[clang] Worse code gen when default constructing with () than {}

[davidstone]

The following translation unit

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:

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:

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

[davidstone]

Note that constructing with the C++20 syntax of s(0, 0) or s(0) is also sufficient to get the good code generation. It is specifically s() that falls into the slow path.

[llvmbot]

@llvm/issue-subscribers-clang-codegen

Author: David Stone (davidstone)

The following translation unit

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:

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:

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