wasm: do not export malloc, calloc, realloc, free

These functions were exported by accident, because the compiler had no
way of saying these functions shouldn't be exported.

This can be a big code size reduction for small programs. Before:

    $ tinygo build -o test.wasm -target=wasi -no-debug -scheduler=none ./testdata/alias.go && ls -l test.wasm
    -rwxrwxr-x 1 ayke ayke 2947  8 sep 13:47 test.wasm

After:

    $ tinygo build -o test.wasm -target=wasi -no-debug -scheduler=none ./testdata/alias.go && ls -l test.wasm
    -rwxrwxr-x 1 ayke ayke 968  8 sep 13:47 test.wasm

This is all because the GC isn't needed anymore.

This commit also adds support for using //go:wasm-module to set the
module name of an exported function (the default remains env).

compiler/compiler.go

@@ -1059,11 +1059,12 @@ func (b *builder) createFunctionStart(intrinsic bool) {
 	if b.info.section != "" {
 		b.llvmFn.SetSection(b.info.section)
 	}
-	if b.info.exported && strings.HasPrefix(b.Triple, "wasm") {
+	if b.info.exported && b.info.module != "" && strings.HasPrefix(b.Triple, "wasm") {
 		// Set the exported name. This is necessary for WebAssembly because
 		// otherwise the function is not exported.
-		functionAttr := b.ctx.CreateStringAttribute("wasm-export-name", b.info.linkName)
+		b.llvmFn.AddFunctionAttr(b.ctx.CreateStringAttribute("wasm-export-name", b.info.linkName))
-		b.llvmFn.AddFunctionAttr(functionAttr)
+		// Set the export module.
+		b.llvmFn.AddFunctionAttr(b.ctx.CreateStringAttribute("wasm-export-module", b.info.module))
 	}
 
 	// Some functions have a pragma controlling the inlining level.

compiler/symbol.go

@@ -210,8 +210,9 @@ func (c *compilerContext) getFunction(fn *ssa.Function) llvm.Value {
 // exported.
 func (c *compilerContext) getFunctionInfo(f *ssa.Function) functionInfo {
 	info := functionInfo{
-		// Pick the default linkName.
+		module:     "env",
-		linkName: f.RelString(nil),
+		importName: f.Name(),
+		linkName:   f.RelString(nil), // pick the default linkName
 	}
 	// Check for //go: pragmas, which may change the link name (among others).
 	info.parsePragmas(f)
@@ -225,10 +226,6 @@ func (info *functionInfo) parsePragmas(f *ssa.Function) {
 		return
 	}
 	if decl, ok := f.Syntax().(*ast.FuncDecl); ok && decl.Doc != nil {
-
-		// Our importName for a wasm module (if we are compiling to wasm), or llvm link name
-		var importName string
-
 		for _, comment := range decl.Doc.List {
 			text := comment.Text
 			if strings.HasPrefix(text, "//export ") {
@@ -246,7 +243,8 @@ func (info *functionInfo) parsePragmas(f *ssa.Function) {
 					continue
 				}
 
-				importName = parts[1]
+				info.importName = parts[1]
+				info.linkName = parts[1]
 				info.exported = true
 			case "//go:interrupt":
 				if hasUnsafeImport(f.Pkg.Pkg) {
@@ -254,10 +252,13 @@ func (info *functionInfo) parsePragmas(f *ssa.Function) {
 				}
 			case "//go:wasm-module":
 				// Alternative comment for setting the import module.
-				if len(parts) != 2 {
+				if len(parts) == 1 {
-					continue
+					// Function must not be exported outside of the WebAssembly
+					// module (but only be made available for linking).
+					info.module = ""
+				} else if len(parts) == 2 {
+					info.module = parts[1]
 				}
-				info.module = parts[1]
 			case "//go:inline":
 				info.inline = inlineHint
 			case "//go:noinline":
@@ -297,17 +298,6 @@ func (info *functionInfo) parsePragmas(f *ssa.Function) {
 				}
 			}
 		}
-
-		// Set the importName for our exported function if we have one
-		if importName != "" {
-			if info.module == "" {
-				info.linkName = importName
-			} else {
-				// WebAssembly import
-				info.importName = importName
-			}
-		}
-
 	}
 }
 

compiler/testdata/pragma.ll

@@ -62,7 +62,7 @@ declare void @main.undefinedFunctionNotInSection(i8*) #0
 
 attributes #0 = { "target-features"="+bulk-memory,+nontrapping-fptoint,+sign-ext" }
 attributes #1 = { nounwind "target-features"="+bulk-memory,+nontrapping-fptoint,+sign-ext" }
-attributes #2 = { nounwind "target-features"="+bulk-memory,+nontrapping-fptoint,+sign-ext" "wasm-export-name"="extern_func" "wasm-import-module"="env" "wasm-import-name"="extern_func" }
+attributes #2 = { nounwind "target-features"="+bulk-memory,+nontrapping-fptoint,+sign-ext" "wasm-export-module"="env" "wasm-export-name"="extern_func" "wasm-import-module"="env" "wasm-import-name"="extern_func" }
 attributes #3 = { inlinehint nounwind "target-features"="+bulk-memory,+nontrapping-fptoint,+sign-ext" }
 attributes #4 = { noinline nounwind "target-features"="+bulk-memory,+nontrapping-fptoint,+sign-ext" }
-attributes #5 = { nounwind "target-features"="+bulk-memory,+nontrapping-fptoint,+sign-ext" "wasm-export-name"="exportedFunctionInSection" "wasm-import-module"="env" "wasm-import-name"="exportedFunctionInSection" }
+attributes #5 = { nounwind "target-features"="+bulk-memory,+nontrapping-fptoint,+sign-ext" "wasm-export-module"="env" "wasm-export-name"="exportedFunctionInSection" "wasm-import-module"="env" "wasm-import-name"="exportedFunctionInSection" }

src/runtime/arch_tinygowasm.go

@@ -68,6 +68,7 @@ func growHeap() bool {
 var allocs = make(map[uintptr][]byte)
 
 //export malloc
+//go:wasm-module
 func libc_malloc(size uintptr) unsafe.Pointer {
 	buf := make([]byte, size)
 	ptr := unsafe.Pointer(&buf[0])
@@ -76,6 +77,7 @@ func libc_malloc(size uintptr) unsafe.Pointer {
 }
 
 //export free
+//go:wasm-module
 func libc_free(ptr unsafe.Pointer) {
 	if ptr == nil {
 		return
@@ -88,12 +90,14 @@ func libc_free(ptr unsafe.Pointer) {
 }
 
 //export calloc
+//go:wasm-module
 func libc_calloc(nmemb, size uintptr) unsafe.Pointer {
 	// No difference between calloc and malloc.
 	return libc_malloc(nmemb * size)
 }
 
 //export realloc
+//go:wasm-module
 func libc_realloc(oldPtr unsafe.Pointer, size uintptr) unsafe.Pointer {
 	// It's hard to optimize this to expand the current buffer with our GC, but
 	// it is theoretically possible. For now, just always allocate fresh.