From 7c8fc8142e990b862333ea62d446618f249f70e3 Mon Sep 17 00:00:00 2001 From: Rui Ueyama Date: Wed, 23 Jan 2019 02:03:26 +0000 Subject: 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 --- llvm/lib/ExecutionEngine/SectionMemoryManager.cpp | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-) (limited to 'llvm/lib/ExecutionEngine/SectionMemoryManager.cpp') 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, -- cgit v1.1