Extend CanEvaluateZExtd to handle and/or/xor more aggressively in the
BitsToClear case. This allows it to promote expressions which have an
and/or/xor after the lshr, promoting cases like test2 (from PR4216)
and test3 (random extample extracted from a spec benchmark).
clang now compiles the code in PR4216 into:
_test_bitfield: ## @test_bitfield
movl %edi, %eax
orl $194, %eax
movl $4294902010, %ecx
andq %rax, %rcx
orl $32768, %edi
andq $39936, %rdi
movq %rdi, %rax
orq %rcx, %rax
ret
instead of:
_test_bitfield: ## @test_bitfield
movl %edi, %eax
orl $194, %eax
movl $4294902010, %ecx
andq %rax, %rcx
shrl $8, %edi
orl $128, %edi
shlq $8, %rdi
andq $39936, %rdi
movq %rdi, %rax
orq %rcx, %rax
ret
which is still not great, but is progress.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@93145 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Transforms/InstCombine/InstCombineCasts.cpp b/lib/Transforms/InstCombine/InstCombineCasts.cpp
index be0734c..939c2b1 100644
--- a/lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ b/lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -634,6 +634,20 @@
if (BitsToClear == 0 && Tmp == 0)
return true;
+ // If the operation is an AND/OR/XOR and the bits to clear are zero in the
+ // other side, BitsToClear is ok.
+ if (Tmp == 0 &&
+ (Opc == Instruction::And || Opc == Instruction::Or ||
+ Opc == Instruction::Xor)) {
+ // We use MaskedValueIsZero here for generality, but the case we care
+ // about the most is constant RHS.
+ unsigned VSize = V->getType()->getScalarSizeInBits();
+ if (MaskedValueIsZero(I->getOperand(1),
+ APInt::getHighBitsSet(VSize, BitsToClear)))
+ return true;
+ }
+
+ // Otherwise, we don't know how to analyze this BitsToClear case yet.
return false;
case Instruction::LShr:
@@ -652,6 +666,8 @@
case Instruction::Select:
if (!CanEvaluateZExtd(I->getOperand(1), Ty, Tmp) ||
!CanEvaluateZExtd(I->getOperand(2), Ty, BitsToClear) ||
+ // TODO: If important, we could handle the case when the BitsToClear are
+ // known zero in the disagreeing side.
Tmp != BitsToClear)
return false;
return true;
@@ -665,6 +681,8 @@
return false;
for (unsigned i = 1, e = PN->getNumIncomingValues(); i != e; ++i)
if (!CanEvaluateZExtd(PN->getIncomingValue(i), Ty, Tmp) ||
+ // TODO: If important, we could handle the case when the BitsToClear
+ // are known zero in the disagreeing input.
Tmp != BitsToClear)
return false;
return true;