cmd/compile: CALLudiv on nacl/arm doesn't clobber R12

On nacl/arm, R12 is clobbered by the RET instruction in function
that has a frame. runtime.udiv doesn't have a frame, so it does
not clobber R12.

Change-Id: I0de448749f615908f6659e92d201ba3eb2f8266d
Reviewed-on: https://go-review.googlesource.com/93116
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

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

@@ -291,9 +291,6 @@ func NewConfig(arch string, types Types, ctxt *obj.Link, optimize bool) *Config
 	if c.nacl {
 		c.noDuffDevice = true // Don't use Duff's device on NaCl
 
-		// runtime call clobber R12 on nacl
-		opcodeTable[OpARMCALLudiv].reg.clobbers |= 1 << 12 // R12
-
 		// Returns clobber BP on nacl/386, so the write
 		// barrier does.
 		opcodeTable[Op386LoweredWB].reg.clobbers |= 1 << 5 // BP

src/runtime/vlop_arm.s

@@ -102,6 +102,9 @@ TEXT runtime·_sfloatpanic(SB),NOSPLIT|NOFRAME,$0
 #define Ra	R11
 
 // Be careful: Ra == R11 will be used by the linker for synthesized instructions.
+// Note: this function does not have a frame. If it ever needs a frame,
+// the RET instruction will clobber R12 on nacl, and the compiler's register
+// allocator needs to know.
 TEXT runtime·udiv(SB),NOSPLIT|NOFRAME,$0
 	MOVBU	runtime·hardDiv(SB), Ra
 	CMP	$0, Ra