[PATCH] D126364: Fix interaction of pragma FENV_ACCESS with other pragmas

[Kevin P. Neal via Phabricator via cfe-commits]

kpn added a comment.

>> On targets that support static rounding mode (like RISCV) dynamic and constant rounding modes may be different and the behavior changes if default mode is replaced by dynamic.
>
> Whether a target supports static rounding modes on floating-point instructions is completely irrelevant.  The user-visible behavior must be the same either way.  If a target doesn't have specialized instructions, the code generator can save/restore the rounding mode.  This should be transparent to the user; the user can't read the rounding mode in between the save and restore.  (We already do this sort of rounding mode manipulation on x86, to implement float-to-int conversion on targets without SSE.)

The rounding mode argument to the constrained intrinsic is a hint; it tells the intrinsic what rounding mode has been set via the normal rounding mode changing mechanism. Constrained intrinsics don't change the rounding mode. I guess we could weaken that by adding "... except if absolutely required" to account for the float-to-int sans SSE case, but the principle applies.

Because of the requirement that the rounding mode be changed already, I don't see how instructions with static rounding modes are generally interesting. Perhaps a target will learn to recognize that a sequence of instructions can use the static rounding in the instructions and elide a change of rounding mode around the sequence? I don't know how common that would be in practice.


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  https://reviews.llvm.org/D126364/new/

https://reviews.llvm.org/D126364