[LLVMdev] [RFC] Stripping unusable intrinsics

[Bob Wilson]

> On Dec 23, 2014, at 10:41 AM, Chris Lattner <clattner at apple.com> wrote:
> 
> On Dec 23, 2014, at 10:28 AM, Chris Bieneman <beanz at apple.com <mailto:beanz at apple.com>> wrote:
>>>> It should be straight-forward to have something like LLVMInitializeX86Target/RegisterTargetMachine install the intrinsics into a registry.
>>> 
>>> I tried doing that a few years ago. It’s not nearly as easy as it sounds because we’ve got hardcoded references to various target intrinsics scattered throughout the code.
>> 
>> I was just writing to say exactly this. There are a number of ways we could work toward something like this. I’m completely in favor of a world where Intrinsics are properties of the targets and don’t leach out, however today they do in a lot of places.
> 
> What are the specific problems here?  Anything that does an equality comparison with the IntrinsicID can be changed to do strcmp with the name.  That would handle the one-off cases like InstCombiner::SimplifyDemandedUseBits in InstCombine.
> 
> The other cases in InstCombine could be handled similarly, but may be better handled by adding a intrinsic behavior query APIs to the intrinsic registry, or would be better handled (eventually) by adding new attributes to the intrinsics themselves.

I don’t think there’s anything fundamentally difficult, but it’s a big change. For example:

$ git grep Intrinsic:: | wc
    3364   12286  281078

The vast majority of those 3,364 lines have hardcoded references to specific intrinsics. Many of them are used in contexts where you can’t easily insert a strcmp (e.g., case values in large switch statements, or worse, the m_Intrinsic PatternMatch templates).
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