[Bug c/54888] New: GCC with -Os is faster than -O3 on some AVR code

[mojo at world3 dot net]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54888



             Bug #: 54888

           Summary: GCC with -Os is faster than -O3 on some AVR code

    Classification: Unclassified

           Product: gcc

           Version: 4.3.3

            Status: UNCONFIRMED

          Severity: normal

          Priority: P3

         Component: c

        AssignedTo: unassig...@gcc.gnu.org

        ReportedBy: m...@world3.net





Created attachment 28411

  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=28411

Compiler output



I am using AVR-GCC to write some very low power RTC related code. The interrupt

"ISR(RTC_OVF_vect)" executes faster with -Os optimization than it does with

-O1, -O2 or -O3. I have measured execution time on an oscilloscope to confirm.



V4.3.3 is the one that comes with Atmel Studio / WinAVR. Command line:



avr-gcc -funsigned-char -funsigned-bitfields -DF_CPU=8000000UL  -O3

-fpack-struct -fshort-enums -g2 -Wall -c -std=gnu99 -MD -MP -MF "rtc.d"

-MT"rtc.d" -MT"rtc.o"  -mmcu=atxmega128d3   -o"rtc.o" ".././rtc.c"



I don't get any warnings etc. when compiling. Build machine is Windows 7 x64.

Target is an XMEGA128D3, same issue confirmed with the 128A3U (unsurprisingly).



The problem appears to be with GCC, rather than avr-libc, but please correct me

if I am wrong.