, %cl sete %al setne %dl sall %cl, %eax sall %cl, %edx But that requires good 8-bit subreg support. Also, this might be better. It's an extra shift, but it's one instruction shorter, and doesn't stress 8-bit subreg support. (From http://gcc.gnu.org/ml/gcc-patches/2004-09/msg01148.html, but without the unnecessary and.) movl %ecx, %eax shrl $5, %eax movl %eax, %edx xorl $1, %edx sall %cl, %eax sall %cl. %edx 64-bit shifts (in general) expand to really bad code. Instead of using cmovs, we should expand to a conditional branch like GCC produces. //===---------------------------------------------------------------------===// Some isel ideas: 1. Dynamic programming based approach when compile time is not an issue. 2. Code duplication (addressing mode) during isel. 3. Other ideas from "Register-Sensitive Selection, Duplication, and Sequencing of Instructions". 4. Scheduling for reduced register pressure. E.g. "Minimum Register Instruction Sequence Problem: Revisiting Optimal Code Generation for DAGs" and other related papers. http://citeseer.ist.psu.edu/govindarajan01minimum.html //===---------------------------------------------------------------------===// Should we promote i16 to i32 to avoid partial register update stalls? //===---------------------------------------------------------------------===// Leave any_extend as pseudo instruction and hint to register allocator. Delay codegen until post register allocation. Note. any_extend is now turned into an INSERT_SUBREG. We still need to teach the coalescer how to deal with it though. //===---------------------------------------------------------------------===// It appears icc use push for parameter passing. Need to investigate. //===---------------------------------------------------------------------===// The instruction selector sometimes misses folding a load into a compare. The pattern is written as (cmp reg, (load p)). Because the compare isn't commutative, it is not matched with the load on both sides. The dag combiner should be made smart enough to canonicalize the load into the RHS of a compare when it can invert the result of the compare for free. //===---------------------------------------------------------------------===// In many cases, LLVM generates code like this: _test: movl 8(%esp), %eax cmpl %eax, 4(%esp) setl %al movzbl %al, %eax ret on some processors (which ones?), it is more efficient to do this: _test: movl 8(%esp), %ebx xor %eax, %eax cmpl %ebx, 4(%esp) setl %al ret Doing this correctly is tricky though, as the xor clobbers the flags. //===---------------------------------------------------------------------===// We should generate bts/btr/etc instructions on targets where they are cheap or when codesize is important. e.g., for: void setbit(int *target, int bit) { *target |= (1 << bit); } void clearbit(int *target, int bit) { *target &= ~(1 << bit); } //===---------------------------------------------------------------------===// Instead of the following for memset char*, 1, 10: movl $16843009, 4(%edx) movl $16843009, (%edx) movw $257, 8(%edx) It might be better to generate movl $16843009, %eax movl %eax, 4(%edx) movl %eax, (%edx) movw al, 8(%edx) when we can spare a register. It reduces code size. //===---------------------------------------------------------------------===// Evaluate what the best way to codegen sdiv X, (2^C) is. For X/8, we currently get this: define i32 @test1(i32 %X) { %Y = sdiv i32 %X, 8 ret i32 %Y } _test1: movl 4(%esp), %eax movl %eax, %ecx sarl $31, %ecx shrl $29, %ecx addl %ecx, %eax sarl $3, %eax ret GCC knows several different ways to codegen it, one of which is this: _test1: movl 4(%esp), %eax cmpl $-1, %eax leal 7(%eax), %ecx cmovle %ecx, %eax sarl $3, %eax ret which is probably slower, but it's interesting at least :) //===---------------------------------------------------------------------===// We are currently lowering large (1MB+) memmove/memcpy to rep/stosl and rep/movsl We should leave these as libcalls for everything over a much lower threshold, since libc is hand tuned for medium and large mem ops (avoiding RFO for large stores, TLB preheating, etc) //===---------------------------------------------------------------------===// Optimize this into something reasonable: x * copysign(1.0, y) * copysign(1.0, z) //===---------------------------------------------------------------------===// Optimize copysign(x, *y) to use an integer load from y. //===---------------------------------------------------------------------===// The following tests perform worse with LSR: lambda, siod, optimizer-eval, ackermann, hash2, nestedloop, strcat, and Treesor. //===---------------------------------------------------------------------===// Adding to the list of cmp / test poor codegen issues: int test(__m128 *A, __m128 *B) { if (_mm_comige_ss(*A, *B)) return 3; else return 4; } _test: movl 8(%esp), %eax movaps (%eax), %xmm0 movl 4(%esp), %eax movaps (%eax), %xmm1 comiss %xmm0, %xmm1 setae %al movzbl %al, %ecx movl $3, %eax movl $4, %edx cmpl $0, %ecx cmove %edx, %eax ret Note the setae, movzbl, cmpl, cmove can be replaced with a single cmovae. There are a number of issues. 1) We are introducing a setcc between the result of the intrisic call and select. 2) The intrinsic is expected to produce a i32 value so a any extend (which becomes a zero extend) is added. We probably need some kind of target DAG combine hook to fix this. //===---------------------------------------------------------------------===// We generate significantly worse code for this than GCC: http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21150 http://gcc.gnu.org/bugzilla/attachment.cgi?id=8701 There is also one case we do worse on PPC. //===---------------------------------------------------------------------===// For this: int test(int a) { return a * 3; } We currently emits imull $3, 4(%esp), %eax Perhaps this is what we really should generate is? Is imull three or four cycles? Note: ICC generates this: movl 4(%esp), %eax leal (%eax,%eax,2), %eax The current instruction priority is based on pattern complexity. The former is more "complex" because it folds a load so the latter will not be emitted. Perhaps we should use AddedComplexity to give LEA32r a higher priority? We should always try to match LEA first since the LEA matching code does some estimate to determine whether the match is profitable. However, if we care more about code size, then imull is better. It's two bytes shorter than movl + leal. On a Pentium M, both variants have the same characteristics with regard to throughput; however, the multiplication has a latency of four cycles, as opposed to two cycles for the movl+lea variant. //===---------------------------------------------------------------------===// It appears gcc place string data with linkonce linkage in .section __TEXT,__const_coal,coalesced instead of .section __DATA,__const_coal,coalesced. Take a look at darwin.h, there are other Darwin assembler directives that we do not make use of. //===---------------------------------------------------------------------===// define i32 @foo(i32* %a, i32 %t) { entry: br label %cond_true cond_true: ; preds = %cond_true, %entry %x.0.0 = phi i32 [ 0, %entry ], [ %tmp9, %cond_true ] ; <i32> [#uses=3] %t_addr.0.0 = phi i32 [ %t, %entry ], [ %tmp7, %cond_true ] ; <i32> [#uses=1] %tmp2 = getelementptr i32* %a, i32 %x.0.0 ; <i32*> [#uses=1] %tmp3 = load i32* %tmp2 ; <i32> [#uses=1] %tmp5 = add i32 %t_addr.0.0, %x.0.0 ; <i32> [#uses=1] %tmp7 = add i32 %tmp5, %tmp3 ; <i32> [#uses=2] %tmp9 = add i32 %x.0.0, 1 ; <i32> [#uses=2] %tmp = icmp sgt i32 %tmp9, 39 ; <i1> [#uses=1] br i1 %tmp, label %bb12, label %cond_true bb12: ; preds = %cond_true ret i32 %tmp7 } is pessimized by -loop-reduce and -indvars //===---------------------------------------------------------------------===// u32 to float conversion improvement: float uint32_2_float( unsigned u ) { float fl = (int) (u & 0xffff); float fh = (int) (u >> 16); fh *= 0x1.0p16f; return fh + fl; } 00000000 subl $0x04,%esp 00000003 movl 0x08(%esp,1),%eax 00000007 movl %eax,%ecx 00000009 shrl $0x10,%ecx 0000000c cvtsi2ss %ecx,%xmm0 00000010 andl $0x0000ffff,%eax 00000015 cvtsi2ss %eax,%xmm1 00000019 mulss 0x00000078,%xmm0 00000021 addss %xmm1,%xmm0 00000025 movss %xmm0,(%esp,1) 0000002a flds (%esp,1) 0000002d addl $0x04,%esp 00000030 ret //===---------------------------------------------------------------------===// When using fastcc abi, align stack slot of argument of type double on 8 byte boundary to improve performance. //===---------------------------------------------------------------------===// GCC's ix86_expand_int_movcc function (in i386.c) has a ton of interesting simplifications for integer "x cmp y ? a : b". //===---------------------------------------------------------------------===// Consider the expansion of: define i32 @test3(i32 %X) { %tmp1 = urem i32 %X, 255 ret i32 %tmp1 } Currently it compiles to: ... movl $2155905153, %ecx movl 8(%esp), %esi movl %esi, %eax mull %ecx ... This could be "reassociated" into: movl $2155905153, %eax movl 8(%esp), %ecx mull %ecx to avoid the copy. In fact, the existing two-address stuff would do this except that mul isn't a commutative 2-addr instruction. I guess this has to be done at isel time based on the #uses to mul? //===---------------------------------------------------------------------===// Make sure the instruction which starts a loop does not cross a cacheline boundary. This requires knowning the exact length of each machine instruction. That is somewhat complicated, but doable. Example 256.bzip2: In the new trace, the hot loop has an instruction which crosses a cacheline boundary. In addition to potential cache misses, this can't help decoding as I imagine there has to be some kind of complicated decoder reset and realignment to grab the bytes from the next cacheline. 532 532 0x3cfc movb (1809(%esp, %esi), %bl <<<--- spans 2 64 byte lines 942 942 0x3d03 movl %dh, (1809(%esp, %esi) 937 937 0x3d0a incl %esi 3 3 0x3d0b cmpb %bl, %dl 27 27 0x3d0d jnz 0x000062db <main+11707> //===---------------------------------------------------------------------===// In c99 mode, the preprocessor doesn't like assembly comments like #TRUNCATE. //===---------------------------------------------------------------------===// This could be a single 16-bit load. int f(char *p) { if ((p[0] == 1) & (p[1] == 2)) return 1; return 0; } //===---------------------------------------------------------------------===// We should inline lrintf and probably other libc functions. //===---------------------------------------------------------------------===// This code: void test(int X) { if (X) abort(); } is currently compiled to: _test: subl $12, %esp cmpl $0, 16(%esp) jne LBB1_1 addl $12, %esp ret LBB1_1: call L_abort$stub It would be better to produce: _test: subl $12, %esp cmpl $0, 16(%esp) jne L_abort$stub addl $12, %esp ret This can be applied to any no-return function call that takes no arguments etc. Alternatively, the stack save/restore logic could be shrink-wrapped, producing something like this: _test: cmpl $0, 4(%esp) jne LBB1_1 ret LBB1_1: subl $12, %esp call L_abort$stub Both are useful in different situations. Finally, it could be shrink-wrapped and tail called, like this: _test: cmpl $0, 4(%esp) jne LBB1_1 ret LBB1_1: pop %eax # realign stack. call L_abort$stub Though this probably isn't worth it. //===---------------------------------------------------------------------===// Sometimes it is better to codegen subtractions from a constant (e.g. 7-x) with a neg instead of a sub instruction. Consider: int test(char X) { return 7-X; } we currently produce: _test: movl $7, %eax movsbl 4(%esp), %ecx subl %ecx, %eax ret We would use one fewer register if codegen'd as: movsbl 4(%esp), %eax neg %eax add $7, %eax ret Note that this isn't beneficial if the load can be folded into the sub. In this case, we want a sub: int test(int X) { return 7-X; } _test: movl $7, %eax subl 4(%esp), %eax ret //===---------------------------------------------------------------------===// Leaf functions that require one 4-byte spill slot have a prolog like this: _foo: pushl %esi subl $4, %esp ... and an epilog like this: addl $4, %esp popl %esi ret It would be smaller, and potentially faster, to push eax on entry and to pop into a dummy register instead of using addl/subl of esp. Just don't pop into any return registers :) //===---------------------------------------------------------------------===// The X86 backend should fold (branch (or (setcc, setcc))) into multiple branches. We generate really poor code for: double testf(double a) { return a == 0.0 ? 0.0 : (a > 0.0 ? 1.0 : -1.0); } For example, the entry BB is: _testf: subl $20, %esp pxor %xmm0, %xmm0 movsd 24(%esp), %xmm1 ucomisd %xmm0, %xmm1 setnp %al sete %cl testb %cl, %al jne LBB1_5 # UnifiedReturnBlock LBB1_1: # cond_true it would be better to replace the last four instructions with: jp LBB1_1 je LBB1_5 LBB1_1: We also codegen the inner ?: into a diamond: cvtss2sd LCPI1_0(%rip), %xmm2 cvtss2sd LCPI1_1(%rip), %xmm3 ucomisd %xmm1, %xmm0 ja LBB1_3 # cond_true LBB1_2: # cond_true movapd %xmm3, %xmm2 LBB1_3: # cond_true movapd %xmm2, %xmm0 ret We should sink the load into xmm3 into the LBB1_2 block. This should be pretty easy, and will nuke all the copies. //===---------------------------------------------------------------------===// This: #include <algorithm> inline std::pair<unsigned, bool> full_add(unsigned a, unsigned b) { return std::make_pair(a + b, a + b < a); } bool no_overflow(unsigned a, unsigned b) { return !full_add(a, b).second; } Should compile to: addl %esi, %edi setae %al movzbl %al, %eax ret on x86-64, instead of the rather stupid-looking: addl %esi, %edi setb %al xorb $1, %al movzbl %al, %eax ret //===---------------------------------------------------------------------===// The following code: bb114.preheader: ; preds = %cond_next94 %tmp231232 = sext i16 %tmp62 to i32 ; <i32> [#uses=1] %tmp233 = sub i32 32, %tmp231232 ; <i32> [#uses=1] %tmp245246 = sext i16 %tmp65 to i32 ; <i32> [#uses=1] %tmp252253 = sext i16 %tmp68 to i32 ; <i32> [#uses=1] %tmp254 = sub i32 32, %tmp252253 ; <i32> [#uses=1] %tmp553554 = bitcast i16* %tmp37 to i8* ; <i8*> [#uses=2] %tmp583584 = sext i16 %tmp98 to i32 ; <i32> [#uses=1] %tmp585 = sub i32 32, %tmp583584 ; <i32> [#uses=1] %tmp614615 = sext i16 %tmp101 to i32 ; <i32> [#uses=1] %tmp621622 = sext i16 %tmp104 to i32 ; <i32> [#uses=1] %tmp623 = sub i32 32, %tmp621622 ; <i32> [#uses=1] br label %bb114 produces: LBB3_5: # bb114.preheader movswl -68(%ebp), %eax movl $32, %ecx movl %ecx, -80(%ebp) subl %eax, -80(%ebp) movswl -52(%ebp), %eax movl %ecx, -84(%ebp) subl %eax, -84(%ebp) movswl -70(%ebp), %eax movl %ecx, -88(%ebp) subl %eax, -88(%ebp) movswl -50(%ebp), %eax subl %eax, %ecx movl %ecx, -76(%ebp) movswl -42(%ebp), %eax movl %eax, -92(%ebp) movswl -66(%ebp), %eax movl %eax, -96(%ebp) movw $0, -98(%ebp) This appears to be bad because the RA is not folding the store to the stack slot into the movl. The above instructions could be: movl $32, -80(%ebp) ... movl $32, -84(%ebp) ... This seems like a cross between remat and spill folding. This has redundant subtractions of %eax from a stack slot. However, %ecx doesn't change, so we could simply subtract %eax from %ecx first and then use %ecx (or vice-versa). //===---------------------------------------------------------------------===// This code: %tmp659 = icmp slt i16 %tmp654, 0 ; <i1> [#uses=1] br i1 %tmp659, label %cond_true662, label %cond_next715 produces this: testw %cx, %cx movswl %cx, %esi jns LBB4_109 # cond_next715 Shark tells us that using %cx in the testw instruction is sub-optimal. It suggests using the 32-bit register (which is what ICC uses). //===---------------------------------------------------------------------===// We compile this: void compare (long long foo) { if (foo < 4294967297LL) abort(); } to: compare: subl $4, %esp cmpl $0, 8(%esp) setne %al movzbw %al, %ax cmpl $1, 12(%esp) setg %cl movzbw %cl, %cx cmove %ax, %cx testb $1, %cl jne .LBB1_2 # UnifiedReturnBlock .LBB1_1: # ifthen call abort .LBB1_2: # UnifiedReturnBlock addl $4, %esp ret (also really horrible code on ppc). This is due to the expand code for 64-bit compares. GCC produces multiple branches, which is much nicer: compare: subl $12, %esp movl 20(%esp), %edx movl 16(%esp), %eax decl %edx jle .L7 .L5: addl $12, %esp ret .p2align 4,,7 .L7: jl .L4 cmpl $0, %eax .p2align 4,,8 ja .L5 .L4: .p2align 4,,9 call abort //===---------------------------------------------------------------------===// Tail call optimization improvements: Tail call optimization currently pushes all arguments on the top of the stack (their normal place for non-tail call optimized calls) that source from the callers arguments or that source from a virtual register (also possibly sourcing from callers arguments). This is done to prevent overwriting of parameters (see example below) that might be used later. example: int callee(int32, int64); int caller(int32 arg1, int32 arg2) { int64 local = arg2 * 2; return callee(arg2, (int64)local); } [arg1] [!arg2 no longer valid since we moved local onto it] [arg2] -> [(int64) [RETADDR] local ] Moving arg1 onto the stack slot of callee function would overwrite arg2 of the caller. Possible optimizations: - Analyse the actual parameters of the callee to see which would overwrite a caller parameter which is used by the callee and only push them onto the top of the stack. int callee (int32 arg1, int32 arg2); int caller (int32 arg1, int32 arg2) { return callee(arg1,arg2); } Here we don't need to write any variables to the top of the stack since they don't overwrite each other. int callee (int32 arg1, int32 arg2); int caller (int32 arg1, int32 arg2) { return callee(arg2,arg1); } Here we need to push the arguments because they overwrite each other. //===---------------------------------------------------------------------===// main () { int i = 0; unsigned long int z = 0; do { z -= 0x00004000; i++; if (i > 0x00040000) abort (); } while (z > 0); exit (0); } gcc compiles this to: _main: subl $28, %esp xorl %eax, %eax jmp L2 L3: cmpl $262144, %eax je L10 L2: addl $1, %eax cmpl $262145, %eax jne L3 call L_abort$stub L10: movl $0, (%esp) call L_exit$stub llvm: _main: subl $12, %esp movl $1, %eax movl $16384, %ecx LBB1_1: # bb cmpl $262145, %eax jge LBB1_4 # cond_true LBB1_2: # cond_next incl %eax addl $4294950912, %ecx cmpl $16384, %ecx jne LBB1_1 # bb LBB1_3: # bb11 xorl %eax, %eax addl $12, %esp ret LBB1_4: # cond_true call L_abort$stub 1. LSR should rewrite the first cmp with induction variable %ecx. 2. DAG combiner should fold leal 1(%eax), %edx cmpl $262145, %edx => cmpl $262144, %eax //===---------------------------------------------------------------------===// define i64 @test(double %X) { %Y = fptosi double %X to i64 ret i64 %Y } compiles to: _test: subl $20, %esp movsd 24(%esp), %xmm0 movsd %xmm0, 8(%esp) fldl 8(%esp) fisttpll (%esp) movl 4(%esp), %edx movl (%esp), %eax addl $20, %esp #FP_REG_KILL ret This should just fldl directly from the input stack slot. //===---------------------------------------------------------------------===// This code: int foo (int x) { return (x & 65535) | 255; } Should compile into: _foo: movzwl 4(%esp), %eax orl $255, %eax ret instead of: _foo: movl $65280, %eax andl 4(%esp), %eax orl $255, %eax ret //===---------------------------------------------------------------------===// We're codegen'ing multiply of long longs inefficiently: unsigned long long LLM(unsigned long long arg1, unsigned long long arg2) { return arg1 * arg2; } We compile to (fomit-frame-pointer): _LLM: pushl %esi movl 8(%esp), %ecx movl 16(%esp), %esi movl %esi, %eax mull %ecx imull 12(%esp), %esi addl %edx, %esi imull 20(%esp), %ecx movl %esi, %edx addl %ecx, %edx popl %esi ret This looks like a scheduling deficiency and lack of remat of the load from the argument area. ICC apparently produces: movl 8(%esp), %ecx imull 12(%esp), %ecx movl 16(%esp), %eax imull 4(%esp), %eax addl %eax, %ecx movl 4(%esp), %eax mull 12(%esp) addl %ecx, %edx ret Note that it remat'd loads from 4(esp) and 12(esp). See this GCC PR: http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17236 //===---------------------------------------------------------------------===// We can fold a store into "zeroing a reg". Instead of: xorl %eax, %eax movl %eax, 124(%esp) we should get: movl $0, 124(%esp) if the flags of the xor are dead. Likewise, we isel "x<<1" into "add reg,reg". If reg is spilled, this should be folded into: shl [mem], 1 //===---------------------------------------------------------------------===// In SSE mode, we turn abs and neg into a load from the constant pool plus a xor or and instruction, for example: xorpd LCPI1_0, %xmm2 However, if xmm2 gets spilled, we end up with really ugly code like this: movsd (%esp), %xmm0 xorpd LCPI1_0, %xmm0 movsd %xmm0, (%esp) Since we 'know' that this is a 'neg', we can actually "fold" the spill into the neg/abs instruction, turning it into an *integer* operation, like this: xorl 2147483648, [mem+4] ## 2147483648 = (1 << 31) you could also use xorb, but xorl is less likely to lead to a partial register stall. Here is a contrived testcase: double a, b, c; void test(double *P) { double X = *P; a = X; bar(); X = -X; b = X; bar(); c = X; } //===---------------------------------------------------------------------===// The generated code on x86 for checking for signed overflow on a multiply the obvious way is much longer than it needs to be. int x(int a, int b) { long long prod = (long long)a*b; return prod > 0x7FFFFFFF || prod < (-0x7FFFFFFF-1); } See PR2053 for more details. //===---------------------------------------------------------------------===// We should investigate using cdq/ctld (effect: edx = sar eax, 31) more aggressively; it should cost the same as a move+shift on any modern processor, but it's a lot shorter. Downside is that it puts more pressure on register allocation because it has fixed operands. Example: int abs(int x) {return x < 0 ? -x : x;} gcc compiles this to the following when using march/mtune=pentium2/3/4/m/etc.: abs: movl 4(%esp), %eax cltd xorl %edx, %eax subl %edx, %eax ret //===---------------------------------------------------------------------===// Take the following code (from http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16541): extern unsigned char first_one[65536]; int FirstOnet(unsigned long long arg1) { if (arg1 >> 48) return (first_one[arg1 >> 48]); return 0; } The following code is currently generated: FirstOnet: movl 8(%esp), %eax cmpl $65536, %eax movl 4(%esp), %ecx jb .LBB1_2 # UnifiedReturnBlock .LBB1_1: # ifthen shrl $16, %eax movzbl first_one(%eax), %eax ret .LBB1_2: # UnifiedReturnBlock xorl %eax, %eax ret We could change the "movl 8(%esp), %eax" into "movzwl 10(%esp), %eax"; this lets us change the cmpl into a testl, which is shorter, and eliminate the shift. //===---------------------------------------------------------------------===// We compile this function: define i32 @foo(i32 %a, i32 %b, i32 %c, i8 zeroext %d) nounwind { entry: %tmp2 = icmp eq i8 %d, 0 ; <i1> [#uses=1] br i1 %tmp2, label %bb7, label %bb bb: ; preds = %entry %tmp6 = add i32 %b, %a ; <i32> [#uses=1] ret i32 %tmp6 bb7: ; preds = %entry %tmp10 = sub i32 %a, %c ; <i32> [#uses=1] ret i32 %tmp10 } to: foo: # @foo # BB#0: # %entry movl 4(%esp), %ecx cmpb $0, 16(%esp) je .LBB0_2 # BB#1: # %bb movl 8(%esp), %eax addl %ecx, %eax ret .LBB0_2: # %bb7 movl 12(%esp), %edx movl %ecx, %eax subl %edx, %eax ret There's an obviously unnecessary movl in .LBB0_2, and we could eliminate a couple more movls by putting 4(%esp) into %eax instead of %ecx. //===---------------------------------------------------------------------===// See rdar://4653682. From flops: LBB1_15: # bb310 cvtss2sd LCPI1_0, %xmm1 addsd %xmm1, %xmm0 movsd 176(%esp), %xmm2 mulsd %xmm0, %xmm2 movapd %xmm2, %xmm3 mulsd %xmm3, %xmm3 movapd %xmm3, %xmm4 mulsd LCPI1_23, %xmm4 addsd LCPI1_24, %xmm4 mulsd %xmm3, %xmm4 addsd LCPI1_25, %xmm4 mulsd %xmm3, %xmm4 addsd LCPI1_26, %xmm4 mulsd %xmm3, %xmm4 addsd LCPI1_27, %xmm4 mulsd %xmm3, %xmm4 addsd LCPI1_28, %xmm4 mulsd %xmm3, %xmm4 addsd %xmm1, %xmm4 mulsd %xmm2, %xmm4 movsd 152(%esp), %xmm1 addsd %xmm4, %xmm1 movsd %xmm1, 152(%esp) incl %eax cmpl %eax, %esi jge LBB1_15 # bb310 LBB1_16: # bb358.loopexit movsd 152(%esp), %xmm0 addsd %xmm0, %xmm0 addsd LCPI1_22, %xmm0 movsd %xmm0, 152(%esp) Rather than spilling the result of the last addsd in the loop, we should have insert a copy to split the interval (one for the duration of the loop, one extending to the fall through). The register pressure in the loop isn't high enough to warrant the spill. Also check why xmm7 is not used at all in the function. //===---------------------------------------------------------------------===// Take the following: target datalayout = "e-p:32:32:32-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:32:64-f32:32:32-f64:32:64-v64:64:64-v128:128:128-a0:0:64-f80:128:128-S128" target triple = "i386-apple-darwin8" @in_exit.4870.b = internal global i1 false ; <i1*> [#uses=2] define fastcc void @abort_gzip() noreturn nounwind { entry: %tmp.b.i = load i1* @in_exit.4870.b ; <i1> [#uses=1] br i1 %tmp.b.i, label %bb.i, label %bb4.i bb.i: ; preds = %entry tail call void @exit( i32 1 ) noreturn nounwind unreachable bb4.i: ; preds = %entry store i1 true, i1* @in_exit.4870.b tail call void @exit( i32 1 ) noreturn nounwind unreachable } declare void @exit(i32) noreturn nounwind This compiles into: _abort_gzip: ## @abort_gzip ## BB#0: ## %entry subl $12, %esp movb _in_exit.4870.b, %al cmpb $1, %al jne LBB0_2 We somehow miss folding the movb into the cmpb. //===---------------------------------------------------------------------===// We compile: int test(int x, int y) { return x-y-1; } into (-m64): _test: decl %edi movl %edi, %eax subl %esi, %eax ret it would be better to codegen as: x+~y (notl+addl) //===---------------------------------------------------------------------===// This code: int foo(const char *str,...) { __builtin_va_list a; int x; __builtin_va_start(a,str); x = __builtin_va_arg(a,int); __builtin_va_end(a); return x; } gets compiled into this on x86-64: subq $200, %rsp movaps %xmm7, 160(%rsp) movaps %xmm6, 144(%rsp) movaps %xmm5, 128(%rsp) movaps %xmm4, 112(%rsp) movaps %xmm3, 96(%rsp) movaps %xmm2, 80(%rsp) movaps %xmm1, 64(%rsp) movaps %xmm0, 48(%rsp) movq %r9, 40(%rsp) movq %r8, 32(%rsp) movq %rcx, 24(%rsp) movq %rdx, 16(%rsp) movq %rsi, 8(%rsp) leaq (%rsp), %rax movq %rax, 192(%rsp) leaq 208(%rsp), %rax movq %rax, 184(%rsp) movl $48, 180(%rsp) movl $8, 176(%rsp) movl 176(%rsp), %eax cmpl $47, %eax jbe .LBB1_3 # bb .LBB1_1: # bb3 movq 184(%rsp), %rcx leaq 8(%rcx), %rax movq %rax, 184(%rsp) .LBB1_2: # bb4 movl (%rcx), %eax addq $200, %rsp ret .LBB1_3: # bb movl %eax, %ecx addl $8, %eax addq 192(%rsp), %rcx movl %eax, 176(%rsp) jmp .LBB1_2 # bb4 gcc 4.3 generates: subq $96, %rsp .LCFI0: leaq 104(%rsp), %rax movq %rsi, -80(%rsp) movl $8, -120(%rsp) movq %rax, -112(%rsp) leaq -88(%rsp), %rax movq %rax, -104(%rsp) movl $8, %eax cmpl $48, %eax jb .L6 movq -112(%rsp), %rdx movl (%rdx), %eax addq $96, %rsp ret .p2align 4,,10 .p2align 3 .L6: mov %eax, %edx addq -104(%rsp), %rdx addl $8, %eax movl %eax, -120(%rsp) movl (%rdx), %eax addq $96, %rsp ret and it gets compiled into this on x86: pushl %ebp movl %esp, %ebp subl $4, %esp leal 12(%ebp), %eax movl %eax, -4(%ebp) leal 16(%ebp), %eax movl %eax, -4(%ebp) movl 12(%ebp), %eax addl $4, %esp popl %ebp ret gcc 4.3 generates: pushl %ebp movl %esp, %ebp movl 12(%ebp), %eax popl %ebp ret //===---------------------------------------------------------------------===// Teach tblgen not to check bitconvert source type in some cases. This allows us to consolidate the following patterns in X86InstrMMX.td: def : Pat<(v2i32 (bitconvert (i64 (vector_extract (v2i64 VR128:$src), (iPTR 0))))), (v2i32 (MMX_MOVDQ2Qrr VR128:$src))>; def : Pat<(v4i16 (bitconvert (i64 (vector_extract (v2i64 VR128:$src), (iPTR 0))))), (v4i16 (MMX_MOVDQ2Qrr VR128:$src))>; def : Pat<(v8i8 (bitconvert (i64 (vector_extract (v2i64 VR128:$src), (iPTR 0))))), (v8i8 (MMX_MOVDQ2Qrr VR128:$src))>; There are other cases in various td files. //===---------------------------------------------------------------------===// Take something like the following on x86-32: unsigned a(unsigned long long x, unsigned y) {return x % y;} We currently generate a libcall, but we really shouldn't: the expansion is shorter and likely faster than the libcall. The expected code is something like the following: movl 12(%ebp), %eax movl 16(%ebp), %ecx xorl %edx, %edx divl %ecx movl 8(%ebp), %eax divl %ecx movl %edx, %eax ret A similar code sequence works for division. //===---------------------------------------------------------------------===// We currently compile this: define i32 @func1(i32 %v1, i32 %v2) nounwind { entry: %t = call {i32, i1} @llvm.sadd.with.overflow.i32(i32 %v1, i32 %v2) %sum = extractvalue {i32, i1} %t, 0 %obit = extractvalue {i32, i1} %t, 1 br i1 %obit, label %overflow, label %normal normal: ret i32 %sum overflow: call void @llvm.trap() unreachable } declare {i32, i1} @llvm.sadd.with.overflow.i32(i32, i32) declare void @llvm.trap() to: _func1: movl 4(%esp), %eax addl 8(%esp), %eax jo LBB1_2 ## overflow LBB1_1: ## normal ret LBB1_2: ## overflow ud2 it would be nice to produce "into" someday. //===---------------------------------------------------------------------===// Test instructions can be eliminated by using EFLAGS values from arithmetic instructions. This is currently not done for mul, and, or, xor, neg, shl, sra, srl, shld, shrd, atomic ops, and others. It is also currently not done for read-modify-write instructions. It is also current not done if the OF or CF flags are needed. The shift operators have the complication that when the shift count is zero, EFLAGS is not set, so they can only subsume a test instruction if the shift count is known to be non-zero. Also, using the EFLAGS value from a shift is apparently very slow on some x86 implementations. In read-modify-write instructions, the root node in the isel match is the store, and isel has no way for the use of the EFLAGS result of the arithmetic to be remapped to the new node. Add and subtract instructions set OF on signed overflow and CF on unsiged overflow, while test instructions always clear OF and CF. In order to replace a test with an add or subtract in a situation where OF or CF is needed, codegen must be able to prove that the operation cannot see signed or unsigned overflow, respectively. //===---------------------------------------------------------------------===// memcpy/memmove do not lower to SSE copies when possible. A silly example is: define <16 x float> @foo(<16 x float> %A) nounwind { %tmp = alloca <16 x float>, align 16 %tmp2 = alloca <16 x float>, align 16 store <16 x float> %A, <16 x float>* %tmp %s = bitcast <16 x float>* %tmp to i8* %s2 = bitcast <16 x float>* %tmp2 to i8* call void @llvm.memcpy.i64(i8* %s, i8* %s2, i64 64, i32 16) %R = load <16 x float>* %tmp2 ret <16 x float> %R } declare void @llvm.memcpy.i64(i8* nocapture, i8* nocapture, i64, i32) nounwind which compiles to: _foo: subl $140, %esp movaps %xmm3, 112(%esp) movaps %xmm2, 96(%esp) movaps %xmm1, 80(%esp) movaps %xmm0, 64(%esp) movl 60(%esp), %eax movl %eax, 124(%esp) movl 56(%esp), %eax movl %eax, 120(%esp) movl 52(%esp), %eax <many many more 32-bit copies> movaps (%esp), %xmm0 movaps 16(%esp), %xmm1 movaps 32(%esp), %xmm2 movaps 48(%esp), %xmm3 addl $140, %esp ret On Nehalem, it may even be cheaper to just use movups when unaligned than to fall back to lower-granularity chunks. //===---------------------------------------------------------------------===// Implement processor-specific optimizations for parity with GCC on these processors. GCC does two optimizations: 1. ix86_pad_returns inserts a noop before ret instructions if immediately preceded by a conditional branch or is the target of a jump. 2. ix86_avoid_jump_misspredicts inserts noops in cases where a 16-byte block of code contains more than 3 branches. The first one is done for all AMDs, Core2, and "Generic" The second one is done for: Atom, Pentium Pro, all AMDs, Pentium 4, Nocona, Core 2, and "Generic" //===---------------------------------------------------------------------===// Testcase: int x(int a) { return (a&0xf0)>>4; } Current output: movl 4(%esp), %eax shrl $4, %eax andl $15, %eax ret Ideal output: movzbl 4(%esp), %eax shrl $4, %eax ret //===---------------------------------------------------------------------===// Re-implement atomic builtins __sync_add_and_fetch() and __sync_sub_and_fetch properly. When the return value is not used (i.e. only care about the value in the memory), x86 does not have to use add to implement these. Instead, it can use add, sub, inc, dec instructions with the "lock" prefix. This is currently implemented using a bit of instruction selection trick. The issue is the target independent pattern produces one output and a chain and we want to map it into one that just output a chain. The current trick is to select it into a MERGE_VALUES with the first definition being an implicit_def. The proper solution is to add new ISD opcodes for the no-output variant. DAG combiner can then transform the node before it gets to target node selection. Problem #2 is we are adding a whole bunch of x86 atomic instructions when in fact these instructions are identical to the non-lock versions. We need a way to add target specific information to target nodes and have this information carried over to machine instructions. Asm printer (or JIT) can use this information to add the "lock" prefix. //===---------------------------------------------------------------------===// struct B { unsigned char y0 : 1; }; int bar(struct B* a) { return a->y0; } define i32 @bar(%struct.B* nocapture %a) nounwind readonly optsize { %1 = getelementptr inbounds %struct.B* %a, i64 0, i32 0 %2 = load i8* %1, align 1 %3 = and i8 %2, 1 %4 = zext i8 %3 to i32 ret i32 %4 } bar: # @bar # BB#0: movb (%rdi), %al andb $1, %al movzbl %al, %eax ret Missed optimization: should be movl+andl. //===---------------------------------------------------------------------===// The x86_64 abi says: Booleans, when stored in a memory object, are stored as single byte objects the value of which is always 0 (false) or 1 (true). We are not using this fact: int bar(_Bool *a) { return *a; } define i32 @bar(i8* nocapture %a) nounwind readonly optsize { %1 = load i8* %a, align 1, !tbaa !0 %tmp = and i8 %1, 1 %2 = zext i8 %tmp to i32 ret i32 %2 } bar: movb (%rdi), %al andb $1, %al movzbl %al, %eax ret GCC produces bar: movzbl (%rdi), %eax ret //===---------------------------------------------------------------------===// Consider the following two functions compiled with clang: _Bool foo(int *x) { return !(*x & 4); } unsigned bar(int *x) { return !(*x & 4); } foo: movl 4(%esp), %eax testb $4, (%eax) sete %al movzbl %al, %eax ret bar: movl 4(%esp), %eax movl (%eax), %eax shrl $2, %eax andl $1, %eax xorl $1, %eax ret The second function generates more code even though the two functions are are functionally identical. //===---------------------------------------------------------------------===// Take the following C code: int f(int a, int b) { return (unsigned char)a == (unsigned char)b; } We generate the following IR with clang: define i32 @f(i32 %a, i32 %b) nounwind readnone { entry: %tmp = xor i32 %b, %a ; <i32> [#uses=1] %tmp6 = and i32 %tmp, 255 ; <i32> [#uses=1] %cmp = icmp eq i32 %tmp6, 0 ; <i1> [#uses=1] %conv5 = zext i1 %cmp to i32 ; <i32> [#uses=1] ret i32 %conv5 } And the following x86 code: xorl %esi, %edi testb $-1, %dil sete %al movzbl %al, %eax ret A cmpb instead of the xorl+testb would be one instruction shorter. //===---------------------------------------------------------------------===// Given the following C code: int f(int a, int b) { return (signed char)a == (signed char)b; } We generate the following IR with clang: define i32 @f(i32 %a, i32 %b) nounwind readnone { entry: %sext = shl i32 %a, 24 ; <i32> [#uses=1] %conv1 = ashr i32 %sext, 24 ; <i32> [#uses=1] %sext6 = shl i32 %b, 24 ; <i32> [#uses=1] %conv4 = ashr i32 %sext6, 24 ; <i32> [#uses=1] %cmp = icmp eq i32 %conv1, %conv4 ; <i1> [#uses=1] %conv5 = zext i1 %cmp to i32 ; <i32> [#uses=1] ret i32 %conv5 } And the following x86 code: movsbl %sil, %eax movsbl %dil, %ecx cmpl %eax, %ecx sete %al movzbl %al, %eax ret It should be possible to eliminate the sign extensions. //===---------------------------------------------------------------------===// LLVM misses a load+store narrowing opportunity in this code: %struct.bf = type { i64, i16, i16, i32 } @bfi = external global %struct.bf* ; <%struct.bf**> [#uses=2] define void @t1() nounwind ssp { entry: %0 = load %struct.bf** @bfi, align 8 ; <%struct.bf*> [#uses=1] %1 = getelementptr %struct.bf* %0, i64 0, i32 1 ; <i16*> [#uses=1] %2 = bitcast i16* %1 to i32* ; <i32*> [#uses=2] %3 = load i32* %2, align 1 ; <i32> [#uses=1] %4 = and i32 %3, -65537 ; <i32> [#uses=1] store i32 %4, i32* %2, align 1 %5 = load %struct.bf** @bfi, align 8 ; <%struct.bf*> [#uses=1] %6 = getelementptr %struct.bf* %5, i64 0, i32 1 ; <i16*> [#uses=1] %7 = bitcast i16* %6 to i32* ; <i32*> [#uses=2] %8 = load i32* %7, align 1 ; <i32> [#uses=1] %9 = and i32 %8, -131073 ; <i32> [#uses=1] store i32 %9, i32* %7, align 1 ret void } LLVM currently emits this: movq bfi(%rip), %rax andl $-65537, 8(%rax) movq bfi(%rip), %rax andl $-131073, 8(%rax) ret It could narrow the loads and stores to emit this: movq bfi(%rip), %rax andb $-2, 10(%rax) movq bfi(%rip), %rax andb $-3, 10(%rax) ret The trouble is that there is a TokenFactor between the store and the load, making it non-trivial to determine if there's anything between the load and the store which would prohibit narrowing. //===---------------------------------------------------------------------===// This code: void foo(unsigned x) { if (x == 0) bar(); else if (x == 1) qux(); } currently compiles into: _foo: movl 4(%esp), %eax cmpl $1, %eax je LBB0_3 testl %eax, %eax jne LBB0_4 the testl could be removed: _foo: movl 4(%esp), %eax cmpl $1, %eax je LBB0_3 jb LBB0_4 0 is the only unsigned number < 1. //===---------------------------------------------------------------------===// This code: %0 = type { i32, i1 } define i32 @add32carry(i32 %sum, i32 %x) nounwind readnone ssp { entry: %uadd = tail call %0 @llvm.uadd.with.overflow.i32(i32 %sum, i32 %x) %cmp = extractvalue %0 %uadd, 1 %inc = zext i1 %cmp to i32 %add = add i32 %x, %sum %z.0 = add i32 %add, %inc ret i32 %z.0 } declare %0 @llvm.uadd.with.overflow.i32(i32, i32) nounwind readnone compiles to: _add32carry: ## @add32carry addl %esi, %edi sbbl %ecx, %ecx movl %edi, %eax subl %ecx, %eax ret But it could be: _add32carry: leal (%rsi,%rdi), %eax cmpl %esi, %eax adcl $0, %eax ret //===---------------------------------------------------------------------===// The hot loop of 256.bzip2 contains code that looks a bit like this: int foo(char *P, char *Q, int x, int y) { if (P[0] != Q[0]) return P[0] < Q[0]; if (P[1] != Q[1]) return P[1] < Q[1]; if (P[2] != Q[2]) return P[2] < Q[2]; return P[3] < Q[3]; } In the real code, we get a lot more wrong than this. However, even in this code we generate: _foo: ## @foo ## BB#0: ## %entry movb (%rsi), %al movb (%rdi), %cl cmpb %al, %cl je LBB0_2 LBB0_1: ## %if.then cmpb %al, %cl jmp LBB0_5 LBB0_2: ## %if.end movb 1(%rsi), %al movb 1(%rdi), %cl cmpb %al, %cl jne LBB0_1 ## BB#3: ## %if.end38 movb 2(%rsi), %al movb 2(%rdi), %cl cmpb %al, %cl jne LBB0_1 ## BB#4: ## %if.end60 movb 3(%rdi), %al cmpb 3(%rsi), %al LBB0_5: ## %if.end60 setl %al movzbl %al, %eax ret Note that we generate jumps to LBB0_1 which does a redundant compare. The redundant compare also forces the register values to be live, which prevents folding one of the loads into the compare. In contrast, GCC 4.2 produces: _foo: movzbl (%rsi), %eax cmpb %al, (%rdi) jne L10 L12: movzbl 1(%rsi), %eax cmpb %al, 1(%rdi) jne L10 movzbl 2(%rsi), %eax cmpb %al, 2(%rdi) jne L10 movzbl 3(%rdi), %eax cmpb 3(%rsi), %al L10: setl %al movzbl %al, %eax ret which is "perfect". //===---------------------------------------------------------------------===// For the branch in the following code: int a(); int b(int x, int y) { if (x & (1<<(y&7))) return a(); return y; } We currently generate: movb %sil, %al andb $7, %al movzbl %al, %eax btl %eax, %edi jae .LBB0_2 movl+andl would be shorter than the movb+andb+movzbl sequence. //===---------------------------------------------------------------------===// For the following: struct u1 { float x, y; }; float foo(struct u1 u) { return u.x + u.y; } We currently generate: movdqa %xmm0, %xmm1 pshufd $1, %xmm0, %xmm0 # xmm0 = xmm0[1,0,0,0] addss %xmm1, %xmm0 ret We could save an instruction here by commuting the addss. //===---------------------------------------------------------------------===// This (from PR9661): float clamp_float(float a) { if (a > 1.0f) return 1.0f; else if (a < 0.0f) return 0.0f; else return a; } Could compile to: clamp_float: # @clamp_float movss .LCPI0_0(%rip), %xmm1 minss %xmm1, %xmm0 pxor %xmm1, %xmm1 maxss %xmm1, %xmm0 ret with -ffast-math. //===---------------------------------------------------------------------===// This function (from PR9803): int clamp2(int a) { if (a > 5) a = 5; if (a < 0) return 0; return a; } Compiles to: _clamp2: ## @clamp2 pushq %rbp movq %rsp, %rbp cmpl $5, %edi movl $5, %ecx cmovlel %edi, %ecx testl %ecx, %ecx movl $0, %eax cmovnsl %ecx, %eax popq %rbp ret The move of 0 could be scheduled above the test to make it is xor reg,reg. //===---------------------------------------------------------------------===// GCC PR48986. We currently compile this: void bar(void); void yyy(int* p) { if (__sync_fetch_and_add(p, -1) == 1) bar(); } into: movl $-1, %eax lock xaddl %eax, (%rdi) cmpl $1, %eax je LBB0_2 Instead we could generate: lock dec %rdi je LBB0_2 The trick is to match "fetch_and_add(X, -C) == C". //===---------------------------------------------------------------------===// unsigned t(unsigned a, unsigned b) { return a <= b ? 5 : -5; } We generate: movl $5, %ecx cmpl %esi, %edi movl $-5, %eax cmovbel %ecx, %eax GCC: cmpl %edi, %esi sbbl %eax, %eax andl $-10, %eax addl $5, %eax //===---------------------------------------------------------------------===//