cmd/compile: don't spill rematerializeable value when resolving merge edges

Fixes #19515.

Change-Id: I4bcce152cef52d00fbb5ab4daf72a6e742bae27c
Reviewed-on: https://go-review.googlesource.com/38158
Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/compile/internal/ssa/regalloc.go

@@ -2227,6 +2227,7 @@ func (e *edgeState) findRegFor(typ Type) Location {
 	// 1) an unused register
 	// 2) a non-unique register not holding a final value
 	// 3) a non-unique register
+	// 4) TODO: a register holding a rematerializeable value
 	x := m &^ e.usedRegs
 	if x != 0 {
 		return &e.s.registers[pickReg(x)]
@@ -2252,10 +2253,12 @@ func (e *edgeState) findRegFor(typ Type) Location {
 		a := e.cache[vid]
 		for _, c := range a {
 			if r, ok := e.s.f.getHome(c.ID).(*Register); ok && m>>uint(r.num)&1 != 0 {
-				x := e.p.NewValue1(c.Pos, OpStoreReg, c.Type, c)
-				e.set(t, vid, x, false, c.Pos)
-				if e.s.f.pass.debug > regDebug {
-					fmt.Printf("  SPILL %s->%s %s\n", r.Name(), t.Name(), x.LongString())
+				if !c.rematerializeable() {
+					x := e.p.NewValue1(c.Pos, OpStoreReg, c.Type, c)
+					e.set(t, vid, x, false, c.Pos)
+					if e.s.f.pass.debug > regDebug {
+						fmt.Printf("  SPILL %s->%s %s\n", r.Name(), t.Name(), x.LongString())
+					}
 				}
 				// r will now be overwritten by the caller. At some point
 				// later, the newly saved value will be moved back to its

test/fixedbugs/issue19515.go

@@ -0,0 +1,51 @@
+// compile
+
+// Copyright 2017 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.
+
+// Issue 19515: compiler panics on spilling int128 constant.
+
+package x
+
+type VScrollPanel struct {
+	x, y int
+}
+
+type Color struct {
+	R, G, B, A float32
+}
+
+func maxF(a, b float32) float32 {
+	if a > b {
+		return 0
+	}
+	return 1
+}
+
+type TransformMatrix [6]float32
+
+type Paint struct {
+	xform      TransformMatrix
+	feather    float32
+	innerColor Color
+	outerColor Color
+}
+
+func BoxGradient(x, y, w, h, f float32, iColor, oColor Color) Paint {
+	return Paint{
+		xform:      TransformMatrix{9, 0, 0, 0, x, y},
+		feather:    maxF(1.0, f),
+		innerColor: iColor,
+		outerColor: oColor,
+	}
+}
+
+func (v *VScrollPanel) Draw() {
+	x := float32(v.x)
+	y := float32(v.y)
+
+	BoxGradient(x+x-2, y-1, 0, 0, 0, Color{}, Color{})
+	BoxGradient(x+y-2, y-1, 0, 0, 0, Color{}, Color{})
+}
+