Slowdowns in code generated by GCC>=3.3

Földy Lajos
Wed Oct 20 17:26:00 GMT 2004

> Hi,
> I am a developer of a bytecode emulator for the Prolog language. With 
> the release of GCC-3.3, our emulator was slowed down by a factor of 3 on 
> x86 with -O3 turned on (we didn't measure other platforms; the 
> optimization flag doesn't seem to matter). We were hoping this was a 
> temporary issue, but the situation didn't improve in any of the newer 
> releases :( I don't know whether i should file this as a bug report, so 
> i first ask for advice her.
> I'll try to explain on a high level what happens. If this isn't 
> sufficient, i can try to give some code, but this will take me some time
> to isolate the code. This is the situation:
> - Since the program counter in our emulator is very crucial, we use the 
>   'register' and 'asm ("bx")' hints.
> - For each instruction in the bytecode, we store the address of the 
>   label of the code which has to be executed for the instruction. 
>   Therefore, the program counter always contains points to an address of
>   code to be executed, and after each instruction we do a 
>         goto  **(void **)program_counter
> Previous versions of GCC keep the program counter in ebx, and do a 
> jmp *(%ebx) after the instructions (as expected). The newer GCCs seem
> to unnecessarily move the program counter around between registers, and
> don't do the jmp*(%ebx) after each instruction, but seem to jump to a
> 'common' piece of code doing this jump.
> Looking at the changelog of gcc-3.3, i can only deduce this has to do 
> with the new DFA scheduler, but of course i can not tell for sure.
> I don't know if any of this information is useful, but we could use some 
> pointers in places to look where things are going wrong in the code 
> generation. The factor 3 of slowdown is really a lot.
> Does anyone have any ideas ?
> thanks a lot,
> Remko


not portable, but on i386 you can try using the good old inline assembly: 

	void* pc;
	__asm__("jmp *%0" : : "a" (pc));

best regards,
lajos foldy

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