Unmerges have the same fundamental problem as G_TRUNC, and G_TRUNC
could be implemented in terms of G_UNMERGE_VALUES. Reducing the number
of elements in unmerge results ends up producing the original unmerge
type profile, so the artifact combiner needs to eliminate the
intermediate illegal registers. This avoids infinite looping in the
legalizer in a future change.
Assuming an unmerge has each result unmerged the same way, this ends
up producing a new unmerge of the source for every definition. I'm not
sure if the artifact combiner should either insert temporary merges
here and erase the original merge, or if the combiner should look at
uses from defs rather than defs from uses for unmerges.
In a few cases this regresses from using 16-bit shifts for 8-bit
values to using 32-bit shifts, but I think these can be legalized
later (the other legalization rules don't try very hard to use 16-bit
shifts either).
Most notably, we were incorrectly reporting <3 x s16> as a legal type
for these. Make sure these aren't legal to help make progress on
fixing the artifact combiner and vector legalizer
rules. Unfortunately, this means spreading the -global-isel-abort=0
hack, although this doesn't change the legalizer result in any
situation.
Summary:
Combine sext(zext x) to (zext x) since the sign-bit is 0
after the zero-extension.
Combine sext(sext x) to (sext x) and ext(zext x) to (zext x)
since the intermediate step is not needed.
Reviewers: arsenm, volkan, aemerson, aditya_nandakumar
Reviewed By: arsenm
Subscribers: jvesely, wdng, nhaehnle, rovka, kerbowa, llvm-commits
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D77210
Matt Arsenault