Re: egcs-971031, sparc-sun-solaris2.5 new bootstrap failure

[Toon Moene <toon@moene.indiv.nluug.nl>]

>  Here is a patch to fix this.  This will be in the next
>  snapshot.
>
>  Thu Nov  6 18:27:36 1997  Jim Wilson  <wilson@cygnus.com>
>
>  	* flags.h (flag_rerun_loop_opt): Declare.
>       * loop.c (invariant_p, case LABEL_REF): Check
>  	flag_rerun_loop_opt.
>       * toplev.c (flag_rerum_loop_opt): Delete static.

Ah, that means that Jeff implemented rerun-loop-opt slightly  
differently from the way g77 had it; we *did* have an externally  
visible flag -frerun-loop-opt.

>  	 We don't know the loop bounds here though, so
>  	 just fail for all labels.  */
>  !       /* ??? This is also necessary if
>  flag_rerun_loop_opt is true, because in
>  !        this
>  case we may be doing loop unrolling the second time we
>  run loop,
>  !        and hence the first loop run also
>  needs this check.  There is no way
>  !        to check here
>  whether the second run will actually do loop unrolling
>  !        though, as that info is in a local var in
>  rest_of_compilation.  */
>  !       if (flag_unroll_loops || flag_rerun_loop_opt)
>  	return 0;
>       else
>       return 1;

Fascinating - this error must have been in g77 too (between 0.5.18,  
April '96 and 0.5.20, March '97).  We never found a problem with it  
though.

For g77-0.5.20 I decided to have loop unrolling only happening in  
the second pass through loop_optimize (as it is in egcs now),  
because the backend is not good at optimising unrolled loops.  One  
can easily check this by compiling the following on a ix86 (for  
simplicity this assumes n % 4 == 0):

      subroutine daxpy(x, y, a, n)
      integer n
      double precision x(n), y(n), a
      do i = 1, n, 4
         y(i  ) = y(i  ) + a * x(i  )
         y(i+1) = y(i+1) + a * x(i+1)
         y(i+2) = y(i+2) + a * x(i+2)
         y(i+3) = y(i+3) + a * x(i+3)
      enddo
      end

This basically is again pointing out that an optimisation that  
combine_givs *should* do isn't done, and we end up using 8 different  
pseudo registers for the array accesses.

HTH,
Toon.