[SmallVector] Reallocate if assigned memory is right after the current vector, created with capacity 0

Potential solution for
https://github.com/llvm/llvm-project/issues/57324.

Differential Revision: https://reviews.llvm.org/D132512

1 files changed, 25 insertions, 4 deletions

diff --git a/llvm/lib/Support/SmallVector.cpp b/llvm/lib/Support/SmallVector.cpp
index 062e965..f7e7e80 100644
--- a/llvm/lib/Support/SmallVector.cpp
+++ b/llvm/lib/Support/SmallVector.cpp
@@ -108,12 +108,29 @@ static size_t getNewCapacity(size_t MinSize, size_t TSize, size_t OldCapacity) {
   return std::clamp(NewCapacity, MinSize, MaxSize);
 }
 
+template <class Size_T>
+void *SmallVectorBase<Size_T>::replaceAllocation(void *NewElts, size_t TSize,
+                                                 size_t NewCapacity,
+                                                 size_t VSize) {
+  void *NewEltsReplace = llvm::safe_malloc(NewCapacity * TSize);
+  if (VSize)
+    memcpy(NewEltsReplace, NewElts, VSize * TSize);
+  free(NewElts);
+  return NewEltsReplace;
+}
+
 // Note: Moving this function into the header may cause performance regression.
 template <class Size_T>
-void *SmallVectorBase<Size_T>::mallocForGrow(size_t MinSize, size_t TSize,
+void *SmallVectorBase<Size_T>::mallocForGrow(void *FirstEl, size_t MinSize,
+                                             size_t TSize,
                                              size_t &NewCapacity) {
   NewCapacity = getNewCapacity<Size_T>(MinSize, TSize, this->capacity());
-  return llvm::safe_malloc(NewCapacity * TSize);
+  // Even if capacity is not 0 now, if the vector was originally created with
+  // capacity 0, it's possible for the malloc to return FirstEl.
+  void *NewElts = llvm::safe_malloc(NewCapacity * TSize);
+  if (NewElts == FirstEl)
+    NewElts = replaceAllocation(NewElts, TSize, NewCapacity);
+  return NewElts;
 }
 
 // Note: Moving this function into the header may cause performance regression.
@@ -123,13 +140,17 @@ void SmallVectorBase<Size_T>::grow_pod(void *FirstEl, size_t MinSize,
   size_t NewCapacity = getNewCapacity<Size_T>(MinSize, TSize, this->capacity());
   void *NewElts;
   if (BeginX == FirstEl) {
-    NewElts = safe_malloc(NewCapacity * TSize);
+    NewElts = llvm::safe_malloc(NewCapacity * TSize);
+    if (NewElts == FirstEl)
+      NewElts = replaceAllocation(NewElts, TSize, NewCapacity);
 
     // Copy the elements over.  No need to run dtors on PODs.
     memcpy(NewElts, this->BeginX, size() * TSize);
   } else {
     // If this wasn't grown from the inline copy, grow the allocated space.
-    NewElts = safe_realloc(this->BeginX, NewCapacity * TSize);
+    NewElts = llvm::safe_realloc(this->BeginX, NewCapacity * TSize);
+    if (NewElts == FirstEl)
+      NewElts = replaceAllocation(NewElts, TSize, NewCapacity, size());
   }
 
   this->BeginX = NewElts;