[llvm][ELF] Add ELF header/section header table size statistics

[aeubanks]

Followup to #102363. This makes the elf-object-writer.*Bytes stats sum up to assembler.ObjectBytes.

[llvmbot]

@llvm/pr-subscribers-backend-x86

@llvm/pr-subscribers-mc

Author: Arthur Eubanks (aeubanks)

Changes

Followup to #102363. This makes the elf-object-writer.*Bytes stats sum up to assembler.ObjectBytes.

---

Full diff: https://github.com/llvm/llvm-project/pull/109345.diff

1 Files Affected:

• (modified) llvm/lib/MC/ELFObjectWriter.cpp (+7)

diff --git a/llvm/lib/MC/ELFObjectWriter.cpp b/llvm/lib/MC/ELFObjectWriter.cpp
index 8127091ad37627..94c7697d250dac 100644
--- a/llvm/lib/MC/ELFObjectWriter.cpp
+++ b/llvm/lib/MC/ELFObjectWriter.cpp
@@ -68,6 +68,8 @@ using namespace llvm;
 namespace {
 namespace stats {
 
+STATISTIC(ELFHeaderBytes, "Total size of ELF headers");
+STATISTIC(SectionHeaderBytes, "Total size of section headers table");
 STATISTIC(AllocTextBytes, "Total size of SHF_ALLOC text sections");
 STATISTIC(AllocROBytes, "Total size of SHF_ALLOC readonly sections");
 STATISTIC(AllocRWBytes, "Total size of SHF_ALLOC read-write sections");
@@ -945,6 +947,7 @@ void ELFWriter::writeSectionHeader(uint32_t GroupSymbolIndex, uint64_t Offset,
 }
 
 void ELFWriter::writeSectionHeaders(const MCAssembler &Asm) {
+  uint64_t Start = W.OS.tell();
   const unsigned NumSections = SectionTable.size();
 
   // Null section first.
@@ -1008,6 +1011,8 @@ void ELFWriter::writeSectionHeaders(const MCAssembler &Asm) {
 
     writeSectionHeader(GroupSymbolIndex, Offsets.first, Size, *Section);
   }
+
+  stats::SectionHeaderBytes += W.OS.tell() - Start;
 }
 
 uint64_t ELFWriter::writeObject(MCAssembler &Asm) {
@@ -1023,6 +1028,8 @@ uint64_t ELFWriter::writeObject(MCAssembler &Asm) {
   // Write out the ELF header ...
   writeHeader(Asm);
 
+  stats::ELFHeaderBytes += W.OS.tell() - StartOffset;
+
   // ... then the sections ...
   SmallVector<std::pair<MCSectionELF *, SmallVector<unsigned>>, 0> Groups;
   // Map from group section index to group

[MaskRay]

Add a test?

If llvm-project ever gets https://maskray.me/blog/2024-03-31-a-compact-section-header-table-for-elf (impl:
https://github.com/MaskRay/llvm-project/tree/demo-cshdr ; which might need certain extent of agreement), this can be used to show the size difference due to -Wa,--cshdr :)

[aeubanks]

I don't really want to hardcode the value of this stat in the test, and I'm not sure how else I'd test it?

[MaskRay]

Encoding the value should be fine. It's very unlikely that the value will ever change.

[aeubanks]

done