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

H.J. Lu hjl.tools@gmail.com
Thu Feb 6 02:28:53 GMT 2025 
On Wed, Feb 5, 2025 at 7:37 PM Jan Beulich <jbeulich@suse.com> wrote:
>
> 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.

It can be used to check pointer alignment.

> 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".

Please don't add a segment register.

> 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



-- 
H.J.
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