tree 06e955f8613b7e431462a12d0e455514f2563ca8
parent be36aa9fb628db1ae50c2085f526cde92937fac3
author Sterling Augustine <saugustine@google.com> 1583368198 -0800
committer Copybara-Service <copybara-worker@google.com> 1598638570 -0700

Lazily save initialState of registers during unwind.

Summary:
Copying all of the saved register state on every entry to
parseInstruction is a severe performance contraint, especially
because most of this saved state is never used. On x86 linux
this is about 560 bytes, and will be more on other platforms.

When performance testing libunwind, this memcpy appears at the
top of nearly all our tests.

By only saving this state as needed, we see increasing in performance
of around 2.5% for the ctak test here.

https://github.com/clasp-developers/ctak

Certain internal extremely exception-heavy tasks run in about 2/3
the time.

Note that by stashing the new boolean inside what had been padding in
the original structure, this uses no additional memory.

Subscribers: fedor.sergeev, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D75692

GitOrigin-RevId: 8ffdabdb61e1e2251b07ee93a4806cde1890f47d