wasm: correctly intcast signed integers

When a signed integer's bitsize is not 32 or 64, but the given
bitsize and wanted bitsize are either both represented by Wasm's i32
or i64, we must either sign extend or wrap the integer.

src/arch/wasm/CodeGen.zig

@@ -4160,7 +4160,7 @@ fn airIntcast(func: *CodeGen, inst: Air.Inst.Index) InnerError!void {
 
     const op_bits = toWasmBits(@as(u16, @intCast(operand_ty.bitSize(mod)))).?;
     const wanted_bits = toWasmBits(@as(u16, @intCast(ty.bitSize(mod)))).?;
-    const result = if (op_bits == wanted_bits)
+    const result = if (op_bits == wanted_bits and !ty.isSignedInt(mod))
         func.reuseOperand(ty_op.operand, operand)
     else
         try (try func.intcast(operand, operand_ty, ty)).toLocal(func, ty);
@@ -4181,7 +4181,19 @@ fn intcast(func: *CodeGen, operand: WValue, given: Type, wanted: Type) InnerErro
 
     const op_bits = toWasmBits(given_bitsize).?;
     const wanted_bits = toWasmBits(wanted_bitsize).?;
-    if (op_bits == wanted_bits) return operand;
+    if (op_bits == wanted_bits) {
+        if (given.isSignedInt(mod)) {
+            if (given_bitsize < wanted_bitsize) {
+                // signed integers are stored as two's complement,
+                // when we upcast from a smaller integer to larger
+                // integers, we must get its absolute value similar to
+                // i64_extend_i32_s instruction.
+                return func.signAbsValue(operand, given);
+            }
+            return func.wrapOperand(operand, wanted);
+        }
+        return operand;
+    }
 
     if (op_bits > 32 and op_bits <= 64 and wanted_bits == 32) {
         try func.emitWValue(operand);

test/behavior/cast_int.zig

@@ -14,3 +14,18 @@ test "@intCast i32 to u7" {
     var z = x >> @as(u7, @intCast(y));
     try expect(z == 0xff);
 }
+
+test "coerce i8 to i32 and @intCast back" {
+    if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest; // TODO
+    if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest; // TODO
+    if (builtin.zig_backend == .stage2_sparc64) return error.SkipZigTest; // TODO
+    if (builtin.zig_backend == .stage2_spirv64) return error.SkipZigTest;
+
+    var x: i8 = -5;
+    var y: i32 = -5;
+    try expect(y == x);
+
+    var x2: i32 = -5;
+    var y2: i8 = -5;
+    try expect(y2 == @as(i8, @intCast(x2)));
+}