[PATCH 4/5] x86-64: Intel64 adjustments for conditional jumps
H.J. Lu
hjl.tools@gmail.com
Thu Jul 16 12:30:03 GMT 2020
On Thu, Jul 16, 2020 at 4:57 AM Jan Beulich <jbeulich@suse.com> wrote:
>
> On 16.07.2020 13:38, H.J. Lu wrote:
> > On Thu, Jul 16, 2020 at 2:53 AM Jan Beulich <jbeulich@suse.com> wrote:
> >>
> >> On 15.07.2020 16:02, H.J. Lu wrote:
> >>> On Tue, Jul 14, 2020 at 11:08 PM Jan Beulich <jbeulich@suse.com> wrote:
> >>>>
> >>>> On 14.07.2020 14:59, H.J. Lu wrote:
> >>>>> On Tue, Jul 14, 2020 at 5:47 AM Jan Beulich <jbeulich@suse.com> wrote:
> >>>>>>
> >>>>>> On 14.07.2020 14:36, H.J. Lu wrote:
> >>>>>>> On Tue, Jul 14, 2020 at 5:20 AM Jan Beulich <jbeulich@suse.com> wrote:
> >>>>>>>>
> >>>>>>>> On 14.07.2020 14:18, Jan Beulich wrote:
> >>>>>>>>> On 14.07.2020 14:00, H.J. Lu wrote:
> >>>>>>>>>> On Tue, Jul 14, 2020 at 3:13 AM Jan Beulich <jbeulich@suse.com> wrote:
> >>>>>>>>>>> --- a/gas/testsuite/gas/i386/opcode-suffix.d
> >>>>>>>>>>> +++ b/gas/testsuite/gas/i386/opcode-suffix.d
> >>>>>>>>>>> @@ -305,22 +305,22 @@ Disassembly of section .text:
> >>>>>>>>>>> *[0-9a-f]+: 0f 77[ ]+emms[ ]+
> >>>>>>>>>>> *[0-9a-f]+: 0f 7e 90 90 90 90 90[ ]+movd[ ]+%mm2,-0x6f6f6f70\(%eax\)
> >>>>>>>>>>> *[0-9a-f]+: 0f 7f 90 90 90 90 90[ ]+movq[ ]+%mm2,-0x6f6f6f70\(%eax\)
> >>>>>>>>>>> - *[0-9a-f]+: 0f 80 90 90 90 90[ ]+jo[ ]+909094e2 <foo\+0x909094e2>
> >>>>>>>>>>> - *[0-9a-f]+: 0f 81 90 90 90 90[ ]+jno[ ]+909094e8 <foo\+0x909094e8>
> >>>>>>>>>>> - *[0-9a-f]+: 0f 82 90 90 90 90[ ]+jb[ ]+909094ee <foo\+0x909094ee>
> >>>>>>>>>>> - *[0-9a-f]+: 0f 83 90 90 90 90[ ]+jae[ ]+909094f4 <foo\+0x909094f4>
> >>>>>>>>>>> - *[0-9a-f]+: 0f 84 90 90 90 90[ ]+je[ ]+909094fa <foo\+0x909094fa>
> >>>>>>>>>>> - *[0-9a-f]+: 0f 85 90 90 90 90[ ]+jne[ ]+90909500 <foo\+0x90909500>
> >>>>>>>>>>> - *[0-9a-f]+: 0f 86 90 90 90 90[ ]+jbe[ ]+90909506 <foo\+0x90909506>
> >>>>>>>>>>> - *[0-9a-f]+: 0f 87 90 90 90 90[ ]+ja[ ]+9090950c <foo\+0x9090950c>
> >>>>>>>>>>> - *[0-9a-f]+: 0f 88 90 90 90 90[ ]+js[ ]+90909512 <foo\+0x90909512>
> >>>>>>>>>>> - *[0-9a-f]+: 0f 89 90 90 90 90[ ]+jns[ ]+90909518 <foo\+0x90909518>
> >>>>>>>>>>> - *[0-9a-f]+: 0f 8a 90 90 90 90[ ]+jp[ ]+9090951e <foo\+0x9090951e>
> >>>>>>>>>>> - *[0-9a-f]+: 0f 8b 90 90 90 90[ ]+jnp[ ]+90909524 <foo\+0x90909524>
> >>>>>>>>>>> - *[0-9a-f]+: 0f 8c 90 90 90 90[ ]+jl[ ]+9090952a <foo\+0x9090952a>
> >>>>>>>>>>> - *[0-9a-f]+: 0f 8d 90 90 90 90[ ]+jge[ ]+90909530 <foo\+0x90909530>
> >>>>>>>>>>> - *[0-9a-f]+: 0f 8e 90 90 90 90[ ]+jle[ ]+90909536 <foo\+0x90909536>
> >>>>>>>>>>> - *[0-9a-f]+: 0f 8f 90 90 90 90[ ]+jg[ ]+9090953c <foo\+0x9090953c>
> >>>>>>>>>>> + *[0-9a-f]+: 0f 80 90 90 90 90[ ]+jol[ ]+909094e2 <foo\+0x909094e2>
> >>>>>>>>>>> + *[0-9a-f]+: 0f 81 90 90 90 90[ ]+jnol[ ]+909094e8 <foo\+0x909094e8>
> >>>>>>>>>>> + *[0-9a-f]+: 0f 82 90 90 90 90[ ]+jbl[ ]+909094ee <foo\+0x909094ee>
> >>>>>>>>>>> + *[0-9a-f]+: 0f 83 90 90 90 90[ ]+jael[ ]+909094f4 <foo\+0x909094f4>
> >>>>>>>>>>> + *[0-9a-f]+: 0f 84 90 90 90 90[ ]+jel[ ]+909094fa <foo\+0x909094fa>
> >>>>>>>>>>> + *[0-9a-f]+: 0f 85 90 90 90 90[ ]+jnel[ ]+90909500 <foo\+0x90909500>
> >>>>>>>>>>> + *[0-9a-f]+: 0f 86 90 90 90 90[ ]+jbel[ ]+90909506 <foo\+0x90909506>
> >>>>>>>>>>> + *[0-9a-f]+: 0f 87 90 90 90 90[ ]+jal[ ]+9090950c <foo\+0x9090950c>
> >>>>>>>>>>> + *[0-9a-f]+: 0f 88 90 90 90 90[ ]+jsl[ ]+90909512 <foo\+0x90909512>
> >>>>>>>>>>> + *[0-9a-f]+: 0f 89 90 90 90 90[ ]+jnsl[ ]+90909518 <foo\+0x90909518>
> >>>>>>>>>>> + *[0-9a-f]+: 0f 8a 90 90 90 90[ ]+jpl[ ]+9090951e <foo\+0x9090951e>
> >>>>>>>>>>> + *[0-9a-f]+: 0f 8b 90 90 90 90[ ]+jnpl[ ]+90909524 <foo\+0x90909524>
> >>>>>>>>>>> + *[0-9a-f]+: 0f 8c 90 90 90 90[ ]+jll[ ]+9090952a <foo\+0x9090952a>
> >>>>>>>>>>> + *[0-9a-f]+: 0f 8d 90 90 90 90[ ]+jgel[ ]+90909530 <foo\+0x90909530>
> >>>>>>>>>>> + *[0-9a-f]+: 0f 8e 90 90 90 90[ ]+jlel[ ]+90909536 <foo\+0x90909536>
> >>>>>>>>>>> + *[0-9a-f]+: 0f 8f 90 90 90 90[ ]+jgl[ ]+9090953c <foo\+0x9090953c>
> >>>>>>>>>>> *[0-9a-f]+: 0f 90 80 90 90 90 90[ ]+seto[ ]+-0x6f6f6f70\(%eax\)
> >>>>>>>>>>> *[0-9a-f]+: 0f 91 80 90 90 90 90[ ]+setno[ ]+-0x6f6f6f70\(%eax\)
> >>>>>>>>>>> *[0-9a-f]+: 0f 92 80 90 90 90 90[ ]+setb[ ]+-0x6f6f6f70\(%eax\)
> >>>>>>>>>>
> >>>>>>>>>> There are instructions like jl and jnl. Will assembler properly
> >>>>>>>>>> handle `l' as a suffix here?
> >>>>>>>>>
> >>>>>>>>> j<cc> as well as jmp (with displacement) have No_lSuf set, so won't
> >>>>>>>>> accept l suffixes (same for the w one). Nevertheless already prior
> >>>>>>>>> to this change the disassembler will produce "jmpl" (and "jmpw").
> >>>>>>>>> IOW a disagreement between disassembler and assembler already exists.
> >>>>>>>
> >>>>>>> We should avoid it as much as we can.
> >>>>>>>
> >>>>>>>>>> If we do need to distinguish them, can we generate {disp32} pseudo prefix
> >>>>>>>>>> instead?
> >>>>>>>>>
> >>>>>>>>> We could, but then consistently for Jcc, JMP, and CALL. But how is
> >>>>>>>>> emitting a pseudo-prefix in line with the name of the controlling
> >>>>>>>>> command line option "-Msuffix"?
> >>>>>>>
> >>>>>>> That works for me.
> >>>>>>
> >>>>>> Okay, but you didn't answer my question.
> >>>>>
> >>>>> We can always generate pseudo prefix.
> >>>>
> >>>> But my question was towards "prefix" != "suffix".
> >>>>
> >>>>>>>> FAOD to achieve consistency I think the preferred route would then
> >>>>>>>> be for the assembler to accept l and w suffixes for Jcc and JMP.
> >>>>>>>> Not sure though what fallout this may mean.
> >>>>>>>
> >>>>>>> That could be quite messy.
> >>>>>>
> >>>>>> I guess I'd still prefer to try that first, and resort to the
> >>>>>> alternative only if it really turns out to be.
> >>>>>
> >>>>> Please give it a try.
> >>>>>
> >>>>>>> I think pseudo prefix is much less invasive.
> >>>>>>
> >>>>>> Maybe to the disassembler; I'm less sure about the testsuites of both
> >>>>>> gas and ld.
> >>>>>>
> >>>>>> Actually - if we were to go this route, then why pseudo prefixes in
> >>>>>> the first place? We already emit data{16,32} prefixes for other
> >>>>>> reasons, so we could do so here as well in place of the suffixes.
> >>>>>
> >>>>> There is data32 prefix. But there is no disp32 prefix. I call it
> >>>>> pseudo prefix.
> >>>>
> >>>> But the prefix _is_ a data size override, i.e. data{16,32} _is_ what
> >>>> we want to express. My question here is why to invent yet another
> >>>> prefix when we already have a suitable one.
> >>>>
> >>>
> >>> We should use a prefix which can be processed by assembler.
> >>> {dispXX} tells assembler to prefer a specific displacement size.
> >>
> >> Funny you should say this: There's no {disp16}. And the assembler
> >> understands (to a certain degree, but this got improved recently)
> >> data16 / data32. IOW yet another argument towards data16 / data32,
> >> if we are to go this route in the first place (which right now is
> >> only the fallback in case allowing the suffixes in the assembler
> >> causes overly difficult problems, and which continues to pend your
> >> response to me pointing out that emitting _prefixes_ is not in
> >> line with _suffix_ in -Msuffix).
> >>
> >
> > If needed, we can add {disp16}.
>
> I've been having this on my todo list for quite some time already,
> but not because of the considerations here.
>
> What I don't understand is why you _still_ don't answer the question
> raised: Do you really think -Msuffix should lead to output like
>
> {disp32} jmp label
>
> ? I could see {dispNN} to have advantages over data16/data32, but
> that's independent of -Msuffix, i.e. the prefix would be printed
> only if a non-default displacement width was in effect. Printing
> {dispNN} for all JMP / CALL / Jcc in suffix-always mode seems like
> an extreme form of clutter to me. (As a follow-on to this, we then
> ought to also always print {dispNN} for memory accesses with a
> word-sized displacement [albeit presumably limited to the no-base-
> and-no-index case] - yet more clutter.)
>
We are inventing things to distinguish different displacement sizes for
branch instructions. I prefer {disp16} over data16/data32. We only
need to do it for branch instructions with -Msuffix or with a new option.
--
H.J.
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