[PDB] Do not record PGO or coverage public symbols

[rnk]

Summary

These symbols are long, and they tend to cause the PDB file size to
overflow. They are generally not necessary when debugging problems in
user code.

This change reduces the size of chrome.dll.pdb with coverage from
6,937,108,480 bytes to 4,690,210,816 bytes.

[thakis]

Nice!

lld/COFF/PDB.cpp

void PDBLinker::addPublicsToPDB() {
   ScopedTimer t3(publicsLayoutTimer);
   // Compute the public symbols.
   auto &gsiBuilder = builder.getGsiBuilder();
   std::vector<pdb::BulkPublic> publics;
   symtab->forEachSymbol([&publics](Symbol *s) {
     // Only emit external, defined, live symbols that have a chunk. Static,
     // non-external symbols do not appear in the symbol table.
     auto *def = dyn_cast<Defined>(s);
-    if (def && def->isLive() && def->getChunk())
+    if (def && def->isLive() && def->getChunk()) {
+      // Don't emit a public symbol for coverage data symbols. LLVM code
+      // coverage (and PGO) create a __profd_ and __profc_ symbol for every
+      // function. C++ mangled names are long, and tend to dominate symbol size.
+      // Including these names triples the size of the public stream, which
+      // results in bloated PDB files. These symbols generally are not helpful
+      // for debugging, so suppress them.
+      StringRef name = def->getName();
+      if (name.data()[0] == '_' && name.data()[1] == '_') {
+        // Drop the '_' prefix for x86.
+        if (config->machine == I386)
+          name = name.drop_front(1);
+        if (name.startswith("__profd_") || name.startswith("__profc_") ||
+            name.startswith("__covrec_")) {
+          return;
+        }
+      }
       publics.push_back(createPublic(def));
+    }
   });
 
   if (!publics.empty()) {
     publicSymbols = publics.size();
     gsiBuilder.addPublicSymbols(std::move(publics));
   }
 }
 

lld/test/COFF/pgo-pubs.s

+# REQUIRES: x86
+# RUN: llvm-mc -triple=x86_64-windows-msvc -filetype=obj -o %t.obj %s
+# RUN: lld-link %t.obj -out:%t.exe -pdb:%t.pdb -debug -entry:main -subsystem:console
+# RUN: llvm-pdbutil dump -publics %t.pdb | FileCheck %s
+
+# Check that there are no __prof[dc] or __covrec public symbols.
+
+# CHECK-NOT: __profd
+# CHECK-NOT: __profc
+# CHECK-NOT: __covrec
+# CHECK: S_PUB32 {{.*}} `main`
+# CHECK-NOT: __profd
+# CHECK-NOT: __profc
+# CHECK-NOT: __covrec
+
+
+# The following assembly is simplified from this C code:
+# int main() {
+#   return 0;
+# }
+
+# Compiled like so:
+# clang-cl -c pgo-pubs.c -fprofile-instr-generate -fcoverage-mapping -clang:-save-temps
+
+
+  .text
+  .intel_syntax noprefix
+  .globl  main                            # -- Begin function main
+main:                                   # @main
+# %bb.0:                                # %entry
+  xor eax, eax
+  ret
+
+  .section  .lcovfun$M,"dr",discard,__covrec_DB956436E78DD5FAu
+  .globl  __covrec_DB956436E78DD5FAu      # @__covrec_DB956436E78DD5FAu
+  .p2align  3
+__covrec_DB956436E78DD5FAu:
+  .quad -2624081020897602054            # 0xdb956436e78dd5fa
+  .long 9                               # 0x9
+  .quad 24                              # 0x18
+  .quad 2164039332547799183             # 0x1e08364eb07c288f
+  .ascii  "\001\001\000\001\001\b\f\002\002"
+
+  .section  .lcovmap$M,"dr"
+  .p2align  3                               # @__llvm_coverage_mapping
+.L__llvm_coverage_mapping:
+  .long 0                               # 0x0
+  .long 40                              # 0x28
+  .long 0                               # 0x0
+  .long 5                               # 0x5
+  .ascii  "\002%\000\031C:\\src\\llvm-project\\build\npgo-pubs.i"
+
+  .section  .lprfc$M,"dw"
+  .p2align  3                               # @__profc_main
+__profc_main:
+  .zero 8
+
+  .section  .lprfd$M,"dw"
+  .p2align  3                               # @__profd_main
+__profd_main:
+  .quad -2624081020897602054            # 0xdb956436e78dd5fa
+  .quad 24                              # 0x18
+  .quad __profc_main
+  .quad main
+  .quad 0
+  .long 1                               # 0x1
+  .zero 4
+
+  .section  .lprfn$M,"dr"
+.L__llvm_prf_nm:                        # @__llvm_prf_nm
+  .ascii  "\004\000main"