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InstCombine: fold (A << C) == (B << C) --> ((A^B) & (~0U >> C)) == 0

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Anding and comparing with zero can be done in a single instruction on
most archs so this is a bit cheaper.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@233291 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Benjamin Kramer 2015-03-26 17:12:06 +00:00
parent 27d12f3e6e
commit 8f820e9495
3 changed files with 45 additions and 38 deletions
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38
lib/Target/README.txt
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@ -1844,44 +1844,6 @@ we remove checking in code like
//===---------------------------------------------------------------------===//
This code (from Benchmarks/Dhrystone/dry.c):
define i32 @Func1(i32, i32) nounwind readnone optsize ssp {
entry:
%sext = shl i32 %0, 24
%conv = ashr i32 %sext, 24
%sext6 = shl i32 %1, 24
%conv4 = ashr i32 %sext6, 24
%cmp = icmp eq i32 %conv, %conv4
%. = select i1 %cmp, i32 10000, i32 0
ret i32 %.
}
Should be simplified into something like:
define i32 @Func1(i32, i32) nounwind readnone optsize ssp {
entry:
%sext = shl i32 %0, 24
%conv = and i32 %sext, 0xFF000000
%sext6 = shl i32 %1, 24
%conv4 = and i32 %sext6, 0xFF000000
%cmp = icmp eq i32 %conv, %conv4
%. = select i1 %cmp, i32 10000, i32 0
ret i32 %.
}
and then to:
define i32 @Func1(i32, i32) nounwind readnone optsize ssp {
entry:
%conv = and i32 %0, 0xFF
%conv4 = and i32 %1, 0xFF
%cmp = icmp eq i32 %conv, %conv4
%. = select i1 %cmp, i32 10000, i32 0
ret i32 %.
}
//===---------------------------------------------------------------------===//
clang -O3 currently compiles this code
int g(unsigned int a) {

15
lib/Transforms/InstCombine/InstCombineCompares.cpp
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@ -3553,6 +3553,21 @@ Instruction *InstCombiner::visitICmpInst(ICmpInst &I) {
}
}
// (A << C) == (B << C) --> ((A^B) & (~0U >> C)) == 0
if (match(Op0, m_OneUse(m_Shl(m_Value(A), m_ConstantInt(Cst1)))) &&
match(Op1, m_OneUse(m_Shl(m_Value(B), m_Specific(Cst1))))) {
unsigned TypeBits = Cst1->getBitWidth();
unsigned ShAmt = (unsigned)Cst1->getLimitedValue(TypeBits);
if (ShAmt < TypeBits && ShAmt != 0) {
Value *Xor = Builder->CreateXor(A, B, I.getName() + ".unshifted");
APInt AndVal = APInt::getLowBitsSet(TypeBits, TypeBits - ShAmt);
Value *And = Builder->CreateAnd(Xor, Builder->getInt(AndVal),
I.getName() + ".mask");
return new ICmpInst(I.getPredicate(), And,
Constant::getNullValue(Cst1->getType()));
}
}
// Transform "icmp eq (trunc (lshr(X, cst1)), cst" to
// "icmp (and X, mask), cst"
uint64_t ShAmt = 0;

30
test/Transforms/InstCombine/icmp.ll
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@ -1573,3 +1573,33 @@ define i32 @f5(i8 %a, i8 %b) {
%sub7.sub = select i1 %cmp4, i32 %sub7, i32 %sub
ret i32 %sub7.sub
}
; CHECK-LABEL: @f6
; CHECK: %cmp.unshifted = xor i32 %a, %b
; CHECK-NEXT: %cmp.mask = and i32 %cmp.unshifted, 255
; CHECK-NEXT: %cmp = icmp eq i32 %cmp.mask, 0
; CHECK-NEXT: %s = select i1 %cmp, i32 10000, i32 0
; CHECK-NEXT: ret i32 %s
define i32 @f6(i32 %a, i32 %b) {
%sext = shl i32 %a, 24
%conv = ashr i32 %sext, 24
%sext6 = shl i32 %b, 24
%conv4 = ashr i32 %sext6, 24
%cmp = icmp eq i32 %conv, %conv4
%s = select i1 %cmp, i32 10000, i32 0
ret i32 %s
}
; CHECK-LABEL: @f7
; CHECK: %cmp.unshifted = xor i32 %a, %b
; CHECK-NEXT: %cmp.mask = and i32 %cmp.unshifted, 511
; CHECK-NEXT: %cmp = icmp ne i32 %cmp.mask, 0
; CHECK-NEXT: %s = select i1 %cmp, i32 10000, i32 0
; CHECK-NEXT: ret i32 %s
define i32 @f7(i32 %a, i32 %b) {
%sext = shl i32 %a, 23
%sext6 = shl i32 %b, 23
%cmp = icmp ne i32 %sext, %sext6
%s = select i1 %cmp, i32 10000, i32 0
ret i32 %s
}