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

[Jan Beulich]

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.

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.

Jan