[dev.ssa] cmd/compile: support zero type for *T

Change-Id: I4c9bcea01e2c4333c2a3592b66f1da9f424747a4
Reviewed-on: https://go-review.googlesource.com/12130Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -652,7 +652,7 @@ func (s *state) assign(op uint8, left *Node, right *Node) {
 		switch {
 		case t.IsString():
 			val = s.entryNewValue0A(ssa.OpConst, left.Type, "")
-		case t.IsInteger():
+		case t.IsInteger() || t.IsPtr():
 			val = s.entryNewValue0(ssa.OpConst, left.Type)
 		case t.IsBoolean():
 			val = s.entryNewValue0A(ssa.OpConst, left.Type, false) // TODO: store bools as 0/1 in AuxInt?

src/cmd/compile/internal/ssa/gen/AMD64.rules

@@ -76,6 +76,7 @@
 (OffPtr [off] ptr) -> (ADDQconst [off] ptr)
 
 (Const <t> [val]) && t.IsInteger() -> (MOVQconst [val])
+(Const <t>) && t.IsPtr() -> (MOVQconst [0]) // nil is the only const pointer
 
 (Addr {sym} base) -> (LEAQ {sym} base)
 

src/cmd/compile/internal/ssa/rewriteAMD64.go

@@ -499,6 +499,24 @@ func rewriteValueAMD64(v *Value, config *Config) bool {
 		goto end4c8bfe9df26fc5aa2bd76b211792732a
 	end4c8bfe9df26fc5aa2bd76b211792732a:
 		;
+		// match: (Const <t>)
+		// cond: t.IsPtr()
+		// result: (MOVQconst [0])
+		{
+			t := v.Type
+			if !(t.IsPtr()) {
+				goto endd23abe8d7061f11c260b162e24eec060
+			}
+			v.Op = OpAMD64MOVQconst
+			v.AuxInt = 0
+			v.Aux = nil
+			v.resetArgs()
+			v.AuxInt = 0
+			return true
+		}
+		goto endd23abe8d7061f11c260b162e24eec060
+	endd23abe8d7061f11c260b162e24eec060:
+		;
 	case OpConvNop:
 		// match: (ConvNop <t> x)
 		// cond: t == x.Type