MC: Don't align COFF section contents.

[pcc]

Summary

Aligning section contents is not required, but only
recommended, by the specification. 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."

However, according to my measurements, aligning section contents has
a neutral to negative effect on performance.

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, so we should expect
tools to broadly accept unaligned sections.

[majnemer]

LGTM

llvm/lib/MC/WinCOFFObjectWriter.cpp

void WinCOFFObjectWriter::writeSection(MCAssembler &Asm,
                                        const MCAsmLayout &Layout,
                                        const COFFSection &Sec,
                                        const MCSection &MCSec) {
   if (Sec.Number == -1)
     return;
 
   // Write the section contents.
   if (Sec.Header.PointerToRawData != 0) {
-    assert(W.OS.tell() <= Sec.Header.PointerToRawData &&
+    assert(W.OS.tell() == Sec.Header.PointerToRawData &&
            "Section::PointerToRawData is insane!");
 
-    unsigned PaddingSize = Sec.Header.PointerToRawData - W.OS.tell();
-    assert(PaddingSize < 4 &&
-           "Should only need at most three bytes of padding!");
-    W.OS.write_zeros(PaddingSize);
-
     uint32_t CRC = writeSectionContents(Asm, Layout, MCSec);
 
     // Update the section definition auxiliary symbol to record the CRC.
     COFFSection *Sec = SectionMap[&MCSec];
     COFFSymbol::AuxiliarySymbols &AuxSyms = Sec->Symbol->Aux;
     assert(AuxSyms.size() == 1 && AuxSyms[0].AuxType == ATSectionDefinition);
     AuxSymbol &SecDef = AuxSyms[0];
     SecDef.Aux.SectionDefinition.CheckSum = CRC;
for (const auto &Section : Asm) {
     COFFSection *Sec = SectionMap[&Section];
 
     if (Sec->Number == -1)
       continue;
 
     Sec->Header.SizeOfRawData = Layout.getSectionAddressSize(&Section);
 
     if (IsPhysicalSection(Sec)) {
-      // Align the section data to a four byte boundary.
-      Offset = alignTo(Offset, 4);
       Sec->Header.PointerToRawData = Offset;
-
       Offset += Sec->Header.SizeOfRawData;
     }
 
     if (!Sec->Relocations.empty()) {
       bool RelocationsOverflow = Sec->Relocations.size() >= 0xffff;
 
       if (RelocationsOverflow) {
         // Signal overflow by setting NumberOfRelocations to max value. Actual

llvm/test/MC/COFF/directive-section-characteristics.ll

 ; RUN: llc -mtriple i686-windows -filetype obj -o - %s | llvm-readobj -sections \
 ; RUN:    | FileCheck %s
 
 define dllexport void @function() {
 entry:
   ret void
 }
 
 ; CHECK: Section {
 ; CHECK:   Name: .text
 ; CHECK:   PointerToRawData: 0xB4
 ; CHECK: }
 
 ; CHECK: Section {
 ; CHECK:   Name: .drectve
-; CHECK:   PointerToRawData: 0xB8
+; CHECK:   PointerToRawData: 0xB5
 ; CHECK:   Characteristics [
 ; CHECK:     IMAGE_SCN_ALIGN_1BYTES
 ; CHECK:     IMAGE_SCN_LNK_INFO
 ; CHECK:     IMAGE_SCN_LNK_REMOVE
 ; CHECK:   ]
 ; CHECK: }