[llvm-dev] [X86] Memory folding tables in x86 backend
Chris Lattner via llvm-dev
llvm-dev at lists.llvm.org
Mon Jul 24 09:35:29 PDT 2017
> On Jul 23, 2017, at 7:09 AM, Simon Pilgrim via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>> On 23 Jul 2017, at 12:19, Joerg Sonnenberger via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>> On Sun, Jul 23, 2017 at 08:48:00AM +0000, Musa, Ayman via llvm-dev wrote:
>>> 3- Give up on the auto-generation idea and manually update the current tables iteratively with new chunks of instructions until full state is achieved.
>>> P.s. The TableGen backend emits more than 5200 entries, while the known exception at this point are ~20.
>> Why can't you do a variation of this by tagging the instructions with a
>> flag? I.e. for the rm variant, add a flag that says "this has an
>> equivalent register operand version". Given that a lot of the instruction
>> patterns are created via multi-classes, I would expect that to require a
>> lot less than 5200 updates.
> +1 An opt-in ‘GenerateMemoryFolding’ style flag (defaulting to false) seems safer to me than than trying to ensure you’ve correctly flagged all illegal instruction fold.
The counterargument to this is that it adds a lot of cruft to the td files. It should be a lot more rare for an instruction to be “unfoldable” than to be “foldable”.
Validation should be straight-foward for this: generate the table automatically using the new tool and see what the differences are. If the differences are due to incorrect additions, just add the flag to them.
BTW, adding a tblgen backend to automate this is totally awesome. Thank you for pushing on this Ayman!
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