[PATCH] D37262: The issues with X86 prefixes: step 2

[Rafael Avila de Espindola via llvm-commits]

The assembler and disassembler should use the same path.

I would be OK with always producing 1 or N instructions, as long as both
the assembler and disassembler do the same. That is, it is OK to have
Flags, as long as the assembler uses that instead of creating a separate
instruction for prefixes.

It seems that allowing the disassembler to create multiple instructions
would have the advantage of not needing Flags, but that is secondary
IMHO.

Cheers,
Rafael

Craig Topper <craig.topper at gmail.com> writes:

> Here's my understanding of what I think happens today.
>
> -For a very select few instructions if the AsmParser sees a repne/repe
> prefix it creates a special version of the instruction that has the REP
> bits set in TSFlags. For any other instruction it emits the repne/rep/repe
> as a separate MCInst.
> -For the disassembler if it sees a repne/repe byte at the start that it
> doesn't think goes with an instruction it will emit a MCInst containing
> just the REP.
> -If the disassembler encounters a repne/repe byte not at the start of the
> instruction that doesn't go with the instruction we drop it and don't print
> anything. The disassembler interface only allows us to return one
> instruction. So we can't return a separate repne/repe instruction and a
> real instruction from the same byte sequence. I don't believe the assembler
> can ever produce a byte sequence that hits this case, but that doesn't mean
> some binary couldn't contain that string of bytes created by hand. So this
> patch is trying to preserve the extra prefix information in the one MCInst
> we're allowed to emit. Maybe another option would be to allow creating
> multiple MCInsts from the disassembler?
>
> ~Craig
>
> On Tue, Sep 26, 2017 at 10:37 AM, Rafael Avila de Espindola <
> rafael.espindola at gmail.com> wrote:
>
>> The question is why it is different for disassembler than for the
>> assembler?
>>
>> How does the assembler handle trepne?
>>
>> Cheers,
>> Rafael
>>
>> Andrew Tischenko <tishenandr at xenzu.com> writes:
>>
>> > It is not a simple flag, it's some data. And this data could be useful
>> > for any other component because it's some opaque info which could be
>> > send via MCInst from one low level target component to another one.
>> > Without this (additional) data MCInst loosing (potentially very useful)
>> > info about the given instruction.
>> >
>> > Andrew
>> >
>> > On 25.09.2017 22:05, Rafael Avila de Espindola wrote:
>> >> Having a flag field that is used only on disassembly seems wrong.
>> >>
>> >> Don't we support parsing our own output? I don't see trepne in any .s
>> >> test for example.
>> >>
>> >> Cheers,
>> >> Rafael
>> >>
>> >> Craig Topper via Phabricator <reviews at reviews.llvm.org> writes:
>> >>
>> >>> craig.topper added a comment.
>> >>>
>> >>> I'm not sure I can approve growing the size of MCInst. Though I can't
>> see anyway around it. @rafael what do you think?
>> >>>
>> >>>
>> >>> https://reviews.llvm.org/D37262
>>