[codeview] Align symbol records to save 441MB during linking clang.pdb

In PDBs, symbol records must be aligned to four bytes. However, in the
object file, symbol records may not be aligned. MSVC does not pad out
symbol records to make sure they are aligned. That means the linker has
to do extra work to insert the padding. Currently, LLD calculates the
required space with alignment, and copies each record one at a time
while padding them out to the correct size. It has a fast path that
avoids this copy when the records are already aligned.

This change fixes a bug in that codepath so that the copy is actually
saved, and tweaks LLVM's symbol record emission to align symbol records.
Here's how things compare when doing a plain clang Release+PDB build:
- objs are 0.65% bigger (negligible)
- link is 3.3% faster (negligible)
- saves allocating 441MB
- new LLD high water mark is ~1.05GB

llvm-svn: 349431

1 files changed, 1 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/AsmPrinter/CodeViewDebug.cpp b/llvm/lib/CodeGen/AsmPrinter/CodeViewDebug.cpp
index 93b76997c07..78e5f5d215b 100644
--- a/llvm/lib/CodeGen/AsmPrinter/CodeViewDebug.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/CodeViewDebug.cpp
@@ -2833,6 +2833,7 @@ MCSymbol *CodeViewDebug::beginSymbolRecord(SymbolKind SymKind) {
 void CodeViewDebug::endSymbolRecord(MCSymbol *SymEnd) {
   // Symbol records in object files are not aligned, although we are considering
   // it for linker performance reasons.
+  OS.EmitValueToAlignment(4);
   OS.EmitLabel(SymEnd);
 }