[llvm-commits] [llvm] r134516 - in /llvm/trunk: include/llvm/ include/llvm/Support/ include/llvm/Transforms/ lib/CodeGen/ lib/CodeGen/SelectionDAG/ lib/Transforms/Scalar/ lib/Transforms/Utils/ lib/VMCore/ test/CodeGen/Generic/ test/Transforms/LowerExpectIntrinsic/

[Andrew Trick]

On Jul 7, 2011, at 1:48 PM, Jakub Staszak wrote:
> Right now clang creates "expect" intrinsic for every __builtin_expect instruction. These intrinsics must be lowered early because in other case they can harm other optimizations. During the lowering we also create "branch_weight" metadata, which can be used by other optimizations using BranchProbability analysis.

Kuba,

Which optimizations are harmed by expect intrinsics?

If this is just a temporary solution, that's fine. In the long term, we need to formalize the properties of intrinsics that serve as positional meta-data. This is needed for debug info, exception handling, lifetimes, profiling, and probably more to come. We should have a uniform framework for these that guarantee they maintain their association with a control flow edge without affecting any optimizations or heuristics. BasicBlock::getFirstNonPHIOrDbgOrLifetime is a glaring example of the need for a general solution.

Three representations of almost exactly the same information (intrinsics, meta-data, and analysis) is also evidence of a design shortcoming.

-Andy

> On Jul 6, 2011, at 5:31 PM, Chris Lattner wrote:
> 
>> 
>> On Jul 6, 2011, at 11:22 AM, Jakub Staszak wrote:
>> 
>>> Author: kuba
>>> Date: Wed Jul  6 13:22:43 2011
>>> New Revision: 134516
>>> 
>>> URL: http://llvm.org/viewvc/llvm-project?rev=134516&view=rev
>>> Log:
>>> Introduce "expect" intrinsic instructions.
>>> 
>> 
>> Hi Kuba,
>> 
>> Why is this introducing a new pass to do lowering?
>> 
>> -Chris
> 
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