[Bug c/19703] New: Poor optimisation of loop test

[andrewhutchinson at cox dot net]

Missed optimization

gcc version 4.0.0 20041205 (experimental)
4.0.0/cc1.exe -quiet -v looprv.c -quiet -dumpbase
looprv.c -mmcu=atmega169 -auxbase looprv -Os -Wall -version -funsigned-char
-funsigned-bitfields -fpack-struct -fshort-enums -o looprv.s

Down counting loop, uses expensive compare EQ (-n) instead of compare >=0.
Testcase as follows:

volatile char  value6;
extern void foo6(char);
void testloop6(void)
{
int i;
for (i=100;i>= 0;i-=10)
{
   if (!value6)
   {
       foo6(i);
   } 
}

Loop test in RTL is compare NE -10. It should be compare GE 0 - which
is(generally) free. First dump of Expanded RTL show the compare.

-- 
           Summary: Poor optimisation of loop test
           Product: gcc
           Version: 4.0.0
            Status: UNCONFIRMED
          Severity: minor
          Priority: P2
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: andrewhutchinson at cox dot net
                CC: gcc-bugs at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19703