https://bugs.llvm.org/show_bug.cgi?id=42358
Bug ID: 42358
Summary: Bad code generation for __builtin_clrsbl
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: enhancement
Priority: P
Component: Backend: X86
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected], [email protected],
[email protected], [email protected]
When compiling this code :
int clrsbl(unsigned long x)
{
return __builtin_clrsbl(x);
}
Clang generates really bad code on several backends (x64, x86, RISCV-32,
RISCV-64), such as (on x64)
mov eax, 64
add eax, -1
or (on RISCV-32)
addi a0, zero, 32
addi a0, a0, -1
This bug, however, is not present on certain backends such as WebAssembly or
PowerPC 64 (le and not le)
I looked at the emitted llvm code, and I think this is probably an optimization
problem related specifically to @llvm.ctlz. It looks like it's expanding ctlz
after the passes that would eliminate the bad code.
Link to see examples of this across backends : https://godbolt.org/z/pbBKgD
I filed this as an x86 backend bug because I do not know how to file a bug for
several backends at the same time
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