cmd/compile: add rule for post-decomposed growslice optimization

The recently added rule only works before decomposing slices.
Add a rule that works after decomposing slices.

The reason we need the latter is because although the length may
be a constant, it can be hidden inside a slice that is not constant
(its pointer or capacity might be changing). By applying this
optimization after decomposing slices, we can find more cases
where it applies.

Fixes #56440

Change-Id: I0094e59eee3065ab4d210defdda8227a6e897420
Reviewed-on: https://go-review.googlesource.com/c/go/+/446277
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@google.com>

1 files changed, 16 insertions, 0 deletions

diff --git a/test/codegen/issue56440.go b/test/codegen/issue56440.go
index 36b52ace03..c6c1e66789 100644
--- a/test/codegen/issue56440.go
+++ b/test/codegen/issue56440.go
@@ -16,3 +16,19 @@ func f(x []int) int {
 	// amd64:`MOVQ\t40\(.*\),`
 	return x[len(s)]
 }
+
+func g(x []int, p *bool) int {
+	s := make([]int, 3)
+	for {
+		s = s[:3]
+		if cap(s) < 5 {
+			s = make([]int, 3, 5)
+		}
+		s = append(s, 4, 5)
+		if *p {
+			// amd64:`MOVQ\t40\(.*\),`
+			return x[len(s)]
+		}
+	}
+	return 0
+}