[PATCH] D41761: Introduce llvm.nospeculateload intrinsic

[Kristof Beyls via Phabricator via llvm-commits]

kristof.beyls added a comment.

The API design has been discussed over the past weeks in detail on the gcc mailing list. As a result of that, we propose to adapt the API, to enable efficient code generation also on architectures that need to generate a barrier instruction to achieve the desired semantics.

The main change in the proposed API is to drop the failval parameter and to tweak the semantics to the below.
There is a more detailed rationale for these changes at https://gcc.gnu.org/ml/gcc-patches/2018-01/msg01546.html

I haven't updated the code to implement the new specification yet, but thought I'd share the new specification as soon as possible, while I find the time to adapt the implementation:

This is an overloaded intrinsic. You can use llvm.speculationsafeload on any integer type that can legally be loaded on the target, and any pointer type. However, not all targets support this intrinsic at the moment.

  declare T @llvm.speculationsafeload.T(T* %ptr, i8* %lower_bound, i8* %upper_bound, i8* %cmpptr)

Arguments:
""""""""""

The first argument is a pointer, the second is a pointer used as a lower bound, the third is a pointer used as an upper bound. The fourth argument is a pointer.

Semantics:
""""""""""

- When the intrinsic is not being executed speculatively:
  - if %lower_bound <= %cmpptr < %upper_bound, the value at address %ptr is returned.
  - if %cmpptr is not within these bounds, the behaviour is undefined.
- When the intrinsic is being executed speculatively, either:
  - Execution of instructions following the intrinsic that have a dependency on the result of the intrinsic will be blocked, until the intrinsic is no longer executing speculatively. At this point, the semantics under point 1 above apply.
  - Speculation may continue using the value at address %ptr as the return value of the intrinsic, if %lower_bound <= %cmpptr < %upper_bound, or an unspecified constant value if %cmpptr is outside these bounds.


https://reviews.llvm.org/D41761