https://llvm.org/bugs/show_bug.cgi?id=26465
Bug ID: 26465
Summary: [InstCombine] Replacing ((xor A, B) != 0) with (A !=
B) is not always good
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: Loop Optimizer
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
Classification: Unclassified
InstCombiner::visitICmpInstWithInstAndIntCst replaces ((xor A, B) != 0) with (A
!= B). However, this replacement generates suboptimal code if xor has uses
other than the compare.
*** Example code
...
char c = lhs ^ rhs;
if (k == 0) return true;
if (k != 32) return false;
...
*** What LLVM generates now
movzbl (%rbx),%eax
movzbl %r8b,%ecx
cmp %ecx,%eax
je 400c50
mov %r8b,%cl
xor %al,%cl
movzbl %cl,%ecx
cmp $0x20,%ecx
jne 400cb0
*** What LLVM generates if the replacement performed only when xor has one use
mov (%rbx),%al
mov %r13b,%cl
xor %al,%cl
je 400c48
movzbl %cl,%ecx
cmp $0x20,%ecx
jne 400ca0
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