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

[Puyan Lotfi via llvm-dev]

Thanks for the feedback.

On Mon, Jan 8, 2018 at 2:30 PM, Sean Silva <chisophugis at gmail.com> wrote:

>
>
> 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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>>> llvm-dev at lists.llvm.org
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>>>
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>>
>>
>
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