all: add the Boehm-Demers-Weiser GC on Linux

This adds support for the well-known Boehm GC. It's significantly faster
than our own naive GC and could be used as an alternative on bigger
systems.

In the future, this GC might also be supported on WebAssembly with some
extra work. Right now it's Linux only (though Windows/MacOS shouldn't be
too difficult to add).

builder/bdwgc.go

@@ -0,0 +1,71 @@
+package builder
+
+// The well-known conservative Boehm-Demers-Weiser GC.
+// This file provides a way to compile this GC for use with TinyGo.
+
+import (
+	"path/filepath"
+
+	"github.com/tinygo-org/tinygo/goenv"
+)
+
+var BoehmGC = Library{
+	name: "bdwgc",
+	cflags: func(target, headerPath string) []string {
+		libdir := filepath.Join(goenv.Get("TINYGOROOT"), "lib/bdwgc")
+		return []string{
+			// use a modern environment
+			"-DUSE_MMAP",              // mmap is available
+			"-DUSE_MUNMAP",            // return memory to the OS using munmap
+			"-DGC_BUILTIN_ATOMIC",     // use compiler intrinsics for atomic operations
+			"-DNO_EXECUTE_PERMISSION", // don't make the heap executable
+
+			// specific flags for TinyGo
+			"-DALL_INTERIOR_POINTERS",  // scan interior pointers (needed for Go)
+			"-DIGNORE_DYNAMIC_LOADING", // we don't support dynamic loading at the moment
+			"-DNO_GETCONTEXT",          // musl doesn't support getcontext()
+
+			// Special flag to work around the lack of __data_start in ld.lld.
+			// TODO: try to fix this in LLVM/lld directly so we don't have to
+			// work around it anymore.
+			"-DGC_DONT_REGISTER_MAIN_STATIC_DATA",
+
+			// Do not scan the stack. We have our own mechanism to do this.
+			"-DSTACK_NOT_SCANNED",
+
+			// Assertions can be enabled while debugging GC issues.
+			//"-DGC_ASSERTIONS",
+
+			// Threading is not yet supported, so these are disabled.
+			//"-DGC_THREADS",
+			//"-DTHREAD_LOCAL_ALLOC",
+
+			"-I" + libdir + "/include",
+		}
+	},
+	sourceDir: func() string {
+		return filepath.Join(goenv.Get("TINYGOROOT"), "lib/bdwgc")
+	},
+	librarySources: func(target string) ([]string, error) {
+		return []string{
+			"allchblk.c",
+			"alloc.c",
+			"blacklst.c",
+			"dbg_mlc.c",
+			"dyn_load.c",
+			"finalize.c",
+			"headers.c",
+			"mach_dep.c",
+			"malloc.c",
+			"mark.c",
+			"mark_rts.c",
+			"misc.c",
+			"new_hblk.c",
+			"obj_map.c",
+			"os_dep.c",
+			"pthread_stop_world.c",
+			"pthread_support.c",
+			"reclaim.c",
+		}, nil
+	},
+}