[PATCH] D123234: [X86] `lowerBuildVectorAsBroadcast()`: with AVX2, allow i64->XMM broadcasts from constant pool

[Simon Pilgrim via Phabricator via llvm-commits]

RKSimon added a comment.

This is why I don't think we want to perform too much of this in the DAG - we quickly get to cases where the decision between broadcast vs vector load of constants can't be easily determined - value tracking, multiple uses, hoisting, lost folds, spilling etc. all get affected.

A while ago I was investigating the use of VPMOVSX/ZX to reduce the size of the constant pool, and hit many of the same problems. And constant rematerialization would be the same if we ever get to that point.

There's probably some minor further tweaks we can do (more hasOneUse checks?), but really we need to think about performing less in the DAG, and more in later passes.


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