cmd/compile/internal: stack slot merging region formation enhancements
This patch revises the algorithm/strategy used for overlapping the
stack slots of disjointly accessed local variables. The main change
here is to allow merging the stack slot of B into the slot for A if
B's size is less then A (prior to this they had to be identical), and
to also allow merging a non-pointer variables into pointer-variable
slots.
The new algorithm sorts the candidate list first by pointerness
(pointer variables first), then by alignment, then by size, and
finally by name. We no longer check that two variables have the same
GC shape before merging: since it should never be the case that we
have two vars X and Y both live across a given callsite where X and Y
share a stack slot, their gc shape doesn't matter.
Doing things this new way increases the total number of bytes saved
(across all functions) from 91256 to 124336 for the sweet benchmarks.
Updates #62737.
Updates #65532.
Updates #65495.
Change-Id: I1daaac1b1240aa47a6975e98ccd24e03304ab602
Reviewed-on: https://go-review.googlesource.com/c/go/+/577615
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Cherry Mui <cherryyz@google.com>