cmd/compile: keep variables alive in testing.B.Loop loops

For the loop body guarded by testing.B.Loop, we disable function inlining and devirtualization inside. The only legal form to be matched is `for b.Loop() {...}`.

For #61515

Change-Id: I2e226f08cb4614667cbded498a7821dffe3f72d8
Reviewed-on: https://go-review.googlesource.com/c/go/+/612043
Reviewed-by: Michael Pratt <mpratt@google.com>
TryBot-Bypass: Junyang Shao <shaojunyang@google.com>
Commit-Queue: Junyang Shao <shaojunyang@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>

src/cmd/compile/internal/inline/interleaved/interleaved.go

@@ -105,6 +105,32 @@ func DevirtualizeAndInlineFunc(fn *ir.Func, profile *pgoir.Profile) {
 	})
 }
 
+// isTestingBLoop returns true if it matches the node as a
+// testing.(*B).Loop. See issue #61515.
+func isTestingBLoop(t ir.Node) bool {
+	if t.Op() != ir.OFOR {
+		return false
+	}
+	nFor, ok := t.(*ir.ForStmt)
+	if !ok || nFor.Cond == nil || nFor.Cond.Op() != ir.OCALLFUNC {
+		return false
+	}
+	n, ok := nFor.Cond.(*ir.CallExpr)
+	if !ok || n.Fun == nil || n.Fun.Op() != ir.OMETHEXPR {
+		return false
+	}
+	name := ir.MethodExprName(n.Fun)
+	if name == nil {
+		return false
+	}
+	if fSym := name.Sym(); fSym != nil && name.Class == ir.PFUNC && fSym.Pkg != nil &&
+		fSym.Name == "(*B).Loop" && fSym.Pkg.Path == "testing" {
+		// Attempting to match a function call to testing.(*B).Loop
+		return true
+	}
+	return false
+}
+
 // fixpoint repeatedly edits a function until it stabilizes.
 //
 // First, fixpoint applies match to every node n within fn. Then it
@@ -133,6 +159,14 @@ func fixpoint(fn *ir.Func, match func(ir.Node) bool, edit func(ir.Node) ir.Node)
 			return n // already visited n.X before wrapping
 		}
 
+		if isTestingBLoop(n) {
+			// No inlining nor devirtualization performed on b.Loop body
+			if base.Flag.LowerM > 1 {
+				fmt.Printf("%v: skip inlining within testing.B.loop for %v\n", ir.Line(n), n)
+			}
+			return n
+		}
+
 		ok := match(n)
 
 		// can't wrap TailCall's child into ParenExpr

test/inline_testingbloop.go

@@ -0,0 +1,31 @@
+// errorcheck -0 -m=2
+
+// Copyright 2024 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Test no inlining of function calls in testing.B.Loop.
+// See issue #61515.
+
+package foo
+
+import "testing"
+
+func caninline(x int) int { // ERROR "can inline caninline"
+	return x
+}
+
+func cannotinline(b *testing.B) { // ERROR "b does not escape" "cannot inline cannotinline.*"
+	for i := 0; i < b.N; i++ {
+		caninline(1) // ERROR "inlining call to caninline"
+	}
+	for b.Loop() { // ERROR "skip inlining within testing.B.loop"
+		caninline(1)
+	}
+	for i := 0; i < b.N; i++ {
+		caninline(1) // ERROR "inlining call to caninline"
+	}
+	for b.Loop() { // ERROR "skip inlining within testing.B.loop"
+		caninline(1)
+	}
+}