runtime: use the main (startup) stack for the main goroutine

Instead of always starting a new goroutine for the main goroutine, run
the main goroutine on the system stack.
The system stack is not occupied with scheduling, instead each goroutine
that wants to pause itself calls into the scheduler which will switch to
the next task (goroutine) to run, or sleeps.

There are various advantages of this over the previous system:

  * When the program doesn't start a goroutine, the code size and RAM
    consumption is close to what you'd get with `-scheduler=none`.
  * When the program does start a goroutine, there is still a reduction
    in RAM consumption because only one extra stack is needed.
  * Because tasks directly switch to the next task to run, only a single
    task switch is needed instead of two (goroutine -> scheduler ->
    goroutine). This should improve task switching performance.

I kept the current behavior for WebAssembly/Asyncify. I looked into how
the same benefits can be realized for WebAssembly but couldn't easily
find how to do that. Maybe this can be done separately, or maybe we'll
just wait for the stack switching proposal to finish.

The code for Cortex-M is currently more complicated than I'd like, and
therefore can sometimes result in a slight increase in code size. I'd
like to fix this eventually but am still looking into good ways to do
this. I still think this change is generally beneficial because many
programs see big reductions in code size when compiling for Cortex-M.

builder/build.go

@@ -1228,10 +1228,15 @@ func determineStackSizes(mod llvm.Module, executable string) ([]string, map[stri
 	// Goroutines need to be started and finished and take up some stack space
 	// that way. This can be measured by measuing the stack size of
 	// tinygo_startTask.
-	if numFuncs := len(functions["tinygo_startTask"]); numFuncs != 1 {
-		return nil, nil, fmt.Errorf("expected exactly one definition of tinygo_startTask, got %d", numFuncs)
+	var baseStackSize uint64
+	var baseStackSizeType stacksize.SizeType
+	var baseStackSizeFailedAt *stacksize.CallNode
+	if len(gowrappers) != 0 {
+		if numFuncs := len(functions["tinygo_startTask"]); numFuncs != 1 {
+			return nil, nil, fmt.Errorf("expected exactly one definition of tinygo_startTask, got %d", numFuncs)
+		}
+		baseStackSize, baseStackSizeType, baseStackSizeFailedAt = functions["tinygo_startTask"][0].StackSize()
 	}
-	baseStackSize, baseStackSizeType, baseStackSizeFailedAt := functions["tinygo_startTask"][0].StackSize()
 
 	sizes := make(map[string]functionStackSize)
 
@@ -1303,6 +1308,11 @@ func modifyStackSizes(executable string, stackSizeLoads []string, stackSizes map
 	if err != nil {
 		return err
 	}
+	if data == nil {
+		// The .tinygo_stacksizes section doesn't exist, so assume this
+		// modification isn't needed.
+		return nil
+	}
 
 	if len(stackSizeLoads)*4 != len(data) {
 		// Note: while AVR should use 2 byte stack sizes, even 64-bit platforms

builder/elfpatch.go

@@ -15,7 +15,7 @@ func getElfSectionData(executable string, sectionName string) ([]byte, elf.FileH
 
 	section := elfFile.Section(sectionName)
 	if section == nil {
-		return nil, elf.FileHeader{}, fmt.Errorf("could not find %s section", sectionName)
+		return nil, elf.FileHeader{}, nil
 	}
 
 	data, err := section.Data()