cmd/compile: make loop guard+rotate conditional on GOEXPERIMENT

Loops of the form "for i,e := range" needed to have their
condition rotated to the "bottom" for the preemptible loops
GOEXPERIMENT, but this caused a performance regression
because it degraded bounds check removal.  For now, make
the loop rotation/guarding conditional on the experiment.

Fixes #20711.
Updates #10958.

Change-Id: Icfba14cb3b13a910c349df8f84838cf4d9d20cf6
Reviewed-on: https://go-review.googlesource.com/46410
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/compile/internal/gc/range.go

@@ -6,6 +6,7 @@ package gc
 
 import (
 	"cmd/compile/internal/types"
+	"cmd/internal/objabi"
 	"unicode/utf8"
 )
 
@@ -211,9 +212,13 @@ func walkrange(n *Node) *Node {
 		} else if v2 == nil {
 			body = []*Node{nod(OAS, v1, hv1)}
 		} else { // for i,a := range thing { body }
+			if objabi.Preemptibleloops_enabled != 0 {
+				// Doing this transformation makes a bounds check removal less trivial; see #20711
+				// TODO enhance the preemption check insertion so that this transformation is not necessary.
 				ifGuard = nod(OIF, nil, nil)
 				ifGuard.Left = nod(OLT, hv1, hn)
 				translatedLoopOp = OFORUNTIL
+			}
 
 			a := nod(OAS2, nil, nil)
 			a.List.Set2(v1, v2)

test/loopbce.go

@@ -31,7 +31,7 @@ func f0c(a []int) int {
 
 func f1(a []int) int {
 	x := 0
-	for _, i := range a { // Change to "for i,e := range array/slice" hides IV report.
+	for _, i := range a { // ERROR "Induction variable with minimum 0 and increment 1"
 		x += i
 	}
 	return x