[wasm] Fix bounds check for zero initial memory.

src/compiler/wasm-compiler.cc

Index: src/compiler/wasm-compiler.cc
diff --git a/src/compiler/wasm-compiler.cc b/src/compiler/wasm-compiler.cc
index 4392896c644b8796b013903a81c22a5804f71e5b..025f3ef37aedd480c157560ce1bea42d36657aa0 100644
--- a/src/compiler/wasm-compiler.cc
+++ b/src/compiler/wasm-compiler.cc
@@ -2861,34 +2861,30 @@ void WasmGraphBuilder::BoundsCheckMem(MachineType memtype, Node* index,
 
   // Check against the effective size.
   size_t effective_size;

[bradn, 2016/10/14 01:42:38]

Move down to the initialization.

[gdeepti, 2016/10/14 01:57:10]

As the check below is gated by if (size > offset + memsize -1), the declaration
here now makes sense. 

-  if (size == 0) {
-    effective_size = 0;
-  } else if (offset >= size ||
-             (static_cast<uint64_t>(offset) + memsize) > size) {
+  if (size <= offset || size < (static_cast<uint64_t>(offset) + memsize)) {
     // Two checks are needed in the case where the offset is statically
     // out of bounds; one check for the offset being in bounds, and the next for
     // the offset + index being out of bounds for code to be patched correctly
     // on relocation.
-    effective_size = size - memsize + 1;
+    size_t effective_offset = offset + memsize - 1;
     Node* cond = graph()->NewNode(jsgraph()->machine()->Uint32LessThan(),
-                                  jsgraph()->IntPtrConstant(offset),
+                                  jsgraph()->IntPtrConstant(effective_offset),
                                   jsgraph()->RelocatableInt32Constant(
-                                      static_cast<uint32_t>(effective_size),
+                                      static_cast<uint32_t>(size),
                                       RelocInfo::WASM_MEMORY_SIZE_REFERENCE));
     trap_->AddTrapIfFalse(wasm::kTrapMemOutOfBounds, cond, position);
-    DCHECK(offset >= effective_size);
-    effective_size = offset - effective_size;
-  } else {
-    effective_size = size - offset - memsize + 1;
-    CHECK(effective_size <= kMaxUInt32);
-
-    Uint32Matcher m(index);
-    if (m.HasValue()) {
-      uint32_t value = m.Value();
-      if (value < effective_size) {
-        // The bounds check will always succeed.
-        return;
-      }
+  }
+  // For offset > effective size, this relies on check above to fail when it
+  // wraps around.
+  effective_size = size - offset - memsize + 1;
+  CHECK(effective_size <= kMaxUInt32);

[bradn, 2016/10/14 01:42:38]

This check won't work now (it could be negative).
You need to wrap this and the below matcher check in an:
if (size > offset + memsize -1)
Maybe also clarify that effective_size can be negative in the comment?

[gdeepti, 2016/10/14 01:57:10]

Done.

+
+  Uint32Matcher m(index);
+  if (m.HasValue()) {
+    uint32_t value = m.Value();
+    if (value < effective_size) {
+      // The bounds check will always succeed.
+      return;
     }
   }