[PATCH] D110588: [X86] decomposeMulByConstant - decompose legal vXi32 mutliplies on SlowPMULLD targets and all vXi64 mutliplies

Sat Oct 2 04:08:59 PDT 2021

RKSimon added a comment.

In D110588#3038019 <https://reviews.llvm.org/D110588#3038019>, @xbolva00 wrote:

> Can this patch solve first part of PR52039?
>
> https://bugs.llvm.org/show_bug.cgi?id=52039

I think this will only help that on Silvermont :)

We might have to redefine SlowPMULLD slightly so we can enable it on Haswell/Broadwell (and maybe Jaguar) (see PR35948). As well as this patch its used in reduceVMULWidth for PMULLD->PMULLW+PMULHW+SHUFFLE expansion of small values which is good for SLM (which has REALLY bad microcoded PMULLD) but not usually for anything else where its just a 2/3uop instruction.

I've been trying to improve PMADDWD combines as an alternative to reduceVMULWidth but there's a still some work to do (PR47437 and PR45897).

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