runtime: refactor markGlobals to findGlobals

Instead of markGlobals calling markRoots unconditionally (which doesn't
make sense for -gc=none and -gc=leaking), provide markRoots as a
callback function.

This is in preparation for -gc=boehm, where the previous design is even
more awkward and a callback makes far more sense.

I've tested the size impact using `make smoketest XTENSA=0`. There is
none, except for two cases:

  * One with `-opt=0` so const-propagation for the callback didn't take
    place.
  * One other on AVR, I don't know why but as it's only 16 bytes in a
    very specific case I'm going to assume it's just a random change in
    compiler output that caused a size difference.

src/runtime/gc_blocks.go

@@ -420,7 +420,7 @@ func runGC() (freeBytes uintptr) {
 
 	// Mark phase: mark all reachable objects, recursively.
 	markStack()
-	markGlobals()
+	findGlobals(markRoots)
 
 	if baremetal && hasScheduler {
 		// Channel operations in interrupts may move task pointers around while we are marking.

src/runtime/gc_globals.go

@@ -2,13 +2,14 @@
 
 package runtime
 
-// This file implements markGlobals for all the files that don't have a more
+// This file implements findGlobals for all systems where the start and end of
-// specific implementation.
+// the globals section can be found through linker-defined symbols.
 
-// markGlobals marks all globals, which are reachable by definition.
+// findGlobals finds all globals (which are reachable by definition) and calls
+// the callback for them.
 //
 // This implementation marks all globals conservatively and assumes it can use
 // linker-defined symbols for the start and end of the .data section.
-func markGlobals() {
+func findGlobals(found func(start, end uintptr)) {
-	markRoots(globalsStart, globalsEnd)
+	found(globalsStart, globalsEnd)
 }

src/runtime/gc_leaking.go

@@ -100,7 +100,3 @@ func setHeapEnd(newHeapEnd uintptr) {
 	// enough.
 	heapEnd = newHeapEnd
 }
-
-func markRoots(start, end uintptr) {
-	// dummy, so that markGlobals will compile
-}

src/runtime/gc_none.go

@@ -37,7 +37,3 @@ func initHeap() {
 func setHeapEnd(newHeapEnd uintptr) {
 	// Nothing to do here, this function is never actually called.
 }
-
-func markRoots(start, end uintptr) {
-	// dummy, so that markGlobals will compile
-}

src/runtime/os_darwin.go

@@ -54,12 +54,12 @@ type segmentLoadCommand struct {
 //go:extern _mh_execute_header
 var libc_mh_execute_header machHeader
 
-// Mark global variables.
+// Find global variables in .data/.bss sections.
 // The MachO linker doesn't seem to provide symbols for the start and end of the
 // data section. There is get_etext, get_edata, and get_end, but these are
 // undocumented and don't work with ASLR (which is enabled by default).
 // Therefore, read the MachO header directly.
-func markGlobals() {
+func findGlobals(found func(start, end uintptr)) {
 	// Here is a useful blog post to understand the MachO file format:
 	// https://h3adsh0tzz.com/2020/01/macho-file-format/
 
@@ -103,7 +103,7 @@ func markGlobals() {
 				// This could be improved by only reading the memory areas
 				// covered by sections. That would reduce the amount of memory
 				// scanned a little bit (up to a single VM page).
-				markRoots(offset+cmd.vmaddr, offset+cmd.vmaddr+cmd.vmsize)
+				found(offset+cmd.vmaddr, offset+cmd.vmaddr+cmd.vmsize)
 			}
 		}
 

src/runtime/os_linux.go

@@ -77,9 +77,9 @@ type elfProgramHeader32 struct {
 //go:extern __ehdr_start
 var ehdr_start elfHeader
 
-// markGlobals marks all globals, which are reachable by definition.
+// findGlobals finds globals in the .data/.bss sections.
 // It parses the ELF program header to find writable segments.
-func markGlobals() {
+func findGlobals(found func(start, end uintptr)) {
 	// Relevant constants from the ELF specification.
 	// See: https://refspecs.linuxfoundation.org/elf/elf.pdf
 	const (
@@ -99,14 +99,14 @@ func markGlobals() {
 			if header._type == PT_LOAD && header.flags&PF_W != 0 {
 				start := header.vaddr
 				end := start + header.memsz
-				markRoots(start, end)
+				found(start, end)
 			}
 		} else {
 			header := (*elfProgramHeader32)(headerPtr)
 			if header._type == PT_LOAD && header.flags&PF_W != 0 {
 				start := header.vaddr
 				end := start + header.memsz
-				markRoots(start, end)
+				found(start, end)
 			}
 		}
 		headerPtr = unsafe.Add(headerPtr, ehdr_start.phentsize)

src/runtime/os_windows.go

@@ -52,7 +52,7 @@ var module *exeHeader
 // around 160 bytes of amd64 instructions.
 // Most of this function is based on the documentation in
 // https://docs.microsoft.com/en-us/windows/win32/debug/pe-format.
-func markGlobals() {
+func findGlobals(found func(start, end uintptr)) {
 	// Constants used in this function.
 	const (
 		// https://docs.microsoft.com/en-us/windows/win32/api/libloaderapi/nf-libloaderapi-getmodulehandleexa
@@ -85,7 +85,7 @@ func markGlobals() {
 			// Found a writable section. Scan the entire section for roots.
 			start := uintptr(unsafe.Pointer(module)) + uintptr(section.virtualAddress)
 			end := uintptr(unsafe.Pointer(module)) + uintptr(section.virtualAddress) + uintptr(section.virtualSize)
-			markRoots(start, end)
+			found(start, end)
 		}
 		section = (*peSection)(unsafe.Add(unsafe.Pointer(section), unsafe.Sizeof(peSection{})))
 	}

src/runtime/runtime_nintendoswitch.go

@@ -245,17 +245,17 @@ var bssStartSymbol [0]byte
 //go:extern __bss_end
 var bssEndSymbol [0]byte
 
-// Mark global variables.
+// Find global variables.
 // The linker script provides __*_start and __*_end symbols that can be used to
 // scan the given sections. They are already aligned so don't need to be
 // manually aligned here.
-func markGlobals() {
+func findGlobals(found func(start, end uintptr)) {
 	dataStart := uintptr(unsafe.Pointer(&dataStartSymbol))
 	dataEnd := uintptr(unsafe.Pointer(&dataEndSymbol))
-	markRoots(dataStart, dataEnd)
+	found(dataStart, dataEnd)
 	bssStart := uintptr(unsafe.Pointer(&bssStartSymbol))
 	bssEnd := uintptr(unsafe.Pointer(&bssEndSymbol))
-	markRoots(bssStart, bssEnd)
+	found(bssStart, bssEnd)
 }
 
 // getContextPtr returns the hblauncher context