https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110035
--- Comment #4 from Pontakorn Prasertsuk <ptk.prasertsuk at gmail dot com> ---
(In reply to Richard Biener from comment #1)
> Ick - convoluted C++. We end up with
>
> void ff (struct MyClass & obj)
> {
> vector(2) long unsigned int vect_SR.16;
> vector(2) long unsigned int vect_SR.15;
> vector(2) long unsigned int vect_SR.14;
> void * _6;
>
> <bb 2> [local count: 1073741824]:
> vect_SR.14_5 = MEM <vector(2) long unsigned int> [(struct MyClass
> &)obj_2(D)];
> vect_SR.15_28 = MEM <vector(2) long unsigned int> [(struct MyClass
> &)obj_2(D) + 16];
> vect_SR.16_30 = MEM <vector(2) long unsigned int> [(struct MyClass
> &)obj_2(D) + 32];
> _6 = operator new (48);
> MEM <vector(2) long unsigned int> [(struct MyClass2 *)_6] = vect_SR.14_5;
> MEM <vector(2) long unsigned int> [(struct MyClass2 *)_6 + 16B] =
> vect_SR.15_28;
> MEM <vector(2) long unsigned int> [(struct MyClass2 *)_6 + 32B] =
> vect_SR.16_30;
> HandleMyClass2 (_6); [tail call]
>
> and the issue is that 'operator new (48)' can alter what 'obj' points to,
> so we cannot move the loads across the call and we get spilling.
>
> There is no inter-procedural analysis in GCC that would tell us that
> 'obj_2(D)' (the MyClass & obj argument of ff) does not point to an
> object that did not escape. In fact 'ff' has global visibility
> and it might have other callers.
>
> If you add -fwhole-program then you get the function inlined to main and
>
> main:
> .LFB652:
> .cfi_startproc
> subq $8, %rsp
> .cfi_def_cfa_offset 16
> movl $48, %edi
> call _Znwm
> movq $0, (%rax)
> movq %rax, %rdi
> movq $0, 8(%rax)
> movq $0, 16(%rax)
> movq $0, 24(%rax)
> movq $0, 32(%rax)
> movq $0, 40(%rax)
> call _Z14HandleMyClass2Pv
> xorl %eax, %eax
> addq $8, %rsp
> .cfi_def_cfa_offset 8
> ret
>
> (not using vectors because 'main' is considered cold). Do you cite an
> inline copy of ff() for clang?
Hi Richard,
The clang snippet I provided is not inlined into 'main' function.
