[llvm] r228879 - MC, COFF: Align section contents to a four byte boundary

[Peter Collingbourne via llvm-commits]

I think we should switch back to not aligning them. I measured the median
run time of 100 links of Chromium's base_unittests on Linux with lld-link
and on Windows with link.exe with both aligned and unaligned sections. On
Linux I didn't see a measurable performance difference, and on Windows the
link was slightly faster with unaligned sections (presumably because on
Windows the bottleneck is I/O). Also, the sections created by cl.exe are
unaligned.

Peter

On Wed, Aug 22, 2018 at 5:59 PM David Majnemer <david.majnemer at gmail.com>
wrote:

> Microsoft's documentation says (
> https://docs.microsoft.com/en-us/windows/desktop/debug/pe-format#section-table-section-headers
> ):
> "For object files, the value should be aligned on a 4-byte boundary for
> best performance."
>
> It should be legal to relax this IIRC.
>
> On Wed, Aug 22, 2018 at 5:37 PM Peter Collingbourne <peter at pcc.me.uk>
> wrote:
>
>> Hi David,
>>
>> Do you remember why you made this change?
>>
>> Peter
>>
>> > Author: majnemer
>> > Date: Wed Feb 11 16:22:30 2015
>> > New Revision: 228879
>> >
>> > URL: http://llvm.org/viewvc/llvm-project?rev=228879&view=rev
>> > Log:
>> > MC, COFF: Align section contents to a four byte boundary
>> >
>> > Modified:
>> >     llvm/trunk/lib/MC/WinCOFFObjectWriter.cpp
>> >     llvm/trunk/test/MC/COFF/directive-section-characteristics.ll
>> >
>> > Modified: llvm/trunk/lib/MC/WinCOFFObjectWriter.cpp
>> > URL:
>> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/MC/WinCOFFObjectWriter.cpp?rev=228879&r1=228878&r2=228879&view=diff
>> >
>> ==============================================================================
>> > --- llvm/trunk/lib/MC/WinCOFFObjectWriter.cpp (original)
>> > +++ llvm/trunk/lib/MC/WinCOFFObjectWriter.cpp Wed Feb 11 16:22:30 2015
>> > @@ -939,7 +939,8 @@ void WinCOFFObjectWriter::WriteObject(MC
>> >      Sec->Header.SizeOfRawData = Layout.getSectionAddressSize(&Section);
>> >
>> >      if (IsPhysicalSection(Sec)) {
>> > -      Sec->Header.PointerToRawData = offset;
>> > +      // Align the section data to a four byte boundary.
>> > +      Sec->Header.PointerToRawData = RoundUpToAlignment(offset, 4);
>> >
>> >        offset += Sec->Header.SizeOfRawData;
>> >      }
>> > @@ -1009,9 +1010,15 @@ void WinCOFFObjectWriter::WriteObject(MC
>> >          continue;
>> >
>> >        if ((*i)->Header.PointerToRawData != 0) {
>> > -        assert(OS.tell() == (*i)->Header.PointerToRawData &&
>> > +        assert(OS.tell() <= (*i)->Header.PointerToRawData &&
>> >                 "Section::PointerToRawData is insane!");
>> >
>> > +        unsigned SectionDataPadding = (*i)->Header.PointerToRawData -
>> OS.tell();
>> > +        assert(SectionDataPadding < 4 &&
>> > +               "Should only need at most three bytes of padding!");
>> > +
>> > +        WriteZeros(SectionDataPadding);
>> > +
>> >          Asm.writeSectionData(j, Layout);
>> >        }
>> >
>> >
>> > Modified: llvm/trunk/test/MC/COFF/directive-section-characteristics.ll
>> > URL:
>> http://llvm.org/viewvc/llvm-project/llvm/trunk/test/MC/COFF/directive-section-characteristics.ll?rev=228879&r1=228878&r2=228879&view=diff
>> >
>> ==============================================================================
>> > --- llvm/trunk/test/MC/COFF/directive-section-characteristics.ll
>> (original)
>> > +++ llvm/trunk/test/MC/COFF/directive-section-characteristics.ll Wed
>> Feb 11 16:22:30 2015
>> > @@ -7,7 +7,13 @@ entry:
>> >  }
>> >
>> >  ; CHECK: Section {
>> > +; CHECK:   Name: .text
>> > +; CHECK:   PointerToRawData: 0xB4
>> > +; CHECK: }
>> > +
>> > +; CHECK: Section {
>> >  ; CHECK:   Name: .drectve
>> > +; CHECK:   PointerToRawData: 0xB8
>> >  ; CHECK:   Characteristics [
>> >  ; CHECK:     IMAGE_SCN_ALIGN_1BYTES
>> >  ; CHECK:     IMAGE_SCN_LNK_INFO
>>
>> --
>> --
>> Peter
>>
>

-- 
-- 
Peter
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