[llvm][ELF] Add statistics on various section sizes (#102363)

Useful with other infrastructure that consume LLVM statistics to get an
idea of distribution of section sizes.

The breakdown of various section types is subject to change, this is
just an initial go at gather some sort of stats.

Example stats compiling X86ISelLowering.cpp (-g1):

```
        "elf-object-writer.AllocROBytes": 308268,
        "elf-object-writer.AllocRWBytes": 6240,
        "elf-object-writer.AllocTextBytes": 1659203,
        "elf-object-writer.DebugBytes": 3180386,
        "elf-object-writer.OtherBytes": 5862,
        "elf-object-writer.RelocationBytes": 2623440,
        "elf-object-writer.StrtabBytes": 228599,
        "elf-object-writer.SymtabBytes": 120336,
        "elf-object-writer.UnwindBytes": 85216,
```

Author: aeubanks

llvm/lib/MC/ELFObjectWriter.cpp

@@ -14,6 +14,7 @@
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/SmallVector.h"
+#include "llvm/ADT/Statistic.h"
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/ADT/Twine.h"
@@ -62,10 +63,23 @@
 
 using namespace llvm;
 
-#undef  DEBUG_TYPE
-#define DEBUG_TYPE "reloc-info"
+#define DEBUG_TYPE "elf-object-writer"
 
 namespace {
+namespace stats {
+
+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");
+STATISTIC(StrtabBytes, "Total size of SHT_STRTAB sections");
+STATISTIC(SymtabBytes, "Total size of SHT_SYMTAB sections");
+STATISTIC(RelocationBytes, "Total size of relocation sections");
+STATISTIC(DynsymBytes, "Total size of SHT_DYNSYM sections");
+STATISTIC(DebugBytes, "Total size of debug info sections");
+STATISTIC(UnwindBytes, "Total size of unwind sections");
+STATISTIC(OtherBytes, "Total size of uncategorized sections");
+
+} // namespace stats
 
 struct ELFWriter;
 
@@ -951,6 +965,44 @@ void ELFWriter::writeSectionHeader(const MCAssembler &Asm) {
     else
       Size = Offsets.second - Offsets.first;
 
+    auto SectionHasFlag = [&](uint64_t Flag) -> bool {
+      return Section->getFlags() & Flag;
+    };
+
+    if (Section->getName().starts_with(".debug")) {
+      stats::DebugBytes += Size;
+    } else if (Section->getName().starts_with(".eh_frame")) {
+      stats::UnwindBytes += Size;
+    } else if (SectionHasFlag(ELF::SHF_ALLOC)) {
+      if (SectionHasFlag(ELF::SHF_EXECINSTR)) {
+        stats::AllocTextBytes += Size;
+      } else if (SectionHasFlag(ELF::SHF_WRITE)) {
+        stats::AllocRWBytes += Size;
+      } else {
+        stats::AllocROBytes += Size;
+      }
+    } else {
+      switch (Section->getType()) {
+      case ELF::SHT_STRTAB:
+        stats::StrtabBytes += Size;
+        break;
+      case ELF::SHT_SYMTAB:
+        stats::SymtabBytes += Size;
+        break;
+      case ELF::SHT_DYNSYM:
+        stats::DynsymBytes += Size;
+        break;
+      case ELF::SHT_REL:
+      case ELF::SHT_RELA:
+      case ELF::SHT_CREL:
+        stats::RelocationBytes += Size;
+        break;
+      default:
+        stats::OtherBytes += Size;
+        break;
+      }
+    }
+
     writeSection(GroupSymbolIndex, Offsets.first, Size, *Section);
   }
 }

llvm/test/CodeGen/X86/section-stats.ll

@@ -0,0 +1,13 @@
+; REQUIRES: asserts
+; RUN: llc -o /dev/null -filetype=obj -stats %s 2>&1 | FileCheck %s
+
+; CHECK-DAG: 1 elf-object-writer - Total size of SHF_ALLOC text sections
+; CHECK-DAG: 1 elf-object-writer - Total size of SHF_ALLOC read-write sections
+
+target triple = "x86_64-unknown-linux-gnu"
+
+@g = global i8 1
+
+define void @f() {
+    ret void
+}