[dev.ssa] cmd/compile: fix registers for in-place instructions

Some of these were right; others weren't.

Fixes 'GOGC=off GOSSAPKG=mime go test -a mime'.

The right long term fix is probably to teach the
register allocator about in-place instructions.
In the meantime, all the tests that we can run
now pass.

Change-Id: I8e37b00a5f5e14f241b427d45d5f5cc1064883a2
Reviewed-on: https://go-review.googlesource.com/12664Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -1492,7 +1492,7 @@ func genValue(v *ssa.Value) {
 		p.From.Reg = regnum(v.Args[1]) // should be CX
 		p.To.Type = obj.TYPE_REG
 		p.To.Reg = r
-	case ssa.OpAMD64SHLQconst, ssa.OpAMD64SHRQconst, ssa.OpAMD64SARQconst:
+	case ssa.OpAMD64SHLQconst, ssa.OpAMD64SHRQconst, ssa.OpAMD64SARQconst, ssa.OpAMD64XORQconst:
 		x := regnum(v.Args[0])
 		r := regnum(v)
 		if x != r {
@@ -1686,12 +1686,6 @@ func genValue(v *ssa.Value) {
 		p := Prog(obj.ACALL)
 		p.To.Type = obj.TYPE_REG
 		p.To.Reg = regnum(v.Args[0])
-	case ssa.OpAMD64XORQconst:
-		p := Prog(x86.AXORQ)
-		p.From.Type = obj.TYPE_CONST
-		p.From.Offset = v.AuxInt
-		p.To.Type = obj.TYPE_REG
-		p.To.Reg = regnum(v.Args[0])
 	case ssa.OpAMD64NEGQ, ssa.OpAMD64NEGL, ssa.OpAMD64NEGW, ssa.OpAMD64NEGB:
 		p := Prog(v.Op.Asm())
 		p.To.Type = obj.TYPE_REG

src/cmd/compile/internal/ssa/TODO

@@ -28,6 +28,8 @@ Regalloc
  - Floating point registers
  - Make calls clobber all registers
  - Make liveness analysis non-quadratic.
+ - Handle in-place instructions (like XORQconst) directly:
+   Use XORQ AX, 1 rather than MOVQ AX, BX; XORQ BX, 1.
 
 StackAlloc:
  - Sort variables so all ptr-containing ones are first (so stack