reflect: support big-endian systems

The reflect package needs to know the endianness of the system in a few
places. Before this patch, it assumed little-endian systems. But with
GOARCH=mips we now have a big-endian system which also needs to be
supported. So this patch fixes the reflect package to work on big-endian
systems.

Also, I've updated the tests for MIPS: instead of running the
little-endian tests, I've changed it to run the big-endian tests
instead. The two are very similar except for endianness so this should
be fine. To be sure we won't accidentally break little-endian support,
I've kept a single MIPS little-endian test (the CGo test, which doesn't
yet work on big-endian systems anyway).

main_test.go

@@ -36,7 +36,7 @@ var supportedLinuxArches = map[string]string{
 	"X86Linux":   "linux/386",
 	"ARMLinux":   "linux/arm/6",
 	"ARM64Linux": "linux/arm64",
-	"MIPSLinux":  "linux/mipsle/hardfloat",
+	"MIPSLinux":  "linux/mips/hardfloat",
 	"WASIp1":     "wasip1/wasm",
 }
 
@@ -177,17 +177,14 @@ func TestBuild(t *testing.T) {
 				})
 			}
 		}
-		t.Run("MIPS big-endian", func(t *testing.T) {
-			// Run a single test for GOARCH=mips to see whether it works at all.
-			// Big-endian MIPS isn't fully supported yet, but simple examples
-			// should work.
-			// Once big-endian is fully supported, we can probably flip this
-			// around and do full testing of MIPS big-endian support and only do
-			// limited testing of MIPS little-endian (because the two are some
-			// similar).
+		t.Run("MIPS little-endian", func(t *testing.T) {
+			// Run a single test for GOARCH=mipsle to see whether it works at
+			// all. It is already mostly tested because GOARCH=mips and
+			// GOARCH=mipsle are so similar, but it's good to have an extra test
+			// to be sure.
 			t.Parallel()
-			options := optionsFromOSARCH("linux/mips/softfloat", sema)
-			runTest("map.go", options, t, nil, nil)
+			options := optionsFromOSARCH("linux/mipsle/softfloat", sema)
+			runTest("cgo/", options, t, nil, nil)
 		})
 		t.Run("WebAssembly", func(t *testing.T) {
 			t.Parallel()
@@ -232,6 +229,13 @@ func runPlatTests(options compileopts.Options, tests []string, t *testing.T) {
 				continue
 			}
 		}
+		if options.GOOS == "linux" && options.GOARCH == "mips" {
+			if name == "cgo/" {
+				// CGo isn't supported yet on big-endian systems (needs updates
+				// to bitfield access methods).
+				continue
+			}
+		}
 		if options.Target == "simavr" {
 			// Not all tests are currently supported on AVR.
 			// Skip the ones that aren't.

src/reflect/endian-big.go

@@ -0,0 +1,36 @@
+//go:build mips
+
+package reflect
+
+import "unsafe"
+
+// loadValue loads a value that may or may not be word-aligned. The number of
+// bytes given in size are loaded. The biggest possible size it can load is that
+// of an uintptr.
+func loadValue(ptr unsafe.Pointer, size uintptr) uintptr {
+	loadedValue := uintptr(0)
+	for i := uintptr(0); i < size; i++ {
+		loadedValue <<= 8
+		loadedValue |= uintptr(*(*byte)(ptr))
+		ptr = unsafe.Add(ptr, 1)
+	}
+	return loadedValue
+}
+
+// storeValue is the inverse of loadValue. It stores a value to a pointer that
+// doesn't need to be aligned.
+func storeValue(ptr unsafe.Pointer, size, value uintptr) {
+	// This could perhaps be optimized using bits.ReverseBytes32 if needed.
+	value <<= (unsafe.Sizeof(uintptr(0)) - size) * 8
+	for i := uintptr(0); i < size; i++ {
+		*(*byte)(ptr) = byte(value >> ((unsafe.Sizeof(uintptr(0)) - 1) * 8))
+		ptr = unsafe.Add(ptr, 1)
+		value <<= 8
+	}
+}
+
+// maskAndShift cuts out a part of a uintptr. Note that the offset may not be 0.
+func maskAndShift(value, offset, size uintptr) uintptr {
+	mask := ^uintptr(0) >> ((unsafe.Sizeof(uintptr(0)) - size) * 8)
+	return (uintptr(value) >> ((unsafe.Sizeof(uintptr(0)) - offset - size) * 8)) & mask
+}

src/reflect/endian-little.go

@@ -0,0 +1,35 @@
+//go:build !mips
+
+package reflect
+
+import "unsafe"
+
+// loadValue loads a value that may or may not be word-aligned. The number of
+// bytes given in size are loaded. The biggest possible size it can load is that
+// of an uintptr.
+func loadValue(ptr unsafe.Pointer, size uintptr) uintptr {
+	loadedValue := uintptr(0)
+	shift := uintptr(0)
+	for i := uintptr(0); i < size; i++ {
+		loadedValue |= uintptr(*(*byte)(ptr)) << shift
+		shift += 8
+		ptr = unsafe.Add(ptr, 1)
+	}
+	return loadedValue
+}
+
+// storeValue is the inverse of loadValue. It stores a value to a pointer that
+// doesn't need to be aligned.
+func storeValue(ptr unsafe.Pointer, size, value uintptr) {
+	for i := uintptr(0); i < size; i++ {
+		*(*byte)(ptr) = byte(value)
+		ptr = unsafe.Add(ptr, 1)
+		value >>= 8
+	}
+}
+
+// maskAndShift cuts out a part of a uintptr. Note that the offset may not be 0.
+func maskAndShift(value, offset, size uintptr) uintptr {
+	mask := ^uintptr(0) >> ((unsafe.Sizeof(uintptr(0)) - size) * 8)
+	return (uintptr(value) >> (offset * 8)) & mask
+}

src/reflect/value.go

@@ -887,26 +887,6 @@ func (v Value) Index(i int) Value {
 	}
 }
 
-// loadValue loads a value that may or may not be word-aligned. The number of
-// bytes given in size are loaded. The biggest possible size it can load is that
-// of an uintptr.
-func loadValue(ptr unsafe.Pointer, size uintptr) uintptr {
-	loadedValue := uintptr(0)
-	shift := uintptr(0)
-	for i := uintptr(0); i < size; i++ {
-		loadedValue |= uintptr(*(*byte)(ptr)) << shift
-		shift += 8
-		ptr = unsafe.Add(ptr, 1)
-	}
-	return loadedValue
-}
-
-// maskAndShift cuts out a part of a uintptr. Note that the offset may not be 0.
-func maskAndShift(value, offset, size uintptr) uintptr {
-	mask := ^uintptr(0) >> ((unsafe.Sizeof(uintptr(0)) - size) * 8)
-	return (uintptr(value) >> (offset * 8)) & mask
-}
-
 func (v Value) NumMethod() int {
 	return v.typecode.NumMethod()
 }
@@ -1088,9 +1068,7 @@ func (v Value) Set(x Value) {
 	if v.typecode.Kind() == Interface && x.typecode.Kind() != Interface {
 		// move the value of x back into the interface, if possible
 		if x.isIndirect() && x.typecode.Size() <= unsafe.Sizeof(uintptr(0)) {
-			var value uintptr
-			memcpy(unsafe.Pointer(&value), x.value, x.typecode.Size())
-			x.value = unsafe.Pointer(value)
+			x.value = unsafe.Pointer(loadValue(x.value, x.typecode.Size()))
 		}
 
 		intf := composeInterface(unsafe.Pointer(x.typecode), x.value)
@@ -1101,12 +1079,11 @@ func (v Value) Set(x Value) {
 	}
 
 	size := v.typecode.Size()
-	xptr := x.value
 	if size <= unsafe.Sizeof(uintptr(0)) && !x.isIndirect() {
-		value := x.value
-		xptr = unsafe.Pointer(&value)
+		storeValue(v.value, size, uintptr(x.value))
+	} else {
+		memcpy(v.value, x.value, size)
 	}
-	memcpy(v.value, xptr, size)
 }
 
 func (v Value) SetZero() {