cmd/compile/internal/ssa: simplify nil checks in opt.

* Simplify the nilcheck generated by
for _, e := range a {}
* No effect on the generated code because these nil checks
don't end up in the generated code.
* Useful for other analysis, e.g. it'll remove one dependecy
on the induction variable.

Change-Id: I6ee66ddfdc010ae22aea8dca48163303d93de7a9
Reviewed-on: https://go-review.googlesource.com/20307
Run-TryBot: Alexandru Moșoi <alexandru@mosoi.ro>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -19,6 +19,10 @@
 // succ* fields must be variables
 // For now, the generated successors must be a permutation of the matched successors.
 
+// Simplify nil checks.
+// These are inserted by for _, e := range a {}
+(NilCheck (Phi x (Add64 (Const64 [c]) y)) mem) && c > 0 && v.Args[0] == y -> (NilCheck x mem)
+
 // constant folding
 (Trunc16to8 (Const16 [c])) -> (Const8 [int64(int8(c))])
 (Trunc32to8 (Const32 [c])) -> (Const8 [int64(int8(c))])

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

@@ -206,6 +206,8 @@ func rewriteValuegeneric(v *Value, config *Config) bool {
 		return rewriteValuegeneric_OpNeqPtr(v, config)
 	case OpNeqSlice:
 		return rewriteValuegeneric_OpNeqSlice(v, config)
+	case OpNilCheck:
+		return rewriteValuegeneric_OpNilCheck(v, config)
 	case OpOffPtr:
 		return rewriteValuegeneric_OpOffPtr(v, config)
 	case OpOr16:
@@ -4823,6 +4825,39 @@ func rewriteValuegeneric_OpNeqSlice(v *Value, config *Config) bool {
 	}
 	return false
 }
+func rewriteValuegeneric_OpNilCheck(v *Value, config *Config) bool {
+	b := v.Block
+	_ = b
+	// match: (NilCheck (Phi x (Add64 (Const64 [c]) y)) mem)
+	// cond: c > 0 && v.Args[0] == y
+	// result: (NilCheck x mem)
+	for {
+		if v.Args[0].Op != OpPhi {
+			break
+		}
+		x := v.Args[0].Args[0]
+		if v.Args[0].Args[1].Op != OpAdd64 {
+			break
+		}
+		if v.Args[0].Args[1].Args[0].Op != OpConst64 {
+			break
+		}
+		c := v.Args[0].Args[1].Args[0].AuxInt
+		y := v.Args[0].Args[1].Args[1]
+		if len(v.Args[0].Args) != 2 {
+			break
+		}
+		mem := v.Args[1]
+		if !(c > 0 && v.Args[0] == y) {
+			break
+		}
+		v.reset(OpNilCheck)
+		v.AddArg(x)
+		v.AddArg(mem)
+		return true
+	}
+	return false
+}
 func rewriteValuegeneric_OpOffPtr(v *Value, config *Config) bool {
 	b := v.Block
 	_ = b