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From: "ptk.prasertsuk at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/110035] Missed optimization for dependent
 assignment statements
Date: Wed, 31 May 2023 01:20:08 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D110035

--- 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
>=20
> 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;
>=20
>   <bb 2> [local count: 1073741824]:
>   vect_SR.14_5 =3D MEM <vector(2) long unsigned int> [(struct MyClass
> &)obj_2(D)];
>   vect_SR.15_28 =3D MEM <vector(2) long unsigned int> [(struct MyClass
> &)obj_2(D) + 16];
>   vect_SR.16_30 =3D MEM <vector(2) long unsigned int> [(struct MyClass
> &)obj_2(D) + 32];
>   _6 =3D operator new (48);
>   MEM <vector(2) long unsigned int> [(struct MyClass2 *)_6] =3D vect_SR.1=
4_5;
>   MEM <vector(2) long unsigned int> [(struct MyClass2 *)_6 + 16B] =3D
> vect_SR.15_28;
>   MEM <vector(2) long unsigned int> [(struct MyClass2 *)_6 + 32B] =3D
> vect_SR.16_30;
>   HandleMyClass2 (_6); [tail call]
>=20
> 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.
>=20
> 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.
>=20
> If you add -fwhole-program then you get the function inlined to main and
>=20
> 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
>=20
> (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.=