cmd/compile: simplify shift lowering on s390x Use conditional moves instead of subtractions with borrow to handle saturation cases. This allows us to delete the SUBE/SUBEW ops and associated rules from the SSA backend. Using conditional moves also means we can detect when shift values are masked so I've added some new rules to constant fold the relevant comparisons and masking ops. Also use the new shiftIsBounded() function to avoid generating code to handle saturation cases where possible. Updates #25167 for s390x. Change-Id: Ief9991c91267c9151ce4c5ec07642abb4dcc1c0d Reviewed-on: https://go-review.googlesource.com/110070 Run-TryBot: Michael Munday <mike.munday@ibm.com> TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
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