MemoryBlock: Do not automatically extend a given size to a multiple of page size.

Previously, MemoryBlock automatically extends a requested buffer size to a
multiple of page size because (I believe) doing it was thought to be harmless
and with that you could get more memory (on average 2KiB on 4KiB-page systems)
"for free".

That programming interface turned out to be error-prone. If you request N
bytes, you usually expect that a resulting object returns N for `size()`.
That's not the case for MemoryBlock.

Looks like there is only one place where we take the advantage of
allocating more memory than the requested size. So, with this patch, I
simply removed the automatic size expansion feature from MemoryBlock
and do it on the caller side when needed. MemoryBlock now always
returns a buffer whose size is equal to the requested size.

Differential Revision: https://reviews.llvm.org/D56941

llvm-svn: 351916

1 files changed, 5 insertions, 1 deletions

diff --git a/llvm/lib/ExecutionEngine/SectionMemoryManager.cpp b/llvm/lib/ExecutionEngine/SectionMemoryManager.cpp
index 5f542f4..ba27779 100644
--- a/llvm/lib/ExecutionEngine/SectionMemoryManager.cpp
+++ b/llvm/lib/ExecutionEngine/SectionMemoryManager.cpp
@@ -242,7 +242,11 @@ public:
   allocateMappedMemory(SectionMemoryManager::AllocationPurpose Purpose,
                        size_t NumBytes, const sys::MemoryBlock *const NearBlock,
                        unsigned Flags, std::error_code &EC) override {
-    return sys::Memory::allocateMappedMemory(NumBytes, NearBlock, Flags, EC);
+    // allocateMappedMemory calls mmap(2). We round up a request size
+    // to page size to get extra space for free.
+    static const size_t PageSize = sys::Process::getPageSize();
+    size_t ReqBytes = (NumBytes + PageSize - 1) & ~(PageSize - 1);
+    return sys::Memory::allocateMappedMemory(ReqBytes, NearBlock, Flags, EC);
   }
 
   std::error_code protectMappedMemory(const sys::MemoryBlock &Block,