[gfortran,patch] PR31399: handle large DO loops

[FX Coudert]

Hi all,

Attached patch fixes PR31399 by changing the code we generate for DO  
loops. Except for simple loops (with step 1 or -1), we compute the  
loop count and iterate by decreasing it until it reaches zero.  
However, for the largest loop, the count can overflow the  
corresponding unsigned type. (For example, the loop going from -huge 
(0)-1 to huge(0) has 2*huge(0)+2 elements, while the largest unsigned  
integer of this kind is 2*huge(0)+1.) The solution is to store the  
loop count minus one, instead, and reorder the generated code.

Bootstrapped and regtested on i686-linux, it comes with a testcase  
for both integer and real loops. OK for mainline? (It's a regression  
wrt g77, but it affects very little user code, so I don't think it's  
worth a backport.)

FX



:ADDPATCH fortran:

2007-05-02  Francois-Xavier Coudert  <fxcoudert@gcc.gnu.org>

         PR fortran/31399
         * trans-stmt.c (gfc_trans_do): Handle large loop counts.


2007-05-02  Francois-Xavier Coudert  <fxcoudert@gcc.gnu.org>

         PR fortran/31399
         * gfortran.dg/do_3.F90: New test.


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