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

[Jan Beulich]

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:
>>
>> 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?

"Is your concern only about 32-bit code, or also about 64-bit?"

>> 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.

Can it? Why would it? (IOW: Again you didn't really address my request.)

Jan