wasm: fix C realloc and optimize it a bit

  - Do not use make([]byte, ...) to allocate, instead allocate a slice
    of pointers. This makes sure the precise GC will scan the contents
    of the allocation, since C could very well put pointers in there.
  - Simplify the map to use the pointer as the key and the size as the
    value, instead of storing the slices directly in the map.

Author: aykevl

src/runtime/arch_tinygowasm_malloc.go

@@ -8,16 +8,19 @@ import "unsafe"
 // code linked from other languages can allocate memory without colliding with
 // our GC allocations.
 
-var allocs = make(map[uintptr][]byte)
+// Map of allocations, where the key is the allocated pointer and the value is
+// the size of the allocation.
+var allocs = make(map[unsafe.Pointer]uintptr)
 
 //export malloc
 func libc_malloc(size uintptr) unsafe.Pointer {
 	if size == 0 {
 		return nil
 	}
-	buf := make([]byte, size)
+	const wordSize = unsafe.Sizeof(unsafe.Pointer(nil))
+	buf := make([]unsafe.Pointer, (size+wordSize-1)/wordSize)
 	ptr := unsafe.Pointer(&buf[0])
-	allocs[uintptr(ptr)] = buf
+	allocs[ptr] = size
 	return ptr
 }
 
@@ -26,8 +29,8 @@ func libc_free(ptr unsafe.Pointer) {
 	if ptr == nil {
 		return
 	}
-	if _, ok := allocs[uintptr(ptr)]; ok {
-		delete(allocs, uintptr(ptr))
+	if _, ok := allocs[ptr]; ok {
+		delete(allocs, ptr)
 	} else {
 		panic("free: invalid pointer")
 	}
@@ -48,18 +51,22 @@ func libc_realloc(oldPtr unsafe.Pointer, size uintptr) unsafe.Pointer {
 
 	// It's hard to optimize this to expand the current buffer with our GC, but
 	// it is theoretically possible. For now, just always allocate fresh.
-	buf := make([]byte, size)
+	// TODO: we could skip this if the new allocation is smaller than the old.
+	const wordSize = unsafe.Sizeof(unsafe.Pointer(nil))
+	buf := make([]unsafe.Pointer, (size+wordSize-1)/wordSize)
+	ptr := unsafe.Pointer(&buf[0])
 
 	if oldPtr != nil {
-		if oldBuf, ok := allocs[uintptr(oldPtr)]; ok {
-			copy(buf, oldBuf)
-			delete(allocs, uintptr(oldPtr))
+		if oldSize, ok := allocs[oldPtr]; ok {
+			oldBuf := unsafe.Slice((*byte)(oldPtr), oldSize)
+			newBuf := unsafe.Slice((*byte)(ptr), size)
+			copy(newBuf, oldBuf)
+			delete(allocs, oldPtr)
 		} else {
 			panic("realloc: invalid pointer")
 		}
 	}
 
-	ptr := unsafe.Pointer(&buf[0])
-	allocs[uintptr(ptr)] = buf
+	allocs[ptr] = size
 	return ptr
 }