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InstCombine: fold (A >> C) == (B >> C) --> (A^B) < (1 << C) for constant Cs.

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This is common in bitfield code.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194925 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Benjamin Kramer 2013-11-16 16:00:48 +00:00
parent bd9f36f4db
commit e9cdbf68e5
3 changed files with 46 additions and 8 deletions
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18
lib/Transforms/InstCombine/InstCombineCompares.cpp
View File

@ -2948,6 +2948,24 @@ Instruction *InstCombiner::visitICmpInst(ICmpInst &I) {
Builder->CreateTrunc(B, A->getType()));
}
// (A >> C) == (B >> C) --> (A^B) u< (1 << C)
// For lshr and ashr pairs.
if ((match(Op0, m_OneUse(m_LShr(m_Value(A), m_ConstantInt(Cst1)))) &&
match(Op1, m_OneUse(m_LShr(m_Value(B), m_Specific(Cst1))))) ||
(match(Op0, m_OneUse(m_AShr(m_Value(A), m_ConstantInt(Cst1)))) &&
match(Op1, m_OneUse(m_AShr(m_Value(B), m_Specific(Cst1)))))) {
unsigned TypeBits = Cst1->getBitWidth();
unsigned ShAmt = (unsigned)Cst1->getLimitedValue(TypeBits);
if (ShAmt < TypeBits && ShAmt != 0) {
ICmpInst::Predicate Pred = I.getPredicate() == ICmpInst::ICMP_NE
? ICmpInst::ICMP_UGE
: ICmpInst::ICMP_ULT;
Value *Xor = Builder->CreateXor(A, B, I.getName() + ".unshifted");
APInt CmpVal = APInt::getOneBitSet(TypeBits, ShAmt);
return new ICmpInst(Pred, Xor, Builder->getInt(CmpVal));
}
}
// Transform "icmp eq (trunc (lshr(X, cst1)), cst" to
// "icmp (and X, mask), cst"
uint64_t ShAmt = 0;

8
test/Transforms/InstCombine/compare-signs.ll
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@ -24,9 +24,9 @@
define i32 @test3(i32 %a, i32 %b) nounwind readnone {
; CHECK-LABEL: @test3(
entry:
; CHECK: xor i32 %a, %b
; CHECK: lshr i32 %0, 31
; CHECK: xor i32 %1, 1
; CHECK: [[XOR1:%.*]] = xor i32 %a, %b
; CHECK: [[SHIFT:%.*]] = lshr i32 [[XOR1]], 31
; CHECK: [[XOR2:%.*]] = xor i32 [[SHIFT]], 1
%0 = lshr i32 %a, 31 ; <i32> [#uses=1]
%1 = lshr i32 %b, 31 ; <i32> [#uses=1]
%2 = icmp eq i32 %0, %1 ; <i1> [#uses=1]
@ -34,7 +34,7 @@ entry:
ret i32 %3
; CHECK-NOT: icmp
; CHECK-NOT: zext
; CHECK: ret i32 %2
; CHECK: ret i32 [[XOR2]]
}
; Variation on @test3: checking the 2nd bit in a situation where the 5th bit

28
test/Transforms/InstCombine/icmp.ll
View File

@ -523,12 +523,12 @@ define i1 @test52(i32 %x1) nounwind {
; PR9838
; CHECK-LABEL: @test53(
; CHECK-NEXT: ashr exact
; CHECK-NEXT: ashr
; CHECK-NEXT: sdiv exact
; CHECK-NEXT: sdiv
; CHECK-NEXT: icmp
define i1 @test53(i32 %a, i32 %b) nounwind {
%x = ashr exact i32 %a, 30
%y = ashr i32 %b, 30
%x = sdiv exact i32 %a, 30
%y = sdiv i32 %b, 30
%z = icmp eq i32 %x, %y
ret i1 %z
}
@ -1336,3 +1336,23 @@ end:
%res = phi i1 [%restrue, %true], [%resfalse, %false]
ret i1 %res
}
; CHECK-LABEL: @icmp_lshr_lshr_eq
; CHECK: %z.unshifted = xor i32 %a, %b
; CHECK: %z = icmp ult i32 %z.unshifted, 1073741824
define i1 @icmp_lshr_lshr_eq(i32 %a, i32 %b) nounwind {
%x = lshr i32 %a, 30
%y = lshr i32 %b, 30
%z = icmp eq i32 %x, %y
ret i1 %z
}
; CHECK-LABEL: @icmp_ashr_ashr_ne
; CHECK: %z.unshifted = xor i32 %a, %b
; CHECK: %z = icmp ugt i32 %z.unshifted, 255
define i1 @icmp_ashr_ashr_ne(i32 %a, i32 %b) nounwind {
%x = ashr i32 %a, 8
%y = ashr i32 %b, 8
%z = icmp ne i32 %x, %y
ret i1 %z
}