[dev.ssa] cmd/compile: don't flush a value derived from the current value

If flushing a value from a register that might be used by the current
old-schedule value, save it to the home location.

This resolves the error that was changed from panic to unimplemented in
CL 12655.

Change-Id: If864be34abcd6e11d6117a061376e048a3e29b3a
Reviewed-on: https://go-review.googlesource.com/12682Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

src/cmd/compile/internal/gc/testdata/arith_ssa.go

@@ -8,6 +8,8 @@
 
 package main
 
+// test64BitConstMulti tests that rewrite rules don't fold 64 bit constants
+// into multiply instructions.
 func test64BitConstMult(a, b int64) {
 	want := 34359738369*a + b*34359738370
 	if got := test64BitConstMult_ssa(a, b); want != got {
@@ -21,6 +23,8 @@ func test64BitConstMult_ssa(a, b int64) int64 {
 	return 34359738369*a + b*34359738370
 }
 
+// test64BitConstAdd tests that rewrite rules don't fold 64 bit constants
+// into add instructions.
 func test64BitConstAdd(a, b int64) {
 	want := a + 575815584948629622 + b + 2991856197886747025
 	if got := test64BitConstAdd_ssa(a, b); want != got {
@@ -34,12 +38,28 @@ func test64BitConstAdd_ssa(a, b int64) int64 {
 	return a + 575815584948629622 + b + 2991856197886747025
 }
 
+// testRegallocCVSpill tests that regalloc spills a value whose last use is the
+// current value.
+func testRegallocCVSpill(a, b, c, d int8) {
+	want := a + -32 + b + 63*c*-87*d
+	if got := testRegallocCVSpill_ssa(a, b, c, d); want != got {
+		println("testRegallocCVSpill failed, wanted", want, "got", got)
+		failed = true
+	}
+}
+func testRegallocCVSpill_ssa(a, b, c, d int8) int8 {
+	switch {
+	}
+	return a + -32 + b + 63*c*-87*d
+}
+
 var failed = false
 
 func main() {
 
 	test64BitConstMult(1, 2)
 	test64BitConstAdd(1, 2)
+	testRegallocCVSpill(1, 2, 3, 4)
 
 	if failed {
 		panic("failed")

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

@@ -238,7 +238,7 @@ func regalloc(f *Func) {
 					if regs[r].v != nil {
 						x := regs[r].v
 						c := regs[r].c
-						if regs[r].dirty && lastUse[x.ID] > idx {
+						if regs[r].dirty && lastUse[x.ID] >= idx {
 							// Write x back to home.  Its value is currently held in c.
 							x.Op = OpStoreReg
 							x.Aux = nil
@@ -317,7 +317,7 @@ func regalloc(f *Func) {
 				if regs[r].v != nil {
 					x := regs[r].v
 					c := regs[r].c
-					if regs[r].dirty && lastUse[x.ID] > idx {
+					if regs[r].dirty && lastUse[x.ID] >= idx {
 						// Write x back to home.  Its value is currently held in c.
 						x.Op = OpStoreReg
 						x.Aux = nil