[PATCH 3/5] x86: widen @got{,pcrel} support to PUSH and APX IMUL

[Jan Beulich]

On 04.02.2025 13:14, H.J. Lu wrote:
> On Tue, Feb 4, 2025 at 7:16 PM Jan Beulich <jbeulich@suse.com> wrote:
>> On 04.02.2025 12:12, H.J. Lu wrote:
>>> On Tue, Feb 4, 2025 at 7:02 PM Jan Beulich <jbeulich@suse.com> wrote:
>>>> Thinking of it: With TEST being special-cased in the logic involved, I'm
>>>> also curious to learn of a code sequence where TEST would sensibly be
>>>> used (and where CMP can't be used instead).
>>>
>>> Compiler may generate TEST with GOT.
>>
>> Can it? Why would it? (IOW: Again you didn't really address my request.)
>>
> 
> [hjl@gnu-tgl-3 tmp]$ cat x.c
> extern int foo __attribute__ ((weak));
> 
> extern void bar (void);
> 
> __attribute__ ((regparm(3)))
> void
> _start (long int p)
> {
>   if (((unsigned long) &foo) & p)
>     bar ();
> }
> [hjl@gnu-tgl-3 tmp]$ gcc -c -O2 -m32 x.c -mno-direct-extern-access
> 
> [hjl@gnu-tgl-3 tmp]$ objdump -dwr x.o
> 
> x.o:     file format elf32-i386
> 
> 
> Disassembly of section .text:
> 
> 00000000 <_start>:
>    0: 85 05 00 00 00 00    test   %eax,0x0 2: R_386_GOT32X foo
>    6: 75 08                jne    10 <_start+0x10>
>    8: c3                    ret
>    9: 8d b4 26 00 00 00 00 lea    0x0(%esi,%eiz,1),%esi
>   10: e9 fc ff ff ff        jmp    11 <_start+0x11> 11: R_386_PC32 bar
> [hjl@gnu-tgl-3 tmp]$
> 
> Which request?

"... learn of a code sequence where TEST would sensibly be used" in my
earlier reply. So yes, you provided a contrived example now. I'm afraid
I can't assign meaning to "if (((unsigned long) &foo) & p)", though.
Instead, slightly adjusting your example, I can see PUSH being used by
the compiler in exactly the way we can (but so far didn't) optimize.
That's ordinary argument passing, which I'm inclined to call "sensible".

IOW the question remains as to why optimization of TEST was added,
beyond the "because we can" reasoning. As before, ADD, SUB, and perhaps
CMP make sense when operating on pointers. For AND, OR, XOR, ADC, SBB,
and TEST I'm having a hard time seeing good uses. Since we deal with
them, we also ought to deal with IMUL in the same manner (where
possible). Since beyond these arithmetic ops we also deal with MOV, we
similarly should handle PUSH.

Jan