[llvm-dev] Upper case vs lower case in printed and parsed MIR

[Björn Pettersson A via llvm-dev]

> -----Original Message-----
> From: llvm-dev <llvm-dev-bounces at lists.llvm.org> On Behalf Of Markus Lavin
> via llvm-dev
> Sent: den 11 april 2019 19:40
> To: llvm-dev at lists.llvm.org
> Subject: [llvm-dev] Upper case vs lower case in printed and parsed MIR
> 
> I am confused about the rules for when upper and lower case letters should
> be used in MIR.
> 
> As an example our downstream target has upper case letters in its sub-
> register indices and as a result we cannot import exported MIR without
> manually 'lower casing' it first which is obviously rather annoying.
> 
> Looking in https://llvm.org/docs/MIRLangRef.html it is stated that
> instruction names are case sensitive.
> 
> For register names it appear that they are lower cased before printing (see
> printReg in TargetRegisterInfo.cpp) and to match the definitions are also
> lower cased before loaded into the parser
> (see PerTargetMIParsingState::initNames2Regs in MIParser.cpp). For sub-
> register index names the latter happens but they are currently printed with
> their original casing witch leads to our problem.

Do we know if making register names case sensitive would be a big churn
(e.g. in .mir test cases)? I assume it would have quite big impact.

> 
> What is the right solution here, should they be lower cased when printing
> as well (as I tried to do in https://reviews.llvm.org/D60311)?
> 
> To me it seems that preserving the original casing from the .td file would
> be the most correct thing to do but then it would be inconsistent with e.g.
> register names and would only add to the confusion it seems.

Handling register names and subregister names consistently actually sounds
reasonable. Even though my first feeling was that it would be nice to print
the subregister names with the same casing as used in the code.

There are however more strings so it is hard to know where to draw the line
when it comes to forcing lower case in the API, if instruction names are
still should be case sensitive. Should for example register class names
follow the same rule as for registers?

If we want to force lower case (e.g. for register/subregister names), then
I think we want to avoid doing the lower casing at runtime and instead
tablegen should put the lower case names in the tables already from the start.

There is ofcourse the option of making the parsing case-insensitive (for
register names, subregister names). I guess that would make least churn,
since things would be backwards compatible with existing test cases (and
printouts could be handled just like today, even if it isn't consistent).


@Markus: We could even rename things in the RegisterInfo.td file for our OOT
target to use lower case names (and by that hide the problems we currently see).
But that would not help the community. So it would be nice to hear what
others have to say about this.