[PATCH] D12341: add llvm.unpredictable intrinsic and lower it to metadata

[Philip Reames via llvm-commits]

On 08/28/2015 09:54 AM, Kevin B. Smith via llvm-commits wrote:
> kbsmith1 added a comment.
>
> In http://reviews.llvm.org/D12341#235210, @kparzysz wrote:
>
>> In http://reviews.llvm.org/D12341#235179, @kbsmith1 wrote:
>>
>>> [...]
>>
>> I don't think we completely understand each other.  The profiling data that is attached to MD_prof describes the behavior of the program.  You want to have something that describes the behavior of the hardware.  Those are two different things.  The former is independent from the specific properties of the hardware and can be reused when compiling for different subtargets.  The latter is not.  I'm not opposed to profiling information tied to specific hardware in principle, but I think those two kinds should be easily separable.
>
> No, I really do get that this is for describing the behavior of the program, and I am not trying to use this to describe behavior of the HW.  All I am saying, is that you cannot simply describe the branch pattern, and expect to derive predictability information from that.  I just think that the HW feedback data can be used as a good approximation for this program behavior information, and that the branch pattern will not be usable for deriving that information.
A somewhat horrible though occurred to me after reading this thread.  
Given that many code bases are targeted for multiple distinct hardware 
platforms, and deriving predictability of branch on a particular piece 
of hardware from a description the behavior is somewhat of a 
non-starter, do we need to have a mechanism for describing the 
predictability of a given branch on a particular instance of hardware?  
A branch that's entirely predictable on one compilation target (say 
modern x86 excluding Atom), might be utterly unpredictable to another 
(say PPC).  Do we need to support this either at the source level (Clang 
builtins), or the compiler itself?
>
>
> http://reviews.llvm.org/D12341
>
>
>
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