* Fix DoNotOptimize() GCC copy overhead (#1340)
The issue is that GCC DoNotOptimize() does a full copy of an argument
if it's not a pointer and it slows down a benchmark. If an argument is big
enough there is a memcpy() call for copying the argument. An argument
object can be a big object so DoNotOptimize() could add sufficient
overhead and affects benchmark results.
The cause is in GCC behavior with asm volatile constraints. Looks like GCC
trying to use r(register) constraint for all cases despite object size.
See: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105519
The solution is the split DoNotOptimize() in two cases - value fits
in register and value doesn't fit in register. And use case specific
asm constraint. std::is_trivially_copyable trait is needed because
"+r" constraint doesn't work with non trivial copyable objects.
- Fix requires support C++11 feature std::is_trivially_copyable from GCC
compiler. The feature has been supported since GCC 5
- Fallback for GCC version < 5 still exists but it uses "m" constraint
which means a little bit more overhead in some cases
- Add assembly tests for issued cases
Fixes#1340
* Add supported compiler versions info for assembly tests
- Assembly tests are inherently non-portable. So explicitly add GCC
and Clang versions required for reliable tests passed
- Write a warning message if the current compiler version isn't supported
* Add tests to verify assembler output -- Fix DoNotOptimize.
For things like `DoNotOptimize`, `ClobberMemory`, and even `KeepRunning()`,
it is important exactly what assembly they generate. However, we currently
have no way to test this. Instead it must be manually validated every
time a change occurs -- including a change in compiler version.
This patch attempts to introduce a way to test the assembled output automatically.
It's mirrors how LLVM verifies compiler output, and it uses LLVM FileCheck to run
the tests in a similar way.
The tests function by generating the assembly for a test in CMake, and then
using FileCheck to verify the // CHECK lines in the source file are found
in the generated assembly.
Currently, the tests only run on 64-bit x86 systems under GCC and Clang,
and when FileCheck is found on the system.
Additionally, this patch tries to improve the code gen from DoNotOptimize.
This should probably be a separate change, but I needed something to test.
* Disable assembly tests on Bazel for now
* Link FIXME to github issue
* Fix Tests on OS X
* fix strip_asm.py to work on both Linux and OS X like targets
