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>

src/cmd/compile/internal/ssa/_gen/generic.rules

@@ -2544,7 +2544,10 @@
 // The exception here is that if the new length is a constant, avoiding spilling it
 // is pointless and its constantness is sometimes useful for subsequent optimizations.
 // See issue 56440.
+// Note there are 2 rules here, one for the pre-decomposed []T result and one for
+// the post-decomposed (*T,int,int) result. (The latter is generated after call expansion.)
 (SliceLen (SelectN [0] (StaticLECall {sym} _ newLen:(Const(64|32)) _ _ _ _))) && isSameCall(sym, "runtime.growslice") => newLen
+(SelectN [1] (StaticCall {sym} _ newLen:(Const(64|32)) _ _ _ _)) && v.Type.IsInteger() && isSameCall(sym, "runtime.growslice") => newLen
 
 // Collapse moving A -> B -> C into just A -> C.
 // Later passes (deadstore, elim unread auto) will remove the A -> B move, if possible.

src/cmd/compile/internal/ssa/rewritegeneric.go

@@ -26621,6 +26621,38 @@ func rewriteValuegeneric_OpSelectN(v *Value) bool {
 		v.copyOf(x)
 		return true
 	}
+	// match: (SelectN [1] (StaticCall {sym} _ newLen:(Const64) _ _ _ _))
+	// cond: v.Type.IsInteger() && isSameCall(sym, "runtime.growslice")
+	// result: newLen
+	for {
+		if auxIntToInt64(v.AuxInt) != 1 || v_0.Op != OpStaticCall || len(v_0.Args) != 6 {
+			break
+		}
+		sym := auxToCall(v_0.Aux)
+		_ = v_0.Args[1]
+		newLen := v_0.Args[1]
+		if newLen.Op != OpConst64 || !(v.Type.IsInteger() && isSameCall(sym, "runtime.growslice")) {
+			break
+		}
+		v.copyOf(newLen)
+		return true
+	}
+	// match: (SelectN [1] (StaticCall {sym} _ newLen:(Const32) _ _ _ _))
+	// cond: v.Type.IsInteger() && isSameCall(sym, "runtime.growslice")
+	// result: newLen
+	for {
+		if auxIntToInt64(v.AuxInt) != 1 || v_0.Op != OpStaticCall || len(v_0.Args) != 6 {
+			break
+		}
+		sym := auxToCall(v_0.Aux)
+		_ = v_0.Args[1]
+		newLen := v_0.Args[1]
+		if newLen.Op != OpConst32 || !(v.Type.IsInteger() && isSameCall(sym, "runtime.growslice")) {
+			break
+		}
+		v.copyOf(newLen)
+		return true
+	}
 	return false
 }
 func rewriteValuegeneric_OpSignExt16to32(v *Value) bool {

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
+}