[PATCH] D106191: [clang] Option control afn flag

[Erich Keane via Phabricator via llvm-commits]

erichkeane added inline comments.


================
Comment at: clang/docs/UsersManual.rst:1393
+
+   Allow substitution of approximate calculations for functions.
+   Defaults to ``-fno-approx-func``.
----------------
masoud.ataei wrote:
> erichkeane wrote:
> > masoud.ataei wrote:
> > > erichkeane wrote:
> > > > This seems pretty incomplete to me, I have no idea what it does from this description.  Can you elaborate?
> > > If I didn't miss to say it is about math function calls, then I guess it would be more clear. 
> > > ```
> > > Allow substitution of approximate calculations for math function calls.
> > > ```
> > > This option just adds `afn` fast-math flag to the function calls and in backend the math function call can be replaced by an approximate calculation (either another math function call or not). 
> > > https://llvm.org/docs/LangRef.html#fast-math-flags
> > allow substitution of approximate calculations for math function with what?  I'm not clear as to what this does still.  I'm not sure that I get what the 'afn' documentation means there either.  Typically we'd want the clang frontend flags to be more accurate/descriptive than the LLVM-IR documentation.
> This option adds `afn` fast-math flag to function calls in IR. And in the case of math function calls (like `log`, `sqrt`, `pow`, ...), `afn` flag allows LLVM to substitute the math calls with an "approximate calculation". What an approximate calculation is may differ based on which math call it is, what other fast-math flags are set, what is the target machine, and other factors.
> 
> An approximate calculation can be
>  1. sequence of instructions (inlining): for example, inlining sqrt call needs `afn` flag to be present in the call.
>  2. a substituted math function call (which may be less accurate but faster): 
>      this substitution can be 
>      (a) for general targets: for example: `pow(x,0.25)` to `sqrt(sqrt(x))`. or
>      (b) for a specific target: for example, in the case of PowerPC, we are proposing to substitute math calls to MASS library calls https://reviews.llvm.org/D101759
> 
> Note that, `afn` flag is necessary for these approximate calculation but may not be sufficient. Most of them need at lease one or two other fast-math flags to be set too. 
> 
> I agree the term "approximate calculation" is a very general term. But I guess it is needed to describe this general situation. 
Perhaps then something like:
`Allow certain math function calls (such as log, sqrt, pow, etc) to be replaced with an approximately equivalent set of instructions or alternative math function calls.  For example, a pow(x, 0.25) may be replaced with sqrt(sqrt(x), despite being an inexact result in cases where <expand a little>.`


For Options.td we can be a little more terse, so perhaps something a little more like:

`Allow certain math function calls to be replaced with an approximately equivalent calculation.`

@aaron.ballman  WDYT?



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https://reviews.llvm.org/D106191