commit a8285d7b91523f7efd6162a07526a5352abcf1d7
author Reid Kleckner <rnk@google.com> Tue Dec 18 01:14:05 2018 +0000
committer Reid Kleckner <rnk@google.com> Tue Dec 18 01:14:05 2018 +0000
tree 9690bfed23196925470d81325bac8ed4199f50e2
parent c64cec86d1c7534f42e403183a52a7e571ab01f5

[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

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@349431 91177308-0d34-0410-b5e6-96231b3b80d8