[LLVMdev] Proposal: Move host CPU auto-detection out of the TargetMachine

[Reid Kleckner]

On Thu, Apr 10, 2014 at 6:32 PM, Jim Grosbach <grosbach at apple.com> wrote:

> It’s very important that a run of “llc” on one machine produce the same
> output on two heterogenous machines given the same input and command
> lines*. That’s not true right now, leading to lots of bot failures that
> patch originators can’t reproduce because they’re getting different code
> locally due to the auto-detection. The recent “Test failures with 3.4.1”
> thread for examples.
>

I think we should do this, but only to make llc's behavior more
deterministic and predictable. I don't think we should start checking in
tests that rely on the default subtarget features without explicitly
requesting the relevant features.

Consider somebody who works on an ARM or x86 variant like atom.  Probably
what they'll want to do is set up a bot that runs the LLVM test suite in
such a way that their subtarget features are on by default.  Our test suite
currently "supports" that if the host CPU features happen to be the ones
you want.  Instead, we should probably move to a world where bots can set
different defaults by configuring the tools appropriately.  For example,
they could rewrite 'llc' to 'llc -mcpu=blah' in lit if no mcpu flags exist.

This would be similar to what we do in Clang for the default C++ ABI. If
the test actually needs the MSVC or Itanium C++ ABI, they ask for it
explicitly. Otherwise they test one or the other depending on the default
target triple. I don't think we should double the number of RUN lines to
keep that test coverage, and I don't think the cost of Windows-bot-only
test failures is too high.

2c
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