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================ Comment at: clang/docs/UsersManual.rst:1393 + + Allow substitution of approximate calculations for functions. + Defaults to ``-fno-approx-func``. ---------------- 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. 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
