Re: [fortran] Add support for #pragma GCC unroll v3

[Eric Botcazou <ebotcazou@adacore.com>]

> The patch looks ok to me.

Thanks.

> For documentation, the gfortran manual has 2 sections:
> 
> 6.1 Extensions implemented in GNU Fortran
> 7.2 GNU Fortran Compiler Directives
> 
> 6.1 describes extension covering legacy code and vendor extensions.
> 7.2 describes other !$GCC directives.  Currently, the section is
> mainly calling conventions (CDECL, STDCALL, etc) and library
> macroc (DLLEXPORT).  These should probably be in 7.2.1 and the
> UNROLL directive in 7.2.2.
> 
> I can help with the documentation (although it might take a weekend
> or two to get done), but need to know sematics.  Does the directive
> apply to only the immediately following loop?  Does it apply to all
> loops that follow the directive?

The former.  Here's the documentation for the C & C++ compilers:

`#pragma GCC unroll N'
     You can use this pragma to control how many times a loop should be
     unrolled.  It must be placed immediately before a `for', `while'
     or `do' loop or a `#pragma GCC ivdep', and applies only to the
     loop that follows.  N is an integer constant expression specifying
     the unrolling factor.  The values of 0 and 1 block any unrolling
     of the loop.

> What is the interaction of the directive with -funroll-loops and --param
> max-unroll-times=4?

It's independent and always prevails, i.e. it doesn't need -funroll-loops to 
be effective, #pragma GCC 0 will block unrolling despite -funroll-loops and 
#pragma GCC N wins over --param max-unroll-times=M.

-- 
Eric Botcazou