[PATCH] D113371: [X86] combinePMULH - recognise 'cheap' trunctions via PACKS/PACKUS as well as SEXT/ZEXT

[Simon Pilgrim via Phabricator via llvm-commits]

RKSimon added a comment.

In D113371#3161727 <https://reviews.llvm.org/D113371#3161727>, @lebedev.ri wrote:

> Hm, i'm not sure i'm following, why is the truncation free if the dropped bits are zeros / why it won't be free otherwise?

Its not - IsTruncateFree is only true if we've extended both inputs, in which case truncating the inputs should be free, and we're better off doing that so we can perform the MULH on a smaller vector.


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