cmd/compile: reduce inline cost of OCONVOP

OCONVOP doesn't have effect in the compiled code so, it can be safely
excluded from inline cost calculation.

Also make sequence ODEREF OCONVNOP* OADDR cost 1. This is rather common
conversion, such as *(*uint32)(unsafe.Pointer(&x)).

Fixes #42788

Change-Id: I5001f7e89d985c198b6405694cdd5b819cf3f47a
Reviewed-on: https://go-review.googlesource.com/c/go/+/281232
Reviewed-by: Keith Randall <[email protected]>
Run-TryBot: Keith Randall <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Trust: Elias Naur <[email protected]>

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

@@ -384,6 +384,22 @@ func (v *hairyVisitor) doNode(n ir.Node) bool {
 	case ir.OAPPEND:
 		v.budget -= inlineExtraAppendCost
 
+	case ir.ODEREF:
+		// *(*X)(unsafe.Pointer(&x)) is low-cost
+		n := n.(*ir.StarExpr)
+
+		ptr := n.X
+		for ptr.Op() == ir.OCONVNOP {
+			ptr = ptr.(*ir.ConvExpr).X
+		}
+		if ptr.Op() == ir.OADDR {
+			v.budget += 1 // undo half of default cost of ir.ODEREF+ir.OADDR
+		}
+
+	case ir.OCONVNOP:
+		// This doesn't produce code, but the children might.
+		v.budget++ // undo default cost
+
 	case ir.ODCLCONST, ir.OFALL:
 		// These nodes don't produce code; omit from inlining budget.
 		return false

test/inline.go

@@ -10,6 +10,7 @@
 package foo
 
 import (
+	"math"
 	"runtime"
 	"unsafe"
 )
@@ -262,3 +263,25 @@ func gd2() int { // ERROR "can inline gd2"
 func gd3() func() { // ERROR "can inline gd3"
 	return ii
 }
+
+// Issue #42788 - ensure ODEREF OCONVNOP* OADDR is low cost.
+func EncodeQuad(d []uint32, x [6]float32) { // ERROR "can inline EncodeQuad" "d does not escape"
+	_ = d[:6]
+	d[0] = math.Float32bits(x[0]) // ERROR "inlining call to math.Float32bits"
+	d[1] = math.Float32bits(x[1]) // ERROR "inlining call to math.Float32bits"
+	d[2] = math.Float32bits(x[2]) // ERROR "inlining call to math.Float32bits"
+	d[3] = math.Float32bits(x[3]) // ERROR "inlining call to math.Float32bits"
+	d[4] = math.Float32bits(x[4]) // ERROR "inlining call to math.Float32bits"
+	d[5] = math.Float32bits(x[5]) // ERROR "inlining call to math.Float32bits"
+}
+
+// Ensure OCONVNOP is zero cost.
+func Conv(v uint64) uint64 { // ERROR "can inline Conv"
+	return conv2(conv2(conv2(v))) // ERROR "inlining call to (conv1|conv2)"
+}
+func conv2(v uint64) uint64 { // ERROR "can inline conv2"
+	return conv1(conv1(conv1(conv1(v)))) // ERROR "inlining call to conv1"
+}
+func conv1(v uint64) uint64 { // ERROR "can inline conv1"
+	return uint64(uint64(uint64(uint64(uint64(uint64(uint64(uint64(uint64(uint64(uint64(v)))))))))))
+}