[LLVMdev] Removing ReadOnly from math intrinsics

[Raul Silvera]

Right. Like Chandler is saying, these intrinsics are only used under
fno-math-errno and are already properly modeled as not modifying errno.

The change being proposed only affects the dynamic change of hardware fp
rounding modes, which is not currently supported by llvm (#pragma STDC
FENV_ACCESS ON). We currently appear to be at the worst of both worlds as
we do not have support for this environment but are always being
constrained by it.

On Fri, Jan 24, 2014 at 4:17 PM, Philip Reames <listmail at philipreames.com>wrote:

> On 1/24/14 3:52 PM, Raul Silvera wrote:
>
>  In include/llvm/IR/Intrinsics.td there is code to mark sqrt and several
> other math intrinsics as "ReadOnly", even though they do not read memory.
>
>  According to the comments this was done as an attempt to model changes
> to the FP rounding mode. This is too conservative, and unnecessarily blocks
> transformations such as commoning and vectorization.
>
>  I have heard from others that FP environment changes are not well
> modeled on LLVM anyway, so perhaps it is appropriate to just change these
> from ReadOnly to ReadNone. Any opinions on this? If there are no objections
> I'll prepare a patch.
>
> While our current modeling isn't quite right (e.g. we don't model writes
> to errno and related state), I'm very reluctant to see us move in the
> direction you propose.  I'm leary of having intrinsics which are modeled
> incorrectly and relying on the optimizers not to exploit that fact and
> yield incorrect code.  This seems like a recipe for disaster long term.


errno is a totally separate issue. I started to confuse these when talking
with Raul, and sorry for that.

The intrinsics should (IMO) be modeled as *not* fiddling with errno at all.
That is already the case today, and we don't transform library calls into
intrinsic calls unless compiling under '-fno-math-errno' to explicitly say
this is allowed.

The interesting question is whether the floating point intrinsics "read"
the hardware rounding mode. I would very much like to say "no" because LLVM
has essentially no support for modeling hardware rounding modes in any
other context. For example, we don't model it for multiply and divide, so
modeling it for sqrt seems pointless and in impedes a huge number of
optimizations.
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