cmd/compile: fix OffPtr with negative offset on wasm

The wasm archtecture was missing a rule to handle OffPtr with a
negative offset. This commit makes it so OffPtr always gets lowered
to I64AddConst.

Fixes #25741

Change-Id: I1d48e2954e3ff31deb8cba9a9bf0cab7c4bab71a
Reviewed-on: https://go-review.googlesource.com/116595
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>

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

@@ -46,8 +46,7 @@
 (Not x) -> (I64Eqz x)
 
 // Lowering pointer arithmetic
-(OffPtr [0] ptr) -> ptr
-(OffPtr [off] ptr) && off > 0 -> (I64AddConst [off] ptr)
+(OffPtr [off] ptr) -> (I64AddConst [off] ptr)
 
 // Lowering extension
 // It is unnecessary to extend loads
@@ -388,6 +387,7 @@
 (I64Ne x (I64Const [0])) -> (I64Eqz (I64Eqz x))
 
 (I64Add x (I64Const [y])) -> (I64AddConst [y] x)
+(I64AddConst [0] x) -> x
 (I64Eqz (I64Eqz (I64Eqz x))) -> (I64Eqz x)
 
 ((I64Load|I64Load32U|I64Load32S|I64Load16U|I64Load16S|I64Load8U|I64Load8S) [off] (I64AddConst [off2] ptr) mem)

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

@@ -463,6 +463,8 @@ func rewriteValueWasm(v *Value) bool {
 		return rewriteValueWasm_OpWasmF64Mul_0(v)
 	case OpWasmI64Add:
 		return rewriteValueWasm_OpWasmI64Add_0(v)
+	case OpWasmI64AddConst:
+		return rewriteValueWasm_OpWasmI64AddConst_0(v)
 	case OpWasmI64And:
 		return rewriteValueWasm_OpWasmI64And_0(v)
 	case OpWasmI64Eq:
@@ -3688,34 +3690,17 @@ func rewriteValueWasm_OpNot_0(v *Value) bool {
 	}
 }
 func rewriteValueWasm_OpOffPtr_0(v *Value) bool {
-	// match: (OffPtr [0] ptr)
-	// cond:
-	// result: ptr
-	for {
-		if v.AuxInt != 0 {
-			break
-		}
-		ptr := v.Args[0]
-		v.reset(OpCopy)
-		v.Type = ptr.Type
-		v.AddArg(ptr)
-		return true
-	}
 	// match: (OffPtr [off] ptr)
-	// cond: off > 0
+	// cond:
 	// result: (I64AddConst [off] ptr)
 	for {
 		off := v.AuxInt
 		ptr := v.Args[0]
-		if !(off > 0) {
-			break
-		}
 		v.reset(OpWasmI64AddConst)
 		v.AuxInt = off
 		v.AddArg(ptr)
 		return true
 	}
-	return false
 }
 func rewriteValueWasm_OpOr16_0(v *Value) bool {
 	// match: (Or16 x y)
@@ -5211,6 +5196,22 @@ func rewriteValueWasm_OpWasmI64Add_0(v *Value) bool {
 	}
 	return false
 }
+func rewriteValueWasm_OpWasmI64AddConst_0(v *Value) bool {
+	// match: (I64AddConst [0] x)
+	// cond:
+	// result: x
+	for {
+		if v.AuxInt != 0 {
+			break
+		}
+		x := v.Args[0]
+		v.reset(OpCopy)
+		v.Type = x.Type
+		v.AddArg(x)
+		return true
+	}
+	return false
+}
 func rewriteValueWasm_OpWasmI64And_0(v *Value) bool {
 	b := v.Block
 	_ = b

test/fixedbugs/issue25741.go

@@ -0,0 +1,14 @@
+// compile
+
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+var s []int
+
+func main() {
+	i := -1
+	s[i] = 0
+}