cmd/compile/internal/devirtualize: do not select a zero-weight edge as the hottest one

When both a direct call and an interface call appear on the same line,
PGO devirtualization may make a suboptimal decision. In some cases,
the directly called function becomes a candidate for devirtualization
if no other relevant outgoing edges with non-zero weight exist for the
caller's IRNode in the WeightedCG. The edge to this candidate is
considered the hottest. Despite having zero weight, this edge still
causes the interface call to be devirtualized.

This CL prevents devirtualization when the weight of the hottest edge
is 0.

Fixes #72092

Change-Id: I06c0c5e080398d86f832e09244aceaa4aeb98721
Reviewed-on: https://go-review.googlesource.com/c/go/+/655475
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

1 files changed, 1 insertions, 1 deletions

diff --git a/src/cmd/compile/internal/devirtualize/pgo.go b/src/cmd/compile/internal/devirtualize/pgo.go
index 96c9231be3..a8980bb86b 100644
--- a/src/cmd/compile/internal/devirtualize/pgo.go
+++ b/src/cmd/compile/internal/devirtualize/pgo.go
@@ -741,7 +741,7 @@ func findHotConcreteCallee(p *pgoir.Profile, caller *ir.Func, call *ir.CallExpr,
 		hottest = e
 	}
 
-	if hottest == nil {
+	if hottest == nil || hottest.Weight == 0 {
 		if base.Debug.PGODebug >= 2 {
 			fmt.Printf("%v: call %s:%d: no hot callee\n", ir.Line(call), callerName, callOffset)
 		}