cmd/compile: mark modify ops as both read and write

If the modify ops operate on a variable, we should tell the liveness
pass that the variable is still live before the instruction.

This looks like a bug, but I don't think there's any way to trigger
it at the moment. It only matters for pointer-containing values, and
the modify ops don't normally work on pointers. Even when I reach for
unsafe.Pointer tricks, I can't get ADDLmodify to work on pointers, as
there's always a Convert or VarDef preventing the coalescing.

TL;DR I can't figure out a test for this. But we should probably
fix it anyway.

Change-Id: I971c62616dec51a33788b7634e6478e1bfcd6260
Reviewed-on: https://go-review.googlesource.com/112157Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/compile/internal/ssa/gen/386Ops.go

@@ -346,11 +346,11 @@ func init() {
 		{name: "MOVLstore", argLength: 3, reg: gpstore, asm: "MOVL", aux: "SymOff", typ: "Mem", faultOnNilArg0: true, symEffect: "Write"},    // store 4 bytes in arg1 to arg0+auxint+aux. arg2=mem
 
 		// direct binary-op on memory (read-modify-write)
-		{name: "ADDLmodify", argLength: 3, reg: gpstore, asm: "ADDL", aux: "SymOff", typ: "Mem", faultOnNilArg0: true, symEffect: "Write"}, // *(arg0+auxint+aux) += arg1, arg2=mem
-		{name: "SUBLmodify", argLength: 3, reg: gpstore, asm: "SUBL", aux: "SymOff", typ: "Mem", faultOnNilArg0: true, symEffect: "Write"}, // *(arg0+auxint+aux) -= arg1, arg2=mem
-		{name: "ANDLmodify", argLength: 3, reg: gpstore, asm: "ANDL", aux: "SymOff", typ: "Mem", faultOnNilArg0: true, symEffect: "Write"}, // *(arg0+auxint+aux) &= arg1, arg2=mem
-		{name: "ORLmodify", argLength: 3, reg: gpstore, asm: "ORL", aux: "SymOff", typ: "Mem", faultOnNilArg0: true, symEffect: "Write"},   // *(arg0+auxint+aux) |= arg1, arg2=mem
-		{name: "XORLmodify", argLength: 3, reg: gpstore, asm: "XORL", aux: "SymOff", typ: "Mem", faultOnNilArg0: true, symEffect: "Write"}, // *(arg0+auxint+aux) ^= arg1, arg2=mem
+		{name: "ADDLmodify", argLength: 3, reg: gpstore, asm: "ADDL", aux: "SymOff", typ: "Mem", faultOnNilArg0: true, symEffect: "Read,Write"}, // *(arg0+auxint+aux) += arg1, arg2=mem
+		{name: "SUBLmodify", argLength: 3, reg: gpstore, asm: "SUBL", aux: "SymOff", typ: "Mem", faultOnNilArg0: true, symEffect: "Read,Write"}, // *(arg0+auxint+aux) -= arg1, arg2=mem
+		{name: "ANDLmodify", argLength: 3, reg: gpstore, asm: "ANDL", aux: "SymOff", typ: "Mem", faultOnNilArg0: true, symEffect: "Read,Write"}, // *(arg0+auxint+aux) &= arg1, arg2=mem
+		{name: "ORLmodify", argLength: 3, reg: gpstore, asm: "ORL", aux: "SymOff", typ: "Mem", faultOnNilArg0: true, symEffect: "Read,Write"},   // *(arg0+auxint+aux) |= arg1, arg2=mem
+		{name: "XORLmodify", argLength: 3, reg: gpstore, asm: "XORL", aux: "SymOff", typ: "Mem", faultOnNilArg0: true, symEffect: "Read,Write"}, // *(arg0+auxint+aux) ^= arg1, arg2=mem
 
 		// indexed loads/stores
 		{name: "MOVBloadidx1", argLength: 3, reg: gploadidx, commutative: true, asm: "MOVBLZX", aux: "SymOff", symEffect: "Read"}, // load a byte from arg0+arg1+auxint+aux. arg2=mem

src/cmd/compile/internal/ssa/gen/main.go

@@ -227,7 +227,7 @@ func genOp() {
 				if !needEffect {
 					log.Fatalf("symEffect with aux %s not allowed", v.aux)
 				}
-				fmt.Fprintf(w, "symEffect: Sym%s,\n", v.symEffect)
+				fmt.Fprintf(w, "symEffect: Sym%s,\n", strings.Replace(v.symEffect, ",", "|Sym", -1))
 			} else if needEffect {
 				log.Fatalf("symEffect needed for aux %s", v.aux)
 			}

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

@@ -4432,7 +4432,7 @@ var opcodeTable = [...]opInfo{
 		auxType:        auxSymOff,
 		argLen:         3,
 		faultOnNilArg0: true,
-		symEffect:      SymWrite,
+		symEffect:      SymRead | SymWrite,
 		asm:            x86.AADDL,
 		reg: regInfo{
 			inputs: []inputInfo{
@@ -4446,7 +4446,7 @@ var opcodeTable = [...]opInfo{
 		auxType:        auxSymOff,
 		argLen:         3,
 		faultOnNilArg0: true,
-		symEffect:      SymWrite,
+		symEffect:      SymRead | SymWrite,
 		asm:            x86.ASUBL,
 		reg: regInfo{
 			inputs: []inputInfo{
@@ -4460,7 +4460,7 @@ var opcodeTable = [...]opInfo{
 		auxType:        auxSymOff,
 		argLen:         3,
 		faultOnNilArg0: true,
-		symEffect:      SymWrite,
+		symEffect:      SymRead | SymWrite,
 		asm:            x86.AANDL,
 		reg: regInfo{
 			inputs: []inputInfo{
@@ -4474,7 +4474,7 @@ var opcodeTable = [...]opInfo{
 		auxType:        auxSymOff,
 		argLen:         3,
 		faultOnNilArg0: true,
-		symEffect:      SymWrite,
+		symEffect:      SymRead | SymWrite,
 		asm:            x86.AORL,
 		reg: regInfo{
 			inputs: []inputInfo{
@@ -4488,7 +4488,7 @@ var opcodeTable = [...]opInfo{
 		auxType:        auxSymOff,
 		argLen:         3,
 		faultOnNilArg0: true,
-		symEffect:      SymWrite,
+		symEffect:      SymRead | SymWrite,
 		asm:            x86.AXORL,
 		reg: regInfo{
 			inputs: []inputInfo{