[ADT][SmallVector] Flip an assert comparison to avoid overflows yielding false-negati...

r221973 changed SmallVector::operator[] to use size_t instead of unsigned.

Before that, on 64bit platforms, when a large index (say -1) was passed,
truncating it to unsigned avoided an overflow when computing 'begin() + idx',
and failed the range checking assertion, as expected.
With r221973, idx isn't truncated, so the addition wraps to
'(char*)begin() - 1', and doesn't fire anymore when it should have done so.

This commit changes the comparison to instead compute 'end() - begin()'
(i.e., 'size()'), which avoids potentially overflowing additions, and
correctly triggers the assertion when values such as -1 are passed.
Note that the problem already existed before that revision, on platforms
where sizeof(size_t) == sizeof(unsigned).

llvm-svn: 225338

diff --git a/llvm/include/llvm/ADT/SmallVector.h b/llvm/include/llvm/ADT/SmallVector.h
index 1b52a6d..44a3521 100644 (file)
--- a/llvm/include/llvm/ADT/SmallVector.h
+++ b/llvm/include/llvm/ADT/SmallVector.h
@@ -135,11 +135,11 @@ public:
   const_pointer data() const { return const_pointer(begin()); }
 
   reference operator[](size_type idx) {
-    assert(begin() + idx < end());
+    assert(idx < size());
     return begin()[idx];
   }
   const_reference operator[](size_type idx) const {
-    assert(begin() + idx < end());
+    assert(idx < size());
     return begin()[idx];
   }