cmd/compile/internal/gc: better inliner diagnostics

When debugging inliner with -m -m print cost of complex functions,
instead of simple "function too complex". This helps to understand,
how close to inlining is this particular function.

Change-Id: I6871f69b5b914d23fd0b43a24d7c6fc928f4b716
Reviewed-on: https://go-review.googlesource.com/63330
Run-TryBot: Ilya Tocar <ilya.tocar@intel.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -157,7 +157,7 @@ func caninl(fn *Node) {
 		return
 	}
 	if visitor.budget < 0 {
-		reason = "function too complex"
+		reason = fmt.Sprintf("function too complex: cost %d exceeds budget %d", maxBudget-visitor.budget, maxBudget)
 		return
 	}
 
@@ -297,8 +297,8 @@ func (v *hairyVisitor) visit(n *Node) bool {
 		v.budget -= 2
 	}
 
-	if v.budget < 0 {
-		v.reason = "function too complex"
+	// When debugging, don't stop early, to get full cost of inlining this function
+	if v.budget < 0 && Debug['m'] < 2 {
 		return true
 	}