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? Repository: rG LLVM Github Monorepo CHANGES SINCE LAST ACTION https://reviews.llvm.org/D106191/new/ https://reviews.llvm.org/D106191 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
