[lld] r244691 - COFF: Align sections to 512-byte boundaries on disk.
Sean Silva via llvm-commits
llvm-commits at lists.llvm.org
Wed Aug 12 01:31:09 PDT 2015
On Tue, Aug 11, 2015 at 4:09 PM, Rui Ueyama via llvm-commits <
llvm-commits at lists.llvm.org> wrote:
> Author: ruiu
> Date: Tue Aug 11 18:09:00 2015
> New Revision: 244691
>
> URL: http://llvm.org/viewvc/llvm-project?rev=244691&view=rev
> Log:
> COFF: Align sections to 512-byte boundaries on disk.
>
> Sections must start at page boundaries in memory, but they
> can be aligned to sector boundaries (512-bytes) on disk.
> We aligned them to 4096-byte boundaries even on disk, so we
> wasted disk space a bit.
>
This will likely force the kernel to copy or otherwise do unnecessary work
when loading. Are you sure that isn't happening? The kernel ideally wants
to just create a couple page table entries. But if it needs to move things
around at <4K granularity to make them properly aligned to their load
address when loading (like this patch I think causes) then it will need to
do copies.
This can likely be checked by looking for an increase in real memory usage
for the system when the new binaries are loaded (vs. the old page-aligned
ones), since the kernel will have a copy sitting in page cache and a copy
for alignment mapped into the process address space; alternatively, you can
check for the slowdown from the kernel copies when faulting the memory into
the process's address space (or (less likely) it may do the copies eagerly
which should be easy to measure too).
-- Sean Silva
>
> Modified:
> lld/trunk/COFF/Writer.cpp
> lld/trunk/test/COFF/baserel.test
> lld/trunk/test/COFF/hello32.test
>
> Modified: lld/trunk/COFF/Writer.cpp
> URL:
> http://llvm.org/viewvc/llvm-project/lld/trunk/COFF/Writer.cpp?rev=244691&r1=244690&r2=244691&view=diff
>
> ==============================================================================
> --- lld/trunk/COFF/Writer.cpp (original)
> +++ lld/trunk/COFF/Writer.cpp Tue Aug 11 18:09:00 2015
> @@ -37,8 +37,7 @@ using namespace lld;
> using namespace lld::coff;
>
> static const int PageSize = 4096;
> -static const int FileAlignment = 512;
> -static const int SectionAlignment = 4096;
> +static const int SectorSize = 512;
> static const int DOSStubSize = 64;
> static const int NumberfOfDataDirectory = 16;
>
> @@ -174,7 +173,7 @@ void OutputSection::addChunk(Chunk *C) {
> Off += C->getSize();
> Header.VirtualSize = Off;
> if (C->hasData())
> - Header.SizeOfRawData = RoundUpToAlignment(Off, FileAlignment);
> + Header.SizeOfRawData = RoundUpToAlignment(Off, SectorSize);
> }
>
> void OutputSection::addPermissions(uint32_t C) {
> @@ -507,15 +506,14 @@ void Writer::createSymbolAndStringTable(
> // We position the symbol table to be adjacent to the end of the last
> section.
> uint64_t FileOff =
> LastSection->getFileOff() +
> - RoundUpToAlignment(LastSection->getRawSize(), FileAlignment);
> + RoundUpToAlignment(LastSection->getRawSize(), SectorSize);
> if (!OutputSymtab.empty()) {
> PointerToSymbolTable = FileOff;
> FileOff += OutputSymtab.size() * sizeof(coff_symbol16);
> }
> if (!Strtab.empty())
> FileOff += Strtab.size() + 4;
> - FileSize = SizeOfHeaders +
> - RoundUpToAlignment(FileOff - SizeOfHeaders, FileAlignment);
> + FileSize = RoundUpToAlignment(FileOff, SectorSize);
> }
>
> // Visits all sections to assign incremental, non-overlapping RVAs and
> @@ -526,9 +524,9 @@ void Writer::assignAddresses() {
> sizeof(coff_section) * OutputSections.size();
> SizeOfHeaders +=
> Config->is64() ? sizeof(pe32plus_header) : sizeof(pe32_header);
> - SizeOfHeaders = RoundUpToAlignment(SizeOfHeaders, PageSize);
> + SizeOfHeaders = RoundUpToAlignment(SizeOfHeaders, SectorSize);
> uint64_t RVA = 0x1000; // The first page is kept unmapped.
> - uint64_t FileOff = SizeOfHeaders;
> + FileSize = SizeOfHeaders;
> // Move DISCARDABLE (or non-memory-mapped) sections to the end of file
> because
> // the loader cannot handle holes.
> std::stable_partition(
> @@ -539,13 +537,11 @@ void Writer::assignAddresses() {
> if (Sec->getName() == ".reloc")
> addBaserels(Sec);
> Sec->setRVA(RVA);
> - Sec->setFileOffset(FileOff);
> + Sec->setFileOffset(FileSize);
> RVA += RoundUpToAlignment(Sec->getVirtualSize(), PageSize);
> - FileOff += RoundUpToAlignment(Sec->getRawSize(), FileAlignment);
> + FileSize += RoundUpToAlignment(Sec->getRawSize(), SectorSize);
> }
> SizeOfImage = SizeOfHeaders + RoundUpToAlignment(RVA - 0x1000,
> PageSize);
> - FileSize = SizeOfHeaders +
> - RoundUpToAlignment(FileOff - SizeOfHeaders, FileAlignment);
> }
>
> template <typename PEHeaderTy> void Writer::writeHeader() {
> @@ -584,8 +580,8 @@ template <typename PEHeaderTy> void Writ
> Buf += sizeof(*PE);
> PE->Magic = Config->is64() ? PE32Header::PE32_PLUS : PE32Header::PE32;
> PE->ImageBase = Config->ImageBase;
> - PE->SectionAlignment = SectionAlignment;
> - PE->FileAlignment = FileAlignment;
> + PE->SectionAlignment = PageSize;
> + PE->FileAlignment = SectorSize;
> PE->MajorImageVersion = Config->MajorImageVersion;
> PE->MinorImageVersion = Config->MinorImageVersion;
> PE->MajorOperatingSystemVersion = Config->MajorOSVersion;
>
> Modified: lld/trunk/test/COFF/baserel.test
> URL:
> http://llvm.org/viewvc/llvm-project/lld/trunk/test/COFF/baserel.test?rev=244691&r1=244690&r2=244691&view=diff
>
> ==============================================================================
> --- lld/trunk/test/COFF/baserel.test (original)
> +++ lld/trunk/test/COFF/baserel.test Tue Aug 11 18:09:00 2015
> @@ -61,7 +61,7 @@
> # BASEREL-HEADER-NEXT: VirtualSize: 0x20
> # BASEREL-HEADER-NEXT: VirtualAddress: 0x5000
> # BASEREL-HEADER-NEXT: RawDataSize: 512
> -# BASEREL-HEADER-NEXT: PointerToRawData: 0x1800
> +# BASEREL-HEADER-NEXT: PointerToRawData: 0xC00
> # BASEREL-HEADER-NEXT: PointerToRelocations: 0x0
> # BASEREL-HEADER-NEXT: PointerToLineNumbers: 0x0
> # BASEREL-HEADER-NEXT: RelocationCount: 0
>
> Modified: lld/trunk/test/COFF/hello32.test
> URL:
> http://llvm.org/viewvc/llvm-project/lld/trunk/test/COFF/hello32.test?rev=244691&r1=244690&r2=244691&view=diff
>
> ==============================================================================
> --- lld/trunk/test/COFF/hello32.test (original)
> +++ lld/trunk/test/COFF/hello32.test Tue Aug 11 18:09:00 2015
> @@ -38,8 +38,8 @@ HEADER-NEXT: MajorImageVersion: 0
> HEADER-NEXT: MinorImageVersion: 0
> HEADER-NEXT: MajorSubsystemVersion: 6
> HEADER-NEXT: MinorSubsystemVersion: 0
> -HEADER-NEXT: SizeOfImage: 20480
> -HEADER-NEXT: SizeOfHeaders: 4096
> +HEADER-NEXT: SizeOfImage: 16896
> +HEADER-NEXT: SizeOfHeaders: 512
> HEADER-NEXT: Subsystem: IMAGE_SUBSYSTEM_WINDOWS_CUI (0x3)
> HEADER-NEXT: Characteristics [ (0x8140)
> HEADER-NEXT: IMAGE_DLL_CHARACTERISTICS_DYNAMIC_BASE (0x40)
>
>
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