| Issue |
122560
|
| Summary |
[clang] Worse code gen when default constructing with `()` than `{}`
|
| Labels |
clang
|
| Assignees |
|
| Reporter |
davidstone
|
The following translation unit
```c++
long get();
void unused() noexcept;
struct s {
~s() {
if (m0 != 0) {
unused();
}
}
long m0 = 0;
long m1 = 0;
};
s f() {
#if defined USE_BRACES
auto x = s{};
#else
auto x = s();
#endif
x.m1 = get();
return x;
}
```
Generates the following code when compiled with `-DUSE_BRACES -O3 -emit-llvm`:
```
%struct.s = type { i64, i64 }
define dso_local void @f()(ptr dead_on_unwind noalias nocapture writable writeonly sret(%struct.s) align 8 initializes((0, 16)) %agg.result) local_unnamed_addr #0 personality ptr @__gxx_personality_v0 !dbg !21 {
entry:
#dbg_value(ptr %agg.result, !24, !DIExpression(DW_OP_deref), !25)
%m1 = getelementptr inbounds nuw i8, ptr %agg.result, i64 8, !dbg !26
store i64 0, ptr %agg.result, align 8, !dbg !26
%call = tail call noundef i64 @get()(), !dbg !27
store i64 %call, ptr %m1, align 8, !dbg !28
ret void, !dbg !34
}
declare !dbg !35 noundef i64 @get()() local_unnamed_addr #1
declare i32 @__gxx_personality_v0(...)
attributes #0 = { mustprogress uwtable "min-legal-vector-width"="0" "no-trapping-math"="true" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+cmov,+cx8,+fxsr,+mmx,+sse,+sse2,+x87" "tune-cpu"="generic" }
attributes #1 = { "no-trapping-math"="true" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+cmov,+cx8,+fxsr,+mmx,+sse,+sse2,+x87" "tune-cpu"="generic" }
```
but generates the following when compiled with `-O3 -emit-llvm`:
```
%struct.s = type { i64, i64 }
define dso_local void @f()(ptr dead_on_unwind noalias nocapture writable writeonly sret(%struct.s) align 8 initializes((0, 16)) %agg.result) local_unnamed_addr #0 personality ptr @__gxx_personality_v0 !dbg !21 {
entry:
#dbg_value(ptr %agg.result, !24, !DIExpression(DW_OP_deref), !25)
#dbg_value(ptr %agg.result, !26, !DIExpression(), !31)
tail call void @llvm.memset.p0.i64(ptr noundef nonnull align 8 dereferenceable(16) %agg.result, i8 0, i64 16, i1 false), !dbg !33
%call = tail call noundef i64 @get()(), !dbg !34
%m1 = getelementptr inbounds nuw i8, ptr %agg.result, i64 8, !dbg !35
store i64 %call, ptr %m1, align 8, !dbg !36
ret void, !dbg !42
}
declare void @llvm.memset.p0.i64(ptr nocapture writeonly, i8, i64, i1 immarg) #1
declare !dbg !43 noundef i64 @get()() local_unnamed_addr #2
declare i32 @__gxx_personality_v0(...)
attributes #0 = { mustprogress uwtable "min-legal-vector-width"="0" "no-trapping-math"="true" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+cmov,+cx8,+fxsr,+mmx,+sse,+sse2,+x87" "tune-cpu"="generic" }
attributes #1 = { mustprogress nocallback nofree nounwind willreturn memory(argmem: write) }
attributes #2 = { "no-trapping-math"="true" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+cmov,+cx8,+fxsr,+mmx,+sse,+sse2,+x87" "tune-cpu"="generic" }
```
These correspond to the assembly
`-DUSE_BRACES`:
```asm
f():
push rbx
mov rbx, rdi
mov qword ptr [rdi], 0
call get()@PLT
mov qword ptr [rbx + 8], rax
mov rax, rbx
pop rbx
ret
```
and without:
```asm
f():
push rbx
mov rbx, rdi
xorps xmm0, xmm0
movups xmmword ptr [rdi], xmm0
call get()@PLT
mov qword ptr [rbx + 8], rax
mov rax, rbx
pop rbx
ret
```
See it live: https://godbolt.org/z/hfcPeTsfo
![]()
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