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[X86] When expanding a multiply by a negative of one less than a power of 2, like 31, don't generate a negate of a subtract that we'll never optimize. 

We generated a subtract for the power of 2 minus one then negated the result. The negate can be optimized away by swapping the subtract operands, but DAG combine doesn't know how to do that and we don't add any of the new nodes to the worklist anyway.

This patch makes use explicitly emit the swapped subtract.

llvm-svn: 337858
This commit is contained in:
Craig Topper 2018-07-24 21:31:21 +00:00
parent a3fd116219
commit 5ddc0a2b14
2 changed files with 23 additions and 22 deletions
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22
llvm/lib/Target/X86/X86ISelLowering.cpp
View File

@ -33922,14 +33922,20 @@ static SDValue combineMul(SDNode *N, SelectionDAG &DAG,
DAG.getNode(ISD::SHL, DL, VT, N->getOperand(0),
DAG.getConstant(Log2_64(NumSign * SignMulAmt - 1), DL,
MVT::i8)));
// To negate, subtract the number from zero
if (SignMulAmt < 0)
NewMul = DAG.getNode(ISD::SUB, DL, VT,
DAG.getConstant(0, DL, VT), NewMul);
} else if (IsPowerOf2_64MinusOne) {
// (mul x, 2^N - 1) => (sub (shl x, N), x)
NewMul = DAG.getNode(
ISD::SUB, DL, VT,
DAG.getNode(ISD::SHL, DL, VT, N->getOperand(0),
DAG.getConstant(Log2_64(NumSign * SignMulAmt + 1), DL,
MVT::i8)),
N->getOperand(0));
NewMul = DAG.getNode(ISD::SHL, DL, VT, N->getOperand(0),
DAG.getConstant(Log2_64(NumSign * SignMulAmt + 1),
DL, MVT::i8));
// To negate, reverse the operands of the subtract.
if (SignMulAmt < 0)
NewMul = DAG.getNode(ISD::SUB, DL, VT, N->getOperand(0), NewMul);
else
NewMul = DAG.getNode(ISD::SUB, DL, VT, NewMul, N->getOperand(0));
} else if (IsPowerOf2_64MinusTwo && NumSign == 1) {
// (mul x, 2^N - 1) => (sub (shl x, N), x)
NewMul = DAG.getNode(ISD::SHL, DL, VT, N->getOperand(0),
@ -33938,10 +33944,6 @@ static SDValue combineMul(SDNode *N, SelectionDAG &DAG,
NewMul = DAG.getNode(ISD::SUB, DL, VT, NewMul, N->getOperand(0));
NewMul = DAG.getNode(ISD::SUB, DL, VT, NewMul, N->getOperand(0));
}
// To negate, subtract the number from zero
if (NewMul && NumSign == -1)
NewMul =
DAG.getNode(ISD::SUB, DL, VT, DAG.getConstant(0, DL, VT), NewMul);
}
}

23
llvm/test/CodeGen/X86/imul.ll
View File

@ -324,17 +324,16 @@ define i32 @test1(i32 %a) {
; X64: # %bb.0: # %entry
; X64-NEXT: movl %edi, %eax
; X64-NEXT: shll $5, %eax
; X64-NEXT: subl %edi, %eax
; X64-NEXT: negl %eax
; X64-NEXT: subl %eax, %edi
; X64-NEXT: movl %edi, %eax
; X64-NEXT: retq
;
; X86-LABEL: test1:
; X86: # %bb.0: # %entry
; X86-NEXT: movl {{[0-9]+}}(%esp), %ecx
; X86-NEXT: movl %ecx, %eax
; X86-NEXT: shll $5, %eax
; X86-NEXT: movl {{[0-9]+}}(%esp), %eax
; X86-NEXT: movl %eax, %ecx
; X86-NEXT: shll $5, %ecx
; X86-NEXT: subl %ecx, %eax
; X86-NEXT: negl %eax
; X86-NEXT: retl
entry:
%tmp3 = mul i32 %a, -31
@ -414,8 +413,8 @@ define i64 @test5(i64 %a) {
; X64: # %bb.0: # %entry
; X64-NEXT: movq %rdi, %rax
; X64-NEXT: shlq $5, %rax
; X64-NEXT: subq %rdi, %rax
; X64-NEXT: negq %rax
; X64-NEXT: subq %rax, %rdi
; X64-NEXT: movq %rdi, %rax
; X64-NEXT: retq
;
; X86-LABEL: test5:
@ -424,15 +423,15 @@ define i64 @test5(i64 %a) {
; X86-NEXT: .cfi_def_cfa_offset 8
; X86-NEXT: .cfi_offset %esi, -8
; X86-NEXT: movl {{[0-9]+}}(%esp), %ecx
; X86-NEXT: movl {{[0-9]+}}(%esp), %eax
; X86-NEXT: movl %eax, %esi
; X86-NEXT: shll $5, %esi
; X86-NEXT: movl {{[0-9]+}}(%esp), %esi
; X86-NEXT: movl %esi, %eax
; X86-NEXT: shll $5, %eax
; X86-NEXT: subl %eax, %esi
; X86-NEXT: movl $-31, %edx
; X86-NEXT: movl %ecx, %eax
; X86-NEXT: mull %edx
; X86-NEXT: subl %ecx, %edx
; X86-NEXT: subl %esi, %edx
; X86-NEXT: addl %esi, %edx
; X86-NEXT: popl %esi
; X86-NEXT: .cfi_def_cfa_offset 4
; X86-NEXT: retl