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Teach instcombine to fold compares of loads from constant 

arrays with variable indices into a comparison of the index
with a constant.  The most common occurrence of this that
I see by far is stuff like:

if ("foobar"[i] == '\0') ...

which we compile into: if (i == 6), saving a load and 
materialization of the global address.  This also exposes 
loop trip count information to later passes in many cases.

This triggers hundreds of times in xalancbmk, which is where I first
noticed it, but it also triggers in many other apps.  Here are a few 
interesting ones from various apps:

@must_be_connected_without = internal constant [8 x i8*] [i8* getelementptr inbounds ([3 x i8]* @.str64320, i64 0, i64 0), i8* getelementptr inbounds ([3 x i8]* @.str27283, i64 0, i64 0), i8* getelementptr inbounds ([4 x i8]* @.str71327, i64 0, i64 0), i8* getelementptr inbounds ([4 x i8]* @.str72328, i64 0, i64 0), i8* getelementptr inbounds ([3 x i8]* @.str18274, i64 0, i64 0), i8* getelementptr inbounds ([6 x i8]* @.str11267, i64 0, i64 0), i8* getelementptr inbounds ([3 x i8]* @.str32288, i64 0, i64 0), i8* null], align 32 ; <[8 x i8*]*> [#uses=2]
  %scevgep.i = getelementptr [8 x i8*]* @must_be_connected_without, i64 0, i64 %indvar.i ; <i8**> [#uses=1]
  %17 = load ...
  %18 = icmp eq i8* %17, null                     ; <i1> [#uses=1]
-> icmp eq i64 %indvar.i, 7 


@yytable1095 = internal constant [84 x i8] c"\12\01(\05\06\07\08\09\0A\0B\0C\0D\0E1\0F\10\11266\1D: \10\11,-,0\03'\10\11B6\04\17&\18\1945\05\06\07\08\09\0A\0B\0C\0D\0E\1E\0F\10\11*\1A\1B\1C$3+>#%;<IJ=ADFEGH9KL\00\00\00C", align 32 ; <[84 x i8]*> [#uses=2]
  %57 = getelementptr inbounds [84 x i8]* @yytable1095, i64 0, i64 %56 ; <i8*> [#uses=1]
   %mode.0.in = getelementptr inbounds [9 x i32]* @mb_mode_table, i64 0, i64 %.pn ; <i32*> [#uses=1]
load ...
   %64 = icmp eq i8 %58, 4                         ; <i1> [#uses=1]
-> icmp eq i64 %.pn, 35             ; <i1> [#uses=0]


@gsm_DLB = internal constant [4 x i16] [i16 6554, i16 16384, i16 26214, i16 32767]
%scevgep.i = getelementptr [4 x i16]* @gsm_DLB, i64 0, i64 %indvar.i ; <i16*> [#uses=1]
%425 = load %scevgep.i
%426 = icmp eq i16 %425, -32768                 ; <i1> [#uses=0]
-> false

llvm-svn: 92411
This commit is contained in:
Chris Lattner 2010-01-02 08:12:04 +00:00
parent 8fb74c6ee2
commit a061859ccc
2 changed files with 145 additions and 1 deletions
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121
llvm/lib/Transforms/Scalar/InstructionCombining.cpp
View File

@ -258,6 +258,8 @@ namespace {
Instruction *commonShiftTransforms(BinaryOperator &I);
Instruction *FoldFCmp_IntToFP_Cst(FCmpInst &I, Instruction *LHSI,
Constant *RHSC);
Instruction *FoldCmpLoadFromIndexedGlobal(GetElementPtrInst *GEP,
GlobalVariable *GV, CmpInst &ICI);
Instruction *visitFCmpInst(FCmpInst &I);
Instruction *visitICmpInst(ICmpInst &I);
Instruction *visitICmpInstWithCastAndCast(ICmpInst &ICI);
@ -6015,6 +6017,112 @@ Instruction *InstCombiner::FoldFCmp_IntToFP_Cst(FCmpInst &I,
return new ICmpInst(Pred, LHSI->getOperand(0), RHSInt);
}
/// FoldCmpLoadFromIndexedGlobal - Called we see this pattern:
/// cmp pred (load (gep GV, ...)), cmpcst
/// where GV is a global variable with a constant initializer. Try to simplify
/// this into one or two simpler comparisons that do not need the load. For
/// example, we can optimize "icmp eq (load (gep "foo", 0, i)), 0" into
/// "icmp eq i, 3". We assume that eliminating a load is always goodness.
Instruction *InstCombiner::
FoldCmpLoadFromIndexedGlobal(GetElementPtrInst *GEP, GlobalVariable *GV,
CmpInst &ICI) {
// There are many forms of this optimization we can handle, for now, just do
// the simple index into a single-dimensional array.
//
// Require: GEP GV, 0, i
if (GEP->getNumOperands() != 3 ||
!isa<ConstantInt>(GEP->getOperand(1)) ||
!cast<ConstantInt>(GEP->getOperand(1))->isZero())
return 0;
ConstantArray *Init = dyn_cast<ConstantArray>(GV->getInitializer());
if (Init == 0 || Init->getNumOperands() > 1024) return 0;
// Variables for our state machines.
// OnlyTrueElement - Used to emit a comparison of "i == 47", where 47 is the
// only index the condition is true for. The values are -1 -> undef,
// -2 -> overdef, >= 0 -> that index is true.
int OnlyTrueElement = -1;
// OnlyFalseElement - Used to emit a comparison of "i != 47", where 47 is the
// only index the condition is false for. The values are -1 -> undef,
// -2 -> overdef, >= 0 -> that index is false.
int OnlyFalseElement = -1;
// Scan the array and see if one of our patterns matches.
Constant *CompareRHS = cast<Constant>(ICI.getOperand(1));
for (unsigned i = 0, e = Init->getNumOperands(); i != e; ++i) {
// Find out if the comparison would be true or false for the i'th element.
Constant *C = ConstantFoldCompareInstOperands(ICI.getPredicate(),
Init->getOperand(i),
CompareRHS, TD);
// If the result is undef for this element, ignore it.
if (isa<UndefValue>(C)) continue;
// If we can't compute the result for any of the elements, we have to give
// up evaluating the entire conditional.
if (!isa<ConstantInt>(C)) return 0;
// Otherwise, we know if the comparison is true or false for this element,
// update our state machines.
bool IsTrueForElt = !cast<ConstantInt>(C)->isZero();
// State machine for single index comparison.
if (IsTrueForElt) {
// If undefined -> defined. Otherwise -> overdefined.
OnlyTrueElement = OnlyTrueElement == -1 ? i : -2;
} else {
// If undefined -> defined. Otherwise -> overdefined.
OnlyFalseElement = OnlyFalseElement == -1 ? i : -2;
}
// If all of our states become overdefined, bail out early.
if (OnlyTrueElement == -2 && OnlyFalseElement == -2)
return 0;
}
// Now that we've scanned the entire array, emit our new comparison(s). We
// order the state machines in complexity of the generated code.
if (OnlyTrueElement != -2) {
// None true -> false.
if (OnlyTrueElement == -1)
return ReplaceInstUsesWith(ICI, ConstantInt::getFalse(*Context));
// True for one element -> 'i == 47'.
return new ICmpInst(ICmpInst::ICMP_EQ, GEP->getOperand(2),
ConstantInt::get(GEP->getOperand(2)->getType(),
OnlyTrueElement));
}
if (OnlyFalseElement != -2) {
// None false -> true.
if (OnlyFalseElement == -1)
return ReplaceInstUsesWith(ICI, ConstantInt::getTrue(*Context));
return new ICmpInst(ICmpInst::ICMP_NE, GEP->getOperand(2),
ConstantInt::get(GEP->getOperand(2)->getType(),
OnlyFalseElement));
}
assert(0 && "Should have bailed out early");
// TODO: FCMP.
// TODO: Range check.
// TODO: If the global array has 32 (or 64 if native!) or less entries, we
// can turn this into something like:
// ((magicbitconstant >> i) & 1) != 0)
// where we populate magicbitconstant with 0101010 based on the comparison
// results.
return 0;
}
Instruction *InstCombiner::visitFCmpInst(FCmpInst &I) {
bool Changed = false;
@ -6175,7 +6283,7 @@ Instruction *InstCombiner::visitICmpInst(ICmpInst &I) {
Value *A = 0, *B = 0;
// (icmp ne/eq (sub A B) 0) -> (icmp ne/eq A, B)
if (I.isEquality() && CI->isNullValue() &&
if (I.isEquality() && CI->isZero() &&
match(Op0, m_Sub(m_Value(A), m_Value(B)))) {
// (icmp cond A B) if cond is equality
return new ICmpInst(I.getPredicate(), A, B);
@ -6475,6 +6583,17 @@ Instruction *InstCombiner::visitICmpInst(ICmpInst &I) {
return new ICmpInst(I.getPredicate(), LHSI->getOperand(0),
Constant::getNullValue(LHSI->getOperand(0)->getType()));
break;
case Instruction::Load:
if (GetElementPtrInst *GEP =
dyn_cast<GetElementPtrInst>(LHSI->getOperand(0)))
if (GlobalVariable *GV = dyn_cast<GlobalVariable>(GEP->getOperand(0)))
if (GV->isConstant() && GV->hasDefinitiveInitializer() &&
!cast<LoadInst>(LHSI)->isVolatile()) {
if (Instruction *Res = FoldCmpLoadFromIndexedGlobal(GEP, GV, I))
return Res;
}
break;
}
}

25
llvm/test/Transforms/InstCombine/load-cmp.ll Normal file
View File

@ -0,0 +1,25 @@
; RUN: opt < %s -instcombine -S | FileCheck %s
@G16 = internal constant [10 x i16] [i16 35, i16 82, i16 69, i16 81, i16 85,
i16 73, i16 82, i16 69, i16 68, i16 0]
define i1 @test1(i32 %X) {
%P = getelementptr [10 x i16]* @G16, i32 0, i32 %X
%Q = load i16* %P
%R = icmp eq i16 %Q, 0
ret i1 %R
; CHECK: @test1
; CHECK-NEXT: %R = icmp eq i32 %X, 9
; CHECK-NEXT: ret i1 %R
}
define i1 @test2(i32 %X) {
%P = getelementptr [10 x i16]* @G16, i32 0, i32 %X
%Q = load i16* %P
%R = icmp slt i16 %Q, 85
ret i1 %R
; CHECK: @test2
; CHECK-NEXT: %R = icmp ne i32 %X, 4
; CHECK-NEXT: ret i1 %R
}