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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
This commit is contained in:
Rui Ueyama 2019-01-23 02:03:26 +00:00
parent 02d85149a0
commit 7c8fc8142e
3 changed files with 14 additions and 9 deletions
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7
lld/test/ELF/stdout.s
View File

@ -1,12 +1,15 @@
# REQUIRES: x86
# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t.o
# RUN: ld.lld %t.o -o - > %t
# RUN: llvm-objdump -d %t | FileCheck %s
# RUN: ld.lld %t.o -o - > %t1
# RUN: llvm-objdump -d %t1 | FileCheck %s
# CHECK: 0000000000201000 _start:
# CHECK: 201000: 90 nop
# RUN: ld.lld %t.o -o %t2
# RUN: diff %t1 %t2
.globl _start
_start:
nop

6
llvm/lib/ExecutionEngine/SectionMemoryManager.cpp
View File

@ -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,

10
llvm/lib/Support/Unix/Memory.inc
View File

@ -91,9 +91,6 @@ Memory::allocateMappedMemory(size_t NumBytes,
if (NumBytes == 0)
return MemoryBlock();
static const size_t PageSize = Process::getPageSize();
const size_t NumPages = (NumBytes+PageSize-1)/PageSize;
int fd = -1;
int MMFlags = MAP_PRIVATE |
@ -113,11 +110,12 @@ Memory::allocateMappedMemory(size_t NumBytes,
// Use any near hint and the page size to set a page-aligned starting address
uintptr_t Start = NearBlock ? reinterpret_cast<uintptr_t>(NearBlock->base()) +
NearBlock->size() : 0;
static const size_t PageSize = Process::getPageSize();
if (Start && Start % PageSize)
Start += PageSize - Start % PageSize;
void *Addr = ::mmap(reinterpret_cast<void*>(Start), PageSize*NumPages,
Protect, MMFlags, fd, 0);
void *Addr = ::mmap(reinterpret_cast<void *>(Start), NumBytes, Protect,
MMFlags, fd, 0);
if (Addr == MAP_FAILED) {
if (NearBlock) //Try again without a near hint
return allocateMappedMemory(NumBytes, nullptr, PFlags, EC);
@ -128,7 +126,7 @@ Memory::allocateMappedMemory(size_t NumBytes,
MemoryBlock Result;
Result.Address = Addr;
Result.Size = NumPages*PageSize;
Result.Size = NumBytes;
// Rely on protectMappedMemory to invalidate instruction cache.
if (PFlags & MF_EXEC) {