[Cfp-interest] implementing tgmath for constant rounding modes

[Fred J. Tydeman]

On Sat, 2 Jun 2012 08:22:55 -0700 Jim Thomas wrote:
>
>At our last teleconference, Fred asked about how <tgmath.h> could be written to work correctly
>with the pragma that switch between constant and dynamic rounding modes. 
>
><tgmath.h> aside, the math functions that can be affected by constant rounding modes must be 
>sensitive to whether a constant rounding mode is in effect. 
>Take the example of cbrt. <math.h> could have
>
>#define cbrt(x) __roundwise_cbrt(x)
>
>Then the translator would have to keep track of whether a constant rounding mode is in effect at 
>each call site for __roundwise_cbrt and, if so, what the rounding direction is. 
>
>Compilers already keep track of the state of other FP pragmas, which have the same sort of 
>scoping as the constant rounding mode pragmas.

As far as I recall, neither gcc nor clang keep track of FENV_ACCESS.  So, I am not
sure how easy it is to track state of FP pragmas.

So, in effect, __roundwise_cbrt becomes a special keyword/token/symbol to the compiler.
And the same for all the other math functions (that is a lot of names to treat special).

So, if another compiler did it as
  #define cbrt(x) __RoundWise_cbrt(x)
it will be hard for 3rd party library/header implementors
(such as P.J.Plauger's Dinkumware) to create a header that 
works with all compilers.

Would it be better to add just one new attribute, eg,
  #define cbrt(x) [[ROUNDWISE]] cbrt(x)
that would be the same for all compilers?


---
Fred J. Tydeman        Tydeman Consulting
tydeman at tybor.com      Testing, numerics, programming
+1 (775) 358-9748      Vice-chair of PL22.11 (ANSI "C")
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