unix: use conservative GC by default

This commit does two things:

 1. It makes it possible to grow the heap on Linux and MacOS by
    allocating 1GB of virtual memory on startup and then slowly using it
    as necessary, when running out of available heap space.
 2. It switches the default GC to be the conservative GC (previously
    extalloc). This is good for consistency with other platforms that
    all use this same GC.

This makes the extalloc GC unused by default.

compileopts/config.go

@@ -80,12 +80,7 @@ func (c *Config) GC() string {
 	if c.Target.GC != "" {
 		return c.Target.GC
 	}
-	for _, tag := range c.Target.BuildTags {
-		if tag == "baremetal" || tag == "wasm" {
-			return "conservative"
-		}
-	}
-	return "extalloc"
+	return "conservative"
 }
 
 // NeedsStackObjects returns true if the compiler should insert stack objects

src/runtime/os_darwin.go

@@ -3,3 +3,11 @@
 package runtime
 
 const GOOS = "darwin"
+
+const (
+	// See https://github.com/golang/go/blob/master/src/syscall/zerrors_darwin_amd64.go
+	flag_PROT_READ     = 0x1
+	flag_PROT_WRITE    = 0x2
+	flag_MAP_PRIVATE   = 0x2
+	flag_MAP_ANONYMOUS = 0x1000 // MAP_ANON
+)

src/runtime/os_linux.go

@@ -3,3 +3,11 @@
 package runtime
 
 const GOOS = "linux"
+
+const (
+	// See https://github.com/torvalds/linux/blob/master/include/uapi/asm-generic/mman-common.h
+	flag_PROT_READ     = 0x1
+	flag_PROT_WRITE    = 0x2
+	flag_MAP_PRIVATE   = 0x2
+	flag_MAP_ANONYMOUS = 0x20
+)

src/runtime/runtime_unix.go

@@ -16,6 +16,9 @@ func usleep(usec uint) int
 //export malloc
 func malloc(size uintptr) unsafe.Pointer
 
+//export mmap
+func mmap(addr unsafe.Pointer, length, prot, flags, fd int, offset int) unsafe.Pointer
+
 //export abort
 func abort()
 

src/runtime/runtime_unix_heap.go

@@ -5,20 +5,33 @@
 
 package runtime
 
-const heapSize = 1 * 1024 * 1024 // 1MB to start
+var heapSize uintptr = 128 * 1024          // small amount to start
+const heapMaxSize = 1 * 1024 * 1024 * 1024 // 1GB for the entire heap
 
 var heapStart, heapEnd uintptr
 
 func preinit() {
-	heapStart = uintptr(malloc(heapSize))
+	// Allocate a large chunk of virtual memory. Because it is virtual, it won't
+	// really be allocated in RAM. Memory will only be allocated when it is
+	// first touched.
+	addr := mmap(nil, heapMaxSize, flag_PROT_READ|flag_PROT_WRITE, flag_MAP_PRIVATE|flag_MAP_ANONYMOUS, -1, 0)
+	heapStart = uintptr(addr)
 	heapEnd = heapStart + heapSize
 }
 
 // growHeap tries to grow the heap size. It returns true if it succeeds, false
 // otherwise.
 func growHeap() bool {
-	// At the moment, this is not possible. However it shouldn't be too
-	// difficult (at least on Linux) to allocate a large amount of virtual
-	// memory at startup that is then slowly used.
-	return false
+	if heapSize == heapMaxSize {
+		// Already at the max. If we run out of memory, we should consider
+		// increasing heapMaxSize on 64-bit systems.
+		return false
+	}
+	// Grow the heap size used by the program.
+	heapSize = (heapSize * 4 / 3) &^ 4095 // grow by around 33%
+	if heapSize > heapMaxSize {
+		heapSize = heapMaxSize
+	}
+	setHeapEnd(heapStart + heapSize)
+	return true
 }