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Subject: [Bug c/54888] New: GCC with -Os is faster than -O3 on some AVR code
Date: Wed, 10 Oct 2012 14:51:00 -0000
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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: unassigned@gcc.gnu.org
        ReportedBy: mojo@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.