diff --git a/compileopts/config.go b/compileopts/config.go
index d6979213..6e52c4ff 100644
--- a/compileopts/config.go
+++ b/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
diff --git a/src/runtime/os_darwin.go b/src/runtime/os_darwin.go
index 60110161..ac31b67d 100644
--- a/src/runtime/os_darwin.go
+++ b/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
+)
diff --git a/src/runtime/os_linux.go b/src/runtime/os_linux.go
index ab43c3a4..7613134e 100644
--- a/src/runtime/os_linux.go
+++ b/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
+)
diff --git a/src/runtime/runtime_unix.go b/src/runtime/runtime_unix.go
index 59a28682..584f0338 100644
--- a/src/runtime/runtime_unix.go
+++ b/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()
 
diff --git a/src/runtime/runtime_unix_heap.go b/src/runtime/runtime_unix_heap.go
index 98ad8075..cee1f1e1 100644
--- a/src/runtime/runtime_unix_heap.go
+++ b/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
 }