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
