[fortran,patch] Fix LEADZ/TRAILZ

[FX <fxcoudert at gmail dot com>]

The current implementation of LEADZ/TRAILZ has the following bugs:

  1. Simplification of LEADZ for negative arguments is wrong (it  
should always give 0)

  2. When calling leadz or trailz on a type smaller than "int", we  
should cast to the unsigned type of the same size before casting to  
"unsigned int"; that is, for example, we should do:
           __builtin_clz ((unsigned int) (unsigned short) short_value)
      instead of
           __builtin_clz ((unsigned int) short_value)
Otherwise, for negative values, we get wrong results.

  3. In the translation of these intrisics, kinds <= 4 yield a call  
to __builtin_clz(), kind = 8 calls __builtin_clzl() and kind = 16  
calls __builtin_clzll(). This is wrong, as there is no such hard-wired  
correspondance of integer kinds and C types. In particular, on x86_64,  
integer(kind=16) is larger than "long long int", and thus no builtin  
is available!


The attached patch solves all these issues, by 1. special-casing  
negatives args in LEADZ simplification; 2. performing the correct  
double cast; 3. choosing the right builtin based on type size, and  
call adequate new library functions (_gfortran_clz128 /  
_gfortran_ctz128).

Bootstrapped and regtested on x86_64-linux, comes with testcases. OK  
to commit?

FX


PS: could the reviewer (or another maintainer) please double check  
that I've done the correct modification to libgfortran/gfortran.map?

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