commit 4ed87f5662f8ab77bb9c04bbc4d941c3104e96ed
author Fangrui Song <maskray@google.com> Tue May 14 15:25:21 2019 +0000
committer Fangrui Song <maskray@google.com> Tue May 14 15:25:21 2019 +0000
tree 5fd78dfa3779a7e3300b119293b609f29a8d78d1
parent b8b50ee047c9a03352189aa12a0ddbe186759412

[LLD][ELF] Add the -z ifunc-noplt option

Patch by Mark Johnston!

Summary:
When the option is configured, ifunc calls do not go through the PLT;
rather, they appear as regular function calls with relocations
referencing the ifunc symbol, and the resolver is invoked when
applying the relocation.  This is intended for use in freestanding
environments where text relocations are permissible and is incompatible
with the -z text option.  The option is motivated by ifunc usage in the
FreeBSD kernel, where ifuncs are used to elide CPU feature flag bit
checks in hot paths.  Instead of replacing the cost of a branch with that
of an indirect function call, the -z ifunc-noplt option is used to ensure
that ifunc calls carry no hidden overhead relative to normal function
calls.

Test Plan:
I added a couple of regression tests and tested the FreeBSD kernel
build using the latest lld sources.

To demonstrate the effects of the change, I used a micro-benchmark
which results in frequent invocations of a FreeBSD kernel ifunc.  The
benchmark was run with and without IBRS enabled, and with and without
-zifunc-noplt configured.  The observed speedup is small and consistent,
and is significantly larger with IBRS enabled:

https://people.freebsd.org/~markj/ifunc-noplt/noibrs.txt
https://people.freebsd.org/~markj/ifunc-noplt/ibrs.txt

Reviewed By: ruiu, MaskRay

Differential Revision: https://reviews.llvm.org/D61613

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@360685 91177308-0d34-0410-b5e6-96231b3b80d8