Add tests to verify assembler output -- Fix DoNotOptimize.

[EricWF]

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.

This PR is largely just to run the new patch against the bots to ensure nothing has broken.

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[dmah42]

Can you add a markdown documentation file about AssemblyTests or whatever name you choose, essentially with the contents of your commit message. Maybe more detail.

And then cross-link to it from the README?

[pleroy]

TIL about LLVM FileCheck. Pretty cool!

[EricWF]

@dominichamon Yep. Can do.

@pleroy:

TIL about LLVM FileCheck. Pretty cool!

Indeed. It's an amazing tool to have when you need to test string output.

[EricWF]

@dominichamon I added documentation as you requested. Please take a look at your leisure.

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[dmah42]

'whose'

[dmah42]

this should be ### i think, as you're only at ## above.

[EricWF]

If it's no matter, I actually prefer having the smaller section headers for these bits. IMHO it's looks nicer and flows better.

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[googlebot]

CLAs look good, thanks!

[EricWF]

@dominichamon Any final words?

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[dmah42]

I'd prefer if the bazel TODO linked to a github issue so we can track it, but otherwise, Ship It.

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