[LLVMdev] Removing ReadOnly from math intrinsics

[Philip Reames]

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.
>
> The alternative would be to develop a mechanism to finely model FP 
> environment changes; even further, it might be possible to come up 
> with a unified model of library call side effects, including errno and 
> even I/O.
I know there had been some work discussed on list around modelling errno 
explicitly.  I'm not sure what happened with that or what the current 
status is.

I would be mildly supportive of an effort to add optional explicit alias 
sets to function declarations.  I have an (out-of-tree) use-case which 
might benefit from such as well.

Philip
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