Fix WASM for LLVM6

wee_alloc/src/imp_wasm32.rs

@@ -4,25 +4,15 @@ use core::cell::UnsafeCell;
 use units::Pages;
 
 extern "C" {
-    #[link_name = "llvm.wasm.current.memory.i32"]
-    fn current_memory() -> usize;
-
-    // TODO: this intrinsic actually returns the previous limit, but LLVM
-    // doesn't expose that right now. When we upgrade LLVM stop using
-    // `current_memory` above. Also handle `-1` as an allocation failure.
     #[link_name = "llvm.wasm.grow.memory.i32"]
-    fn grow_memory(pages: usize);
-}
-
-unsafe fn get_base_pointer() -> *mut u8 {
-    (current_memory() * PAGE_SIZE.0) as _
+    fn grow_memory(pages: usize) -> i32;
 }
 
 pub(crate) unsafe fn alloc_pages(n: Pages) -> *mut u8 {
-    let ptr = get_base_pointer();
+    let ptr = grow_memory(n.0);
+    extra_assert!(ptr != -1);
+    let ptr = (ptr as usize * PAGE_SIZE.0) as _;
     assert_is_word_aligned(ptr);
-    extra_assert!(!ptr.is_null());
-    grow_memory(n.0);
     ptr
 }