| Issue |
164782
|
| Summary |
[X86] Failure to fold bit count instructions if EFLAGS is used
|
| Labels |
backend:X86,
missed-optimization
|
| Assignees |
|
| Reporter |
RKSimon
|
https://zig.godbolt.org/z/v6x91zxed
```ll
define i64 @lzcnt64(ptr %p0) {
%a0 = load i64, ptr %p0, align 8
%cnt = tail call range(i64 0, 65) i64 @llvm.ctlz.i64(i64 %a0, i1 true)
ret i64 %cnt
}
define i64 @lzcnt64_64(ptr %p0, i64 %a1) {
%a0 = load i64, ptr %p0, align 8
%cnt = tail call range(i64 0, 65) i64 @llvm.ctlz.i64(i64 %a0, i1 true)
%iszero = icmp eq i64 %a0, 0
%res = select i1 %iszero, i64 %a1, i64 %cnt
ret i64 %res
}
```
```asm
lzcnt64: # @lzcnt64
lzcntq (%rdi), %rax
retq
lzcnt64_64: # @lzcnt64_64
movq (%rdi), %rax
lzcntq %rax, %rax
cmovbq %rsi, %rax
retq
```
but should be:
```asm
lzcnt64_64: # @lzcnt64_64
lzcntq (%rdi), %rax
cmovbq %rsi, %rax
retq
```
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