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

H.J. Lu hjl.tools@gmail.com
Tue Feb 4 11:12:30 GMT 2025 
On Tue, Feb 4, 2025 at 7:02 PM Jan Beulich <jbeulich@suse.com> wrote:
>
> On 04.02.2025 11:41, Jan Beulich wrote:
> > On 04.02.2025 11:17, H.J. Lu wrote:
> >> On Tue, Feb 4, 2025 at 6:14 PM Jan Beulich <jbeulich@suse.com> wrote:
> >>>
> >>> On 03.02.2025 23:40, H.J. Lu wrote:
> >>>> On Mon, Feb 3, 2025 at 7:41 PM Jan Beulich <jbeulich@suse.com> wrote:
> >>>>> --- /dev/null
> >>>>> +++ b/ld/testsuite/ld-i386/load8a.d
> >>>>> @@ -0,0 +1,14 @@
> >>>>> +#source: load8.s
> >>>>> +#as: --32 -mrelax-relocations=yes
> >>>>> +#ld: -melf_i386 -z noseparate-code
> >>>>> +#objdump: -dw
> >>>>> +
> >>>>> +.*: +file format .*
> >>>>> +
> >>>>> +Disassembly of section .text:
> >>>>> +
> >>>>> +0+8048074 <_start>:
> >>>>> +[      ]*[a-f0-9]+:    26 68 86 90 04 08       es push \$0x8049086
> >>>>> +[      ]*[a-f0-9]+:    26 68 87 90 04 08       es push \$0x8049087
> >>>>> +[      ]*[a-f0-9]+:    26 68 87 90 04 08       es push \$0x8049087
> >>>>
> >>>> Please avoid adding the es prefix.  It may not be nop in the future.
> >>>
> >>> Constructive comments please. What other prefix do you suggest we use?
> >>> Is another of the segment overrides okay? If not, all that's left is an
> >>> address size override, if I'm not mistaken. Which overall seems less
> >>> desirable to use.
> >>
> >> You can use 1-byte NOP.
> >
> > That'll have an undue effect on debugging, by splitting a single insn
> > into two.
> >
> >>> Plus - is your concern only about 32-bit code, or also about 64-bit? For
> >>> 32-bit code in particular I'm having difficulty seeing why an ES
> >>> prefix might gain new meaning going forward, when an increasing number
> >>> of ISA extensions are for 64-bit mode only anyway. If the concern
> >>> extends to 64-bit code (it shouldn't, as the pre-386 segment overrides
> >>> are documented as nop prefixes, if I'm not mistaken), earlier changes
> >>> would need adjusting then, too, I think.
> >>
> >> I don't think adding instructions like PUSH is very useful.
>
> Further to my earlier reply: You didn't answer my question. Which is

Which question?

> necessary to determine whether earlier changes need adjustment.
>
> > I was actively waiting for this kind of comment. Why was adding support
> > for e.g. ADC and SBB useful then? Imo it can only be one of two ways:
> > Either we support everything that can be supported, or we limit things
> > strictly to cases that are actively deemed useful.
>
> 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.


-- 
H.J.
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