[PATCH] D51317: Keep BumpPtrAllocator::Allocate(0) from returning nullptr

[Brent Royal-Gordon via Phabricator via llvm-commits]

brentdax updated this revision to Diff 162910.
brentdax added a comment.

Reimplemented to return a unique address for each zero-size allocation.


Repository:
  rL LLVM

https://reviews.llvm.org/D51317

Files:
  include/llvm/Support/Allocator.h
  unittests/Support/AllocatorTest.cpp


Index: unittests/Support/AllocatorTest.cpp
===================================================================
--- unittests/Support/AllocatorTest.cpp
+++ unittests/Support/AllocatorTest.cpp
@@ -185,4 +185,29 @@
   EXPECT_GT(MockSlabAllocator::GetLastSlabSize(), 4096u);
 }
 
+// Test that Allocate() behaves properly when asked to allocate zero bytes.
+// 
+// Optimizations disabled so the compiler does not assume the pointer cannot
+// be null because of Allocate()'s LLVM_ATTRIBUTE_RETURNS_NONNULL attribute.
+#pragma clang optimize off
+TEST(AllocatorTest, EmptyAllocation) {
+  BumpPtrAllocator Alloc;
+
+  // Doesn't return null for an initial zero-size allocation.
+  void *a = Alloc.Allocate(0, 1);
+  EXPECT_NE(a, nullptr);
+
+  // Doesn't return the same pointer for two different allocations.
+  void *b = Alloc.Allocate(0, 1);
+  EXPECT_NE(b, a);
+
+  // Doesn't waste more space than necessary to get unique addresses.
+  EXPECT_EQ((char*)b - (char*)a, 1);
+
+  // Zero-size allocation pointers are aligned.
+  void * c = Alloc.Allocate(0, 8);
+  EXPECT_EQ(alignmentAdjustment(c, 8), 0);
+}
+#pragma clang optimize on
+
 }  // anonymous namespace
Index: include/llvm/Support/Allocator.h
===================================================================
--- include/llvm/Support/Allocator.h
+++ include/llvm/Support/Allocator.h
@@ -213,6 +213,7 @@
   LLVM_ATTRIBUTE_RETURNS_NONNULL LLVM_ATTRIBUTE_RETURNS_NOALIAS void *
   Allocate(size_t Size, size_t Alignment) {
     assert(Alignment > 0 && "0-byte alignnment is not allowed. Use 1 instead.");
+    Size = std::max(size_t(1), Size);
 
     // Keep track of how many bytes we've allocated.
     BytesAllocated += Size;


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