Re: Add __builtin_tgmath for better tgmath.h implementation (bug 81156)

[Joseph Myers <joseph at codesourcery dot com>]

On Wed, 15 Nov 2017, Richard Biener wrote:

> Thanks - I suppose we can't avoid the repeated expansion by sth like
> 
> #define exp(Val) ({ __typeof__ Val tem = Val; __TGMATH_UNARY_REAL_IMAG
> (tem, exp, cexp); })

Well, that still expands its argument twice.  You'd need to use 
__auto_type to avoid the double expansion.  And then you'd still have 
extremely complicated expansions (that are correspondingly unfriendly if a 
user makes a mistake with a call, e.g. an argument of unsupported type), 
and complications around getting the right semantics when decimal floating 
point is involved.  And use of ({ }) doesn't work in sizeof outside 
functions.  And that wouldn't help with cases such as 
__STDC_TGMATH_OPERATOR_EVALUATION__, whereas it would actually be easy to 
add __builtin_tgmath_operator that's handled the same as __builtin_tgmath 
but ends up calling a function based on evaluation formats and producing 
an EXCESS_PRECISION_EXPR.

(Clang overloadable functions in C don't avoid the multiple expansion 
either, or at least Clang's tgmath.h doesn't.)

-- 
Joseph S. Myers
joseph@codesourcery.com