gc: drop support for 'precise' globals

Precise globals require a whole program optimization pass that is hard
to support when building packages separately. This patch removes support
for these globals by converting the last use (Linux) to use
linker-defined symbols instead.

For details, see: https://github.com/tinygo-org/tinygo/issues/2870

src/runtime/gc_globals.go

@@ -4,6 +4,9 @@
 
 package runtime
 
+// This file implements markGlobals for all the files that don't have a more
+// specific implementation.
+
 // markGlobals marks all globals, which are reachable by definition.
 //
 // This implementation marks all globals conservatively and assumes it can use

src/runtime/gc_globals_precise.go

@@ -1,35 +0,0 @@
-//go:build gc.conservative && !baremetal && !darwin && !nintendoswitch && !tinygo.wasm && !windows
-// +build gc.conservative,!baremetal,!darwin,!nintendoswitch,!tinygo.wasm,!windows
-
-package runtime
-
-import (
-	"unsafe"
-)
-
-//go:extern runtime.trackedGlobalsStart
-var trackedGlobalsStart uintptr
-
-//go:extern runtime.trackedGlobalsLength
-var trackedGlobalsLength uintptr
-
-//go:extern runtime.trackedGlobalsBitmap
-var trackedGlobalsBitmap [0]uint8
-
-// markGlobals marks all globals, which are reachable by definition.
-//
-// This implementation relies on a compiler pass that stores all globals in a
-// single global (adjusting all uses of them accordingly) and creates a bit
-// vector with the locations of each pointer. This implementation then walks the
-// bit vector and for each pointer it indicates, it marks the root.
-//
-//go:nobounds
-func markGlobals() {
-	for i := uintptr(0); i < trackedGlobalsLength; i++ {
-		if trackedGlobalsBitmap[i/8]&(1<<(i%8)) != 0 {
-			addr := trackedGlobalsStart + i*unsafe.Alignof(uintptr(0))
-			root := *(*uintptr)(unsafe.Pointer(addr))
-			markRoot(addr, root)
-		}
-	}
-}

src/runtime/os_linux.go

@@ -1,8 +1,13 @@
-//go:build linux
-// +build linux
+//go:build linux && !baremetal && !nintendoswitch && !wasi
+// +build linux,!baremetal,!nintendoswitch,!wasi
 
 package runtime
 
+// This file is for systems that are _actually_ Linux (not systems that pretend
+// to be Linux, like baremetal systems).
+
+import "unsafe"
+
 const GOOS = "linux"
 
 const (
@@ -18,3 +23,20 @@ const (
 	clock_REALTIME      = 0
 	clock_MONOTONIC_RAW = 4
 )
+
+//go:extern _edata
+var globalsStartSymbol [0]byte
+
+//go:extern _end
+var globalsEndSymbol [0]byte
+
+// markGlobals marks all globals, which are reachable by definition.
+//
+// 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() {
+	start := uintptr(unsafe.Pointer(&globalsStartSymbol))
+	end := uintptr(unsafe.Pointer(&globalsEndSymbol))
+	start = (start + unsafe.Alignof(uintptr(0)) - 1) &^ (unsafe.Alignof(uintptr(0)) - 1) // align on word boundary
+	markRoots(start, end)
+}

src/runtime/os_other.go

@@ -0,0 +1,11 @@
+//go:build linux && (baremetal || nintendoswitch || wasi)
+// +build linux
+// +build baremetal nintendoswitch wasi
+
+// Other systems that aren't operating systems supported by the Go toolchain
+// need to pretend to be an existing operating system. Linux seems like a good
+// choice for this for its wide hardware support.
+
+package runtime
+
+const GOOS = "linux"