[llvm-dev] Proposal: On re-purposing/reorganizing MIR sigils ('&', '$', '%').

[Sean Silva via llvm-dev]

On Mon, Jan 8, 2018 at 2:27 PM, Matthias Braun <mbraun at apple.com> wrote:

> When we discussed this our line of thought was like this:
>
> - LLVM IR already uses %name for SSA values which is closer to what a vreg
> is than to what a physreg is. It would be neat to draw that parallel to
> llvm IR.
> - We wanted another sigil for physregs so they are easy to differentiate
> from vregs to allow people to differentiate vregs/physregs even if they
> don't know all the physreg names of a particular architecture.
> - The $ was somewhat arbitrary because we had few characters left without
> a meaning in .mir and I found the ampersand better matches symbol names
> than physregs:
> - The ampersand in `&symbolname` should have familiar semantics for people
> using C/C++.
>
> So I would describe this as being closer to llvm IR than to typical
> assembly syntax, but I think that is apropriate for the LLVM machine
> intermediate language.
>

That rationale makes a lot of sense to me. SGTM.

-- Sean Silva


>
> - Matthias
>
> On Dec 26, 2017, at 3:29 PM, Sean Silva via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
> Can we use %% for vregs? Seems slightly easier to remember %/%% than $/%.
> Also, %eax and $some_symbol are already familiar from typical assembly
> syntax and we probably don't want to break that association.
>
> It's all a bikeshed, but being more consistent with assembly is probably a
> win.
>
> -- Sean Silva
>
> On Dec 25, 2017 11:31 AM, "Puyan Lotfi via llvm-dev" <
> llvm-dev at lists.llvm.org> wrote:
>
>> Hi
>>
>> A few of us have discussed enhancing the MIR vregs to include support for
>> named-vregs. At the moment named regs are only supported for physical
>> registers and number regs are reserved for vregs.
>>
>> We've decided that to properly implement a syntax for MIR named vregs we
>> first need to reorganized the sigils used for physical registers and
>> external symbols so our proposal is to swap the sigil used for external
>> symbols ('$') for the ampersand ('&') and re-purpose dollar-sign for
>> physregs so that physregs have the dollar-sign sigil and vregs have the
>> percentage ('%') sigil:
>>
>> essentially:
>>
>> BL *&*__divsi3 ...
>>
>> ...
>>
>> $eax = ...
>>
>> %123 = ...
>>
>> %vregFooBar = ...
>>
>>
>> I have an initial patch attached (replaces '$' for '&' for external
>> symbols). Hoping to open some dialog with the community before doing more
>> implementation work.
>>
>> Thanks
>>
>> PL
>>
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