------- Comment #5 from ubizjak at gmail dot com 2008-03-21 20:58 -------
The inner loop is compiled (-O2 -march=pentium4 -malign-double) to:
.L4:
movl %ecx, %eax
andl $1, %eax
movl a(,%eax,4), %eax
xorl %edx, %edx
(*) pushl %edx
(*) pushl %eax
(*) fildll (%esp)
addl $8, %esp
faddp %st, %st(1)
addl $1, %ecx
cmpl $100000000, %ecx
jne .L4
Instructions marked with (*) form partial memory access.
Runtime:
time ./a.out
real 0m0.794s
user 0m0.724s
sys 0m0.000s
Patched gcc creates:
.L4:
movl %edx, %eax
andl $1, %eax
movd a(,%eax,4), %xmm0
movq %xmm0, -16(%ebp)
fildll -16(%ebp)
faddp %st, %st(1)
addl $1, %edx
cmpl $100000000, %edx
jne .L4
time ./a.out
real 0m0.123s
user 0m0.124s
sys 0m0.000s
This represents more than 5.8x speedup. The optimization is applicable to
non-TARGET_INTER_UNIT_MOVES target as well, despite extra (store forwarded!)
memory access.
--
ubizjak at gmail dot com changed:
What |Removed |Added
----------------------------------------------------------------------------
URL| |http://gcc.gnu.org/ml/gcc-
| |patches/2008-
| |03/msg01295.html
Status|ASSIGNED |RESOLVED
Resolution| |FIXED
Target Milestone|--- |4.4.0
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=13958
