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

[Smith, Kevin B via llvm-commits]

From: Sean Silva [mailto:chisophugis at gmail.com]
Sent: Thursday, August 27, 2015 5:39 PM
To: reviews+D12341+public+5f663c68720e9677 at reviews.llvm.org; Smith, Kevin B
Cc: Sanjay Patel; Chandler Carruth; Krzysztof Parzyszek; Hal Finkel; Bob Wilson; kubastaszak at gmail.com; llvm-commits
Subject: Re: [PATCH] D12341: add llvm.unpredictable intrinsic and lower it to metadata



On Thu, Aug 27, 2015 at 3:13 PM, Kevin B. Smith via llvm-commits <llvm-commits at lists.llvm.org<mailto:llvm-commits at lists.llvm.org>> wrote:
kbsmith1 added a comment.

No, I am not suggesting you have more history.  I am suggesting that you only have a single number that represents the percentage of time that the direction is mispredicted.  It would make a lot of sense for it to be a simple 0-100 value, and would represent the percentage of mispredictions. Anything else takes more space, and is much too closely tied to the branch predictor.

I have seen 2 independent bug reports recently where the simple annotation to the compiler "assume that this is completely unpredictable" would have been the solution. Some pieces of code very closely approximate this, especially in decompression algorithms where by definition if the encoder is doing its job then the pattern cannot be guessed. Other data-driven branches can approximate this behavior as well.

This is more a declaration about the actual pattern of the branch. A number indicating the percentage of mispredictions (or any empirically collected number) is not a generalization of this annotation.

-- Sean Silva

Sean – Yes, I agree this is about an inherent property of the branch, or condition. But percentage of mispredictions could easily be used
to convey that information.  Simply set the value to 100 to say that 100% of the time the branch/condition is mispredicted, and consider that
the declaration about the actual pattern, rather than an actual number.

To some extent this mimics __builtin_expect, which allows a specification of the probability of a condition having a certain value, but where in actual usage, the user’s usage is likely to be always 0 or always 100 for hinting purposes.

This allows a single type of annotation to be used for both the user specified property, and also for HW feedback.

Kevin Smith


ABAB can be perfectly predicted often, AABB can be perfectly predicted often. The number of transitions from true to false or vice-versa has a poor
correlation to branch predictability, just as the branch probability has a poor correlation to branch predictability.


http://reviews.llvm.org/D12341



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