cmd/8g: fix liveness for 387 build (including plan9)

TBR=khr
CC=golang-codereviews
https://golang.org/cl/84570045

src/cmd/8g/prog.c

@@ -138,11 +138,16 @@ static ProgInfo progtable[ALAST] = {
 	[AFMOVW]=	{SizeW | LeftAddr | RightWrite},
 	[AFMOVV]=	{SizeQ | LeftAddr | RightWrite},
 
-	[AFMOVDP]=	{SizeD | LeftRead | RightAddr},
-	[AFMOVFP]=	{SizeF | LeftRead | RightAddr},
-	[AFMOVLP]=	{SizeL | LeftRead | RightAddr},
-	[AFMOVWP]=	{SizeW | LeftRead | RightAddr},
-	[AFMOVVP]=	{SizeQ | LeftRead | RightAddr},
+	// These instructions are marked as RightAddr
+	// so that the register optimizer does not try to replace the
+	// memory references with integer register references.
+	// But they do not use the previous value at the address, so
+	// we also mark them RightWrite.
+	[AFMOVDP]=	{SizeD | LeftRead | RightWrite | RightAddr},
+	[AFMOVFP]=	{SizeF | LeftRead | RightWrite | RightAddr},
+	[AFMOVLP]=	{SizeL | LeftRead | RightWrite | RightAddr},
+	[AFMOVWP]=	{SizeW | LeftRead | RightWrite | RightAddr},
+	[AFMOVVP]=	{SizeQ | LeftRead | RightWrite | RightAddr},
 
 	[AFMULD]=	{SizeD | LeftAddr | RightRdwr},
 	[AFMULDP]=	{SizeD | LeftAddr | RightRdwr},

src/cmd/gc/plive.c

@@ -755,7 +755,15 @@ Next:
 					if(prog->as == AVARDEF || prog->as == AVARKILL)
 						bvset(varkill, pos);
 				} else {
-					if(info.flags & (RightRead | RightAddr))
+					// RightRead is a read, obviously.
+					// RightAddr by itself is also implicitly a read.
+					//
+					// RightAddr|RightWrite means that the address is being taken
+					// but only so that the instruction can write to the value.
+					// It is not a read. It is equivalent to RightWrite except that
+					// having the RightAddr bit set keeps the registerizer from
+					// trying to substitute a register for the memory location.
+					if((info.flags & RightRead) || (info.flags & (RightAddr|RightWrite)) == RightAddr)
 						bvset(uevar, pos);
 					if(info.flags & RightWrite)
 						if(to->node != nil && (!isfat(to->node->type) || prog->as == AVARDEF))