cmd/compile: ensure we keep top 32 bits zeroed for 32-bit arm64 ops
When rewriting, for example, MSUBW, we need to ensure that the result
has its 32 top bits zeroed. That's what the instruction is spec'd to do.
Normally, we'd only use MSUBW for computations on 32-bit values, and
as such the top 32 bits aren't normally used. But some situations, like
if we cast the result to a uint64, the top 32 bits do matter.
This comes up in 62131 because we have a rule saying, MOVWUreg applied
to a MSUBW is unnecessary, as the arg to MOVWUreg already has zeroed
top 32 bits. But if MSUBW is later rewritten to another op that doesn't
zero the top 32 bits (SUB, probably), getting rid of the MOVWUreg earlier
causes a problem.
So change rewrite rules to always maintain the top 32 bits as zero if the
instruction is spec'd to provide that. We need to introduce a few *W operations
to make that happen.
Fixes #62131
Change-Id: If3d160821e285fd7454746b735a243671bff8894
Reviewed-on: https://go-review.googlesource.com/c/go/+/520916
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Cherry Mui <cherryyz@google.com> Reviewed-by: Keith Randall <khr@google.com>