[Bug c/19676] New: Loop optimizer fails to reverse simple loop

[andrewhutchinson at cox dot net]

AVR Target 20041205 snapshot

gcc version 4.0.0 20041205 (experimental)
 /avrdev/libexec/gcc/avr/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


Loop optimiser fails to reverse simple loop. Example

void testloop5(void)
{
        int i;
        for (i=0;i<100;i++)
        {
                if (!value)
                {
                        foo();
                } 
        }
}

generates RTL setting index to 100 then using decrement/branch at end of loop as
expected. However, adding any kind of while/for loop inside outer loop leaves
index unoptimised. For example

void testloop3(void)
{
        int i;
        for (i=0;i<100;i++)
        {
                while (!value)
                {
                foo();
                }
        }
        
}

Here index starts at 0 and increments to 99.

Problem seems to be related to "maybe_multiple" being set in loop scan. However,
since 'i' is never used inside loop there would seem to be no need to check for
multiple setting.

This was tested with AVR target but looks like it will affect any target - I can
provide RTL etc on demand.

-- 
           Summary: Loop optimizer fails to reverse simple loop
           Product: gcc
           Version: 4.0.0
            Status: UNCONFIRMED
          Severity: normal
          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=19676