[cfe-dev] Loop pragma for hardware loops

[Janek Van Oirschot via cfe-dev]

Hey all,

I'm looking to extend the current clang loop pragmas to also support hardware loops and allow a user to insert (or completely disable) hardware loop intrinsics on a per-loop basis.

One of the questions I have regarding this is how to go about incorporating the different hardware loop intrinsics in the pragma. A few options we came up with:

1. The pragma incorporates which intrinsic to use for a loop:
#pragma loop hwloop(set_loop_i32)
or
#pragma loop hwloop(/*LivesInReg=*/ true, /*AddTestGuard=*/ true, /*NumBits=*/ 32)

2. The pragma adds some target specific info (string?) to use in the hwloop TTI hook/new hwloop TTI hook:
#pragma loop hwloop(target="bdnz") // PPC example
#pragma loop hwloop(target="bdz")  // PPC example
or
#pragma loop hwloop(max-count=42, ...)

Option 1 requires the user to know about llvm's hardware loops internals so I'm leaning more towards option 2 as users are more likely to be aware of target specific information (such as PPC's bdnz/bdz).
These are just some options we came up with, we would love to hear about other (better) options, if any.

Kind regards,
Janek van Oirschot
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