[Bug c/27125] New: optimize array bit shift

["ajrobb at bigfoot dot com" <gcc-bugzilla at gcc dot gnu dot org>]

I was writing a routine to shift bits along an array. I want the compiler to
use the shrdl assembler command. It will do this if I load the values into an
(unsigned long long) value, bit it does a whole lot more - taking 22 seconds
to shift 1024000000 words on an 500MHz Intel PIII. The enclosed C routine
doesn't use (unsigned long long) and is a faster - taking 14 seconds.

1. I would like the optimiser to recognise a shrd like it recognises a ror.

2. I would like the optimiser to respect my choice of decrement to zero. (I
think earlier versions (3) of gcc C compiler would respect my choice to
decrement to zero rather then increment from zero to a limit.)

The hand-optimised assembler, my_bit32.s, takes just 8 seconds. Some 4 seconds
were gained by using shrdl and a further 2 seconds were gained by using decl
instead of incl and cmpl.


-- 
           Summary: optimize array bit shift
           Product: gcc
           Version: 4.1.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: ajrobb at bigfoot dot com
 GCC build triplet: i686-pc-linux-gnu
  GCC host triplet: i686-pc-linux-gnu
GCC target triplet: i686-pc-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=27125