all: add flag to enable i64 parameters in WebAssembly

compiler/compiler.go

@@ -3114,9 +3114,11 @@ func (c *Compiler) NonConstGlobals() {
 	}
 }
 
-// Replace i64 in an external function with a stack-allocated i64*, to work
+// When -wasm-abi flag set to "js" (default),
+// replace i64 in an external function with a stack-allocated i64*, to work
 // around the lack of 64-bit integers in JavaScript (commonly used together with
 // WebAssembly). Once that's resolved, this pass may be avoided.
+// See also the -wasm-abi= flag
 // https://github.com/WebAssembly/design/issues/1172
 func (c *Compiler) ExternalInt64AsPtr() error {
 	int64Type := c.ctx.Int64Type()
@@ -3220,7 +3222,8 @@ func (c *Compiler) ExternalInt64AsPtr() error {
 			c.builder.SetInsertPointAtEnd(entryBlock)
 			var callParams []llvm.Value
 			if fnType.ReturnType() == int64Type {
-				return errors.New("todo: i64 return value in exported function")
+				return errors.New("not yet implemented: exported function returns i64 with -wasm-abi=js; " +
+					"see https://tinygo.org/compiler-internals/calling-convention/")
 			}
 			for i, origParam := range fn.Params() {
 				paramValue := externalFn.Param(i)

docs/internals.rst

@@ -130,8 +130,8 @@ somewhat compatible with the C calling convention but with a few quirks:
     pointers. This avoids some overhead in the C calling convention and makes
     the work of the LLVM optimizers easier.
 
-  * The WebAssembly target never exports or imports a ``i64`` (``int64``,
-    ``uint64``) parameter. Instead, it replaces them with ``i64*``, allocating
+  * The WebAssembly target by default doesn't export or import ``i64`` (``int64``,
+    ``uint64``) parameters. Instead, it replaces them with ``i64*``, allocating
     the value on the stack. In other words, imported functions are called with a
     64-bit integer on the stack and exported functions must be called with a
     pointer to a 64-bit integer somewhere in linear memory.
@@ -144,10 +144,15 @@ somewhat compatible with the C calling convention but with a few quirks:
     calling convention workaround may be removed. Also see `this wasm-bindgen
     issue <https://github.com/rustwasm/wasm-bindgen/issues/35>`_.
 
+    Currently there are also non-browser WebAssembly execution environments
+    that do not have this limitation. Use the `-wasm-abi=generic` flag to remove
+    the behavior described above and enable emitting functions with i64
+    parameters directly.
+
   * The WebAssembly target does not return variables directly that cannot be
-    handled by JavaScript (``struct``, ``i64``, multiple return values, etc).
-    Instead, they are stored into a pointer passed as the first parameter by the
-    caller.
+    handled by JavaScript (see above about ``i64``, also ``struct``, multiple
+    return values, etc). Instead, they are stored into a pointer passed as the
+    first parameter by the caller.
 
     This is the calling convention as implemented by LLVM, with the extension
     that ``i64`` return values are returned in the same way as aggregate types.

main.go

@@ -36,6 +36,7 @@ type BuildConfig struct {
 	initInterp bool
 	cFlags     []string
 	ldFlags    []string
+	wasmAbi    string
 }
 
 // Helper function for Compiler object.
@@ -97,7 +98,8 @@ func Compile(pkgName, outpath string, spec *TargetSpec, config *BuildConfig, act
 	// cannot be represented exactly in JavaScript (JS only has doubles). To
 	// keep functions interoperable, pass int64 types as pointers to
 	// stack-allocated values.
-	if strings.HasPrefix(spec.Triple, "wasm") {
+	// Use -wasm-abi=generic to disable this behaviour.
+	if config.wasmAbi == "js" && strings.HasPrefix(spec.Triple, "wasm") {
 		err := c.ExternalInt64AsPtr()
 		if err != nil {
 			return err
@@ -507,6 +509,7 @@ func main() {
 	port := flag.String("port", "/dev/ttyACM0", "flash port")
 	cFlags := flag.String("cflags", "", "additional cflags for compiler")
 	ldFlags := flag.String("ldflags", "", "additional ldflags for linker")
+	wasmAbi := flag.String("wasm-abi", "js", "WebAssembly ABI conventions: js (no i64 params) or generic")
 
 	if len(os.Args) < 2 {
 		fmt.Fprintln(os.Stderr, "No command-line arguments supplied.")
@@ -524,6 +527,7 @@ func main() {
 		debug:      !*nodebug,
 		printSizes: *printSize,
 		initInterp: *initInterp,
+		wasmAbi:    *wasmAbi,
 	}
 
 	if *cFlags != "" {