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GCC optimisation issues.

 
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pixel



Joined: 30 Jan 2004
Posts: 791
PostPosted: Mon Sep 20, 2004 5:12 pm    Post subject: GCC optimisation issues. Reply with quote
Hello,

I wasn't that much aware that the compiler's -O option was so buggy. That is true that I usually do not test my softwares using that option. Now, please, people, do NOT use -O at all :)

Even better: developp your softwares without any -O, and then, when it seems to be stable enough, retry with various -O settings (3 will most likely hit the 128-bits alignment bug, beware), and please report strange behaviors caused by optimisation option.

Thanks ;)
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Drakonite
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Joined: 17 Jan 2004
Posts: 989
PostPosted: Mon Sep 20, 2004 10:30 pm    Post subject: Reply with quote
On a project I'm working on right now...

-O2 works fine
-O1 works fine
-O0 TLB exception
no -O at all (is this same as -O0? I can't remember) TLB exception
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pixel



Joined: 30 Jan 2004
Posts: 791
PostPosted: Mon Sep 20, 2004 10:46 pm    Post subject: Reply with quote
From gcc's manual:

Code:

       -O1    Optimize.   Optimizing  compilation takes somewhat more time, and a lot more memory for a large func­
              tion.

              Without `-O', the compiler's goal is to reduce the cost of compilation and to make debugging  produce
              the  expected results.  Statements are independent: if you stop the program with a breakpoint between
              statements, you can then assign a new value to any variable or change the program counter to any oth­
              er statement in the function and get exactly the results you would expect from the source code.

              Without  `-O',  only  variables declared register are allocated in registers.  The resulting compiled
              code is a little worse than produced by PCC without `-O'.

              With `-O', the compiler tries to reduce code size and execution time.

              When you specify `-O', the two options `-fthread-jumps' and `-fdefer-pop' are turned on.  On machines
              that  have delay slots, the `-fdelayed-branch' option is turned on.  For those machines that can sup­
              port debugging even without a frame pointer, the `-fomit-frame-pointer' option is turned on.  On some
              machines other flags may also be turned on.

       -O2    Optimize  even  more.   Nearly all supported optimizations that do not involve a space-speed tradeoff
              are performed.  Loop unrolling and function inlining are not done, for example.  As compared  to  -O,
              this option increases both compilation time and the performance of the generated code.

       -O3    Optimize  yet more. This turns on everything -O2 does, along with also turning on -finline-functions.

       -O0    Do not optimize.


So, please, can you try compiling your software without stripping and without -O, and, run it until it fails, then, ee-objdump -dx on your elf file to read at the specified location by ps2link what is the faulty instruction. Bug me here or on IRC if you can't understand the asm code ;)
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tjd



Joined: 27 May 2004
Posts: 23
Location: Austin, TX
PostPosted: Wed Sep 22, 2004 5:03 am    Post subject: Reply with quote
On the IOP, with GCC3.2.2 I've encountered [recently-- like last week] a set of strangeness related to -O2. When I backed off to -O0, the code worked as expected. -O1 didn't work either. Basically, the address of variables seemed to get lost. I'm suspecting a register allocation problem (or perhaps somebody in the call chain is not saving/restoring registers). When I have a chance, I'll dig into it more...

I know, I know. Don't use GCC3.2.2 for the IO. It's broken... it'll never work... it'll eat your children... Well, up until now, it's been pretty good and does support C++ which gives me good compatibility with the EE. Sigh.
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pixel



Joined: 30 Jan 2004
Posts: 791
PostPosted: Wed Sep 22, 2004 5:10 am    Post subject: Reply with quote
Can you provide some code snipplet?
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