[PATCH] D144454: Add builtin for llvm set rounding

[Serge Pavlov via Phabricator via cfe-commits]

sepavloff added a comment.

C standard function `fesetround` accepts rounding mode in the form of target-specific values like `FE_TONEAREST`. They usually represent corresponding bits in control register, so are different for different targets. `llvm.set_rounding` in contrast accepts rounding mode as target-independent value, the encoding is same as used in `FLT_ROUNDS`.

The difference between `fesetround` and `llvm.set_rounding` is same as for `fegetround` and `FLT_ROUNDS`. They differ in how rounding mode is specified, - as target-specific or as target-independent values respectively.

The intrinsic `llvm.set_rounding` was introduced to facilitate work on IR, because IR is (mainly) target-agnostic and defines like `FE_TONEAREST` are not available there. It would be nice to have a C builtin counterpart for `FLT_ROUNDS`, I don't know why the standard does not define it.

Implementation of `fesetround` with `set_rounding` would be very useful, but it requires translation of target-specific values for rounding mode into universal values and also checking availability of FPU, as `fesetround` returns value if rounding mode cannot be set. Both these checks are likely to require separate builtins.


Repository:
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  https://reviews.llvm.org/D144454/new/

https://reviews.llvm.org/D144454