xiongji90 added inline comments.
================ Comment at: clang/docs/LanguageExtensions.rst:3288 +This builtin is restrcted to work on x86 and arm targets currently. When support +for the builtin is added for new targets, the manual should be updated accordingly. + ---------------- rjmccall wrote: > This meta-commentary about when to update the manual should not go into the > manual. > > I'm going to insist that we talk about ``#pragma STDC FENV_ACCESS`` here. It > would be inappropriate not to, given that it's undefined behavior to use this > without also using the pragma or one of those flags. We should not be > documenting features with non-obvious UB without warning. > > How about: > > ``` > By default, these builtins may not be used. The floating point rounding mode > is part of the floating point environment, and it is undefined behavior to > read or modify the floating point environment, or to run code under a > non-default floating point environment, unless that code is compiled under a > special mode. Clang supports three well-defined ways to control the rounding > mode of floating point operations: > > - The standard pragma ``#pragma STDC FENV_ROUND <mode>`` can change the > rounding mode to a specific value for the operations covered by the pragma. > This does not permit dynamic access to the rounding mode, but in many cases > it is sufficient to achieve the desired effect, and it is more optimizable > than the alternatives below. > > - The standard pragma ``#pragma STDC FENV_ROUND FE_DYNAMIC`` allows dynamic > access to the floating point rounding mode (including by these builtins) in > code covered by the pragma. The command line option ``-frounding-math`` > behaves as if the translation unit began with this pragma. This pragma does > not allow changes to the floating point exceptions mode and so may be more > optimizable than the next alternative. > > - The standard pragma ``#pragma STDC FENV_ACCESS ON`` allows dynamic access > to the entire floating point environment (including by these builtins) in > code covered by the pragma. The command line option ``-ffp-model=strict`` > behaves as if the translation unit began with this pragma. > > Code that modifies the floating point rounding mode dynamically must take > care to reset it to the default mode before returning control to code that is > not compiled under one of these last two pragmas. See the C standard for > more information about these pragmas. > ``` > > Serge, please fact-check all that. > This meta-commentary about when to update the manual should not go into the > manual. > > I'm going to insist that we talk about ``#pragma STDC FENV_ACCESS`` here. It > would be inappropriate not to, given that it's undefined behavior to use this > without also using the pragma or one of those flags. We should not be > documenting features with non-obvious UB without warning. > > How about: > > ``` > By default, these builtins may not be used. The floating point rounding mode > is part of the floating point environment, and it is undefined behavior to > read or modify the floating point environment, or to run code under a > non-default floating point environment, unless that code is compiled under a > special mode. Clang supports three well-defined ways to control the rounding > mode of floating point operations: > > - The standard pragma ``#pragma STDC FENV_ROUND <mode>`` can change the > rounding mode to a specific value for the operations covered by the pragma. > This does not permit dynamic access to the rounding mode, but in many cases > it is sufficient to achieve the desired effect, and it is more optimizable > than the alternatives below. > > - The standard pragma ``#pragma STDC FENV_ROUND FE_DYNAMIC`` allows dynamic > access to the floating point rounding mode (including by these builtins) in > code covered by the pragma. The command line option ``-frounding-math`` > behaves as if the translation unit began with this pragma. This pragma does > not allow changes to the floating point exceptions mode and so may be more > optimizable than the next alternative. > > - The standard pragma ``#pragma STDC FENV_ACCESS ON`` allows dynamic access > to the entire floating point environment (including by these builtins) in > code covered by the pragma. The command line option ``-ffp-model=strict`` > behaves as if the translation unit began with this pragma. > > Code that modifies the floating point rounding mode dynamically must take > care to reset it to the default mode before returning control to code that is > not compiled under one of these last two pragmas. See the C standard for > more information about these pragmas. > ``` > > Serge, please fact-check all that. Hi, @sepavloff Could you provide your insight here? Thanks very much. 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