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

[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: 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.

[Bug c/54888] 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

--- Comment #1 from mojo at world3 dot net 2012-10-10 14:51:26 UTC ---
Created attachment 28412
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=28412
Compiler output with -O3

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

[gjl at gcc dot gnu.org]

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

Georg-Johann Lay <gjl at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Target|                            |avr
           Priority|P3                          |P4
             Status|UNCONFIRMED                 |WAITING
   Last reconfirmed|                            |2012-10-21
          Component|c                           |target
     Ever Confirmed|0                           |1

--- Comment #2 from Georg-Johann Lay <gjl at gcc dot gnu.org> 2012-10-21 20:40:45 UTC ---
atxmega128d3 is not supported by avr-gcc, neither in 4.3 nor in 4.4 nor 4.5 nor
4.6.  Please report to the respective bug tracker, obviously some private
toolchain port.

Compiling with a current compiler that supports ATxmega128D3 (4.7 or newer)
stops compilation with errors.

And I actually don't understand teh issue: Optimizing for size does not require
to produce slow code.  The code may run fast.

If your program relies on slow executable code, the program is incorrect.

[Bug target/54888] 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

--- Comment #3 from mojo at world3 dot net 2012-10-22 12:40:57 UTC ---
(In reply to comment #2)

> And I actually don't understand teh issue: Optimizing for size does not require
> to produce slow code.  The code may run fast.

-O3 is supposed to produce the fastest possible code, but it doesn't. -Os is
faster. At the very least the two should be equal.

In other words -O3 is broken.

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

[rguenth at gcc dot gnu.org]

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

--- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> 2012-10-22 12:56:23 UTC ---
(In reply to comment #3)
> (In reply to comment #2)
> 
> > And I actually don't understand teh issue: Optimizing for size does not require
> > to produce slow code.  The code may run fast.
> 
> -O3 is supposed to produce the fastest possible code, but it doesn't. -Os is
> faster. At the very least the two should be equal.

Supposed to?  Where in the documentation is that specified?  I remember
a sentence that -O3 enables optimization that might not always be
profitable (but that sentence seems to be gone from latest docs).

> In other words -O3 is broken.

It's behavior is certainly undesirable, but broken?  For certain targets
-Os might be a win because that's what it is tuned for or icache behavior
is simply more important than anything else.

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

[gjl at gcc dot gnu.org]

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

Georg-Johann Lay <gjl at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |gjl at gcc dot gnu.org

--- Comment #5 from Georg-Johann Lay <gjl at gcc dot gnu.org> 2012-10-22 20:10:44 UTC ---
As a start, you could try to enable us to reproduce your problem.

First of all, it is clear that we don't have your hardware (oscilloscope) to
measure things and even if, it is very unlikely someone will start research to
find out exactly were you lost the ticks.

Second, notice that it is ulikely anybody is inclined to pick up buch of code
you dumped above. It's 3800 lines and around 30 functions.  And it fails to
compile.  Maybe you can be more descriptive and point out what /exactly/ goes
wrong and work out a small example and limit to a critical spot or function and
throw away unneeded stuff.

Third, please notice that 4.3 is no more supported since several years now. 
Please supply code that compiles with a supported version of the compiler which
implies at least 4.7 (because you use -mmcu=atxmega128d3).

Fourth, you use inline assembler that is not correct because of missing memory
barrier and might show malfunction in corner cases.

Thus, you may want to fix at least 3. and 4. and rerun your benchmarks to see
if the problem still exists.  Very likely, that is not the case.

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

[gjl at gcc dot gnu.org]

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

Georg-Johann Lay <gjl at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|WAITING                     |RESOLVED
                 CC|gjl at gcc dot gnu.org      |
         Resolution|                            |INVALID

--- Comment #6 from Georg-Johann Lay <gjl at gcc dot gnu.org> 2013-02-02 14:21:27 UTC ---
Closed as invalid. No answer and no valid test case for over 3 months now.