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

[Jim Grosbach]

We already decide on a default. For these three targets is “whatever the user is running on.” For all the other targets it’s something the backend selects. I’m proposing we, as you say, remove the uncertainty and have the default always be the same from one run to the next even (especially) when those runs are on different machines.

For X86, there’s two options, the “generic” target that backend already supports or “core2” which is what clang typically defaults to for x86_64h. I lean towards the former for simplicity. We also don’t have to change all three of the ports. I’m personally motivated primarily by x86, and was intending to leave it to the maintainers of the other two targets to decide what they want to do here, perhaps with some strong encouragement.

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.

-Jim

* With the caveat of platform differences like elf vs. macho. A similar argument can be made there, but unlike CPU, there’s no sensible way to define a least-common-denominator default, so it’s harder. I’m explicitly avoiding opening that can of worms.

On Apr 9, 2014, at 10:19 PM, Eric Christopher <echristo at gmail.com> wrote:

> I'm not a huge fan of this because then you get to decide on a default
> for all the ports, but I can understand if people want to move this
> way to reduce uncertainty.
> 
> Alternately have a way for the backend to pretty print out the results
> of the auto-detection when asked?
> 
> *shrug*
> 
> -eric
> 
> On Tue, Apr 8, 2014 at 1:59 AM, Jim Grosbach <grosbach at apple.com> wrote:
>> All,
>> 
>> Currently the X86 backend does CPU auto-detection and subtarget feature detection when the TargetMachine is created if no explicit CPU was specified. It's counterintuitive for low level tools like 'llc' to do this, as it means the same .ll file compiled on heterogenous machines generates different results from the same 'llc' command line. It is still useful to be able to opt-in to such behavior to, for example, replicate clang's behavior when -mcpu=native is supplied to clang. My thought is to do something similar here and teach 'llc' to recognize -mcpu=native and probe the host CPU if that is given. The subtarget features will then be filled in according to the feature string for that CPU. This (a) changes the auto-detection from opt-out to opt-in and (b) moves the logic out of the core target backend and into the tools drivers.
>> 
>> Attached are draft patches that do this for X86. Similar but smaller cleanups can also be done for SystemZ and PowerPC if it's agreed this is a good idea.
>> 
>> -Jim
>> 
>> 
>> 
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