all: store data layout as little endian value

This makes it much easier to read the value at runtime, as pointer
indices are naturally little endian. It should not affect anything else
in the program.

compiler/llvm.go

@@ -166,6 +166,7 @@ func (c *compilerContext) createObjectLayout(t llvm.Type, pos token.Pos) llvm.Va
 	// Create the global initializer.
 	bitmapBytes := make([]byte, int(objectSizeWords+7)/8)
 	bitmap.FillBytes(bitmapBytes)
+	reverseBytes(bitmapBytes) // big-endian to little-endian
 	var bitmapByteValues []llvm.Value
 	for _, b := range bitmapBytes {
 		bitmapByteValues = append(bitmapByteValues, llvm.ConstInt(c.ctx.Int8Type(), uint64(b), false))
@@ -314,3 +315,12 @@ func (b *builder) readStackPointer() llvm.Value {
 	}
 	return b.CreateCall(stacksave.GlobalValueType(), stacksave, nil, "")
 }
+
+// Reverse a slice of bytes. From the wiki:
+// https://github.com/golang/go/wiki/SliceTricks#reversing
+func reverseBytes(buf []byte) {
+	for i := len(buf)/2 - 1; i >= 0; i-- {
+		opp := len(buf) - 1 - i
+		buf[i], buf[opp] = buf[opp], buf[i]
+	}
+}

compiler/testdata/gc.ll

@@ -20,8 +20,8 @@ target triple = "wasm32-unknown-wasi"
 @main.slice1 = hidden global { ptr, i32, i32 } zeroinitializer, align 8
 @main.slice2 = hidden global { ptr, i32, i32 } zeroinitializer, align 8
 @main.slice3 = hidden global { ptr, i32, i32 } zeroinitializer, align 8
-@"runtime/gc.layout:62-2000000000000001" = linkonce_odr unnamed_addr constant { i32, [8 x i8] } { i32 62, [8 x i8] c" \00\00\00\00\00\00\01" }
-@"runtime/gc.layout:62-0001" = linkonce_odr unnamed_addr constant { i32, [8 x i8] } { i32 62, [8 x i8] c"\00\00\00\00\00\00\00\01" }
+@"runtime/gc.layout:62-2000000000000001" = linkonce_odr unnamed_addr constant { i32, [8 x i8] } { i32 62, [8 x i8] c"\01\00\00\00\00\00\00 " }
+@"runtime/gc.layout:62-0001" = linkonce_odr unnamed_addr constant { i32, [8 x i8] } { i32 62, [8 x i8] c"\01\00\00\00\00\00\00\00" }
 @"reflect/types.type:basic:complex128" = linkonce_odr constant %runtime.typecodeID { ptr null, i32 0, ptr null, ptr @"reflect/types.type:pointer:basic:complex128", i32 0 }
 @"reflect/types.type:pointer:basic:complex128" = linkonce_odr constant %runtime.typecodeID { ptr @"reflect/types.type:basic:complex128", i32 0, ptr null, ptr null, i32 0 }
 

interp/memory.go

@@ -1289,6 +1289,7 @@ func (r *runner) readObjectLayout(layoutValue value) (uint64, *big.Int) {
 		}
 		rawBytes[i] = byte(v)
 	}
+	reverseBytes(rawBytes) // little-endian to big-endian
 	bitmap := new(big.Int).SetBytes(rawBytes)
 	return objectSizeWords, bitmap
 }
@@ -1338,3 +1339,12 @@ func (r *runner) getLLVMTypeFromLayout(layoutValue value) llvm.Type {
 	}
 	return llvmLayoutType
 }
+
+// Reverse a slice of bytes. From the wiki:
+// https://github.com/golang/go/wiki/SliceTricks#reversing
+func reverseBytes(buf []byte) {
+	for i := len(buf)/2 - 1; i >= 0; i-- {
+		opp := len(buf) - 1 - i
+		buf[i], buf[opp] = buf[opp], buf[i]
+	}
+}

interp/testdata/alloc.ll

@@ -1,7 +1,7 @@
 target datalayout = "e-m:e-p:32:32-i64:64-n32:64-S128"
 target triple = "wasm32--wasi"
 
-@"runtime/gc.layout:62-2000000000000001" = linkonce_odr unnamed_addr constant { i32, [8 x i8] } { i32 62, [8 x i8] c" \00\00\00\00\00\00\01" }
+@"runtime/gc.layout:62-2000000000000001" = linkonce_odr unnamed_addr constant { i32, [8 x i8] } { i32 62, [8 x i8] c"\01\00\00\00\00\00\00 " }
 @pointerFree12 = global i8* null
 @pointerFree7 = global i8* null
 @pointerFree3 = global i8* null