cmd/compile: try to rewrite loops to count down

Fixes #61629

This reduce the pressure on regalloc because then the loop only keep alive
one value (the iterator) instead of the iterator and the upper bound since
the comparison now acts against an immediate, often zero which can be skipped.

This optimize things like:
  for i := 0; i < n; i++ {
Or a range over a slice where the index is not used:
  for _, v := range someSlice {
Or the new range over int from #61405:
  for range n {

It is hit in 975 unique places while doing ./make.bash.

Change-Id: I5facff8b267a0b60ea3c1b9a58c4d74cdb38f03f
Reviewed-on: https://go-review.googlesource.com/c/go/+/512935
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Jorropo <jorropo.pgm@gmail.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Auto-Submit: Keith Randall <khr@golang.org>

1 files changed, 10 insertions, 0 deletions

diff --git a/test/prove_invert_loop_with_unused_iterators.go b/test/prove_invert_loop_with_unused_iterators.go
new file mode 100644
index 0000000000..f278e5aee0
--- /dev/null
+++ b/test/prove_invert_loop_with_unused_iterators.go
@@ -0,0 +1,10 @@
+// +build amd64
+// errorcheck -0 -d=ssa/prove/debug=1
+
+package main
+
+func invert(b func(), n int) {
+	for i := 0; i < n; i++ { // ERROR "(Inverted loop iteration|Induction variable: limits \[0,\?\), increment 1)"
+		b()
+	}
+}