https://bugs.llvm.org/show_bug.cgi?id=36551
Bug ID: 36551
Summary: Bool vs unsigned result gives very different codegen
for bit tests
Product: libraries
Version: trunk
Hardware: PC
OS: Windows NT
Status: NEW
Severity: enhancement
Priority: P
Component: Common Code Generator Code
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected], [email protected],
[email protected], [email protected]
https://godbolt.org/g/EFjrMf
Depending on whether we return as a bool or an unsigned, we get quite different
codegen for a !(x & 4) bit-test:
bool foo(int x) {
return !(x & 4);
}
unsigned bar(int x) {
return !(x & 4);
}
clang/gcc comes up with 4 different implementations here:
clang -g0 -O3:
define i1 @_Z3fooi(i32) {
%2 = and i32 %0, 4
%3 = icmp eq i32 %2, 0
ret i1 %3
}
define i32 @_Z3bari(i32) {
%2 = lshr i32 %0, 2
%3 = and i32 %2, 1
%4 = xor i32 %3, 1
ret i32 %4
}
_Z3fooi: # @_Z3fooi
testb $4, %dil
sete %al
retq
_Z3bari: # @_Z3bari
shrl $2, %edi
notl %edi
andl $1, %edi
movl %edi, %eax
retq
gcc -g0 -O3:
_Z3fooi:
movl %edi, %eax
shrl $2, %eax
xorl $1, %eax
andl $1, %eax
ret
_Z3bari:
xorl %eax, %eax
andl $4, %edi
sete %al
ret
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