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Re: [Qemu-devel] Stupid (probably) idea wrt dyngen & gcc 3.4 & 4.0

From: André Braga
Subject: Re: [Qemu-devel] Stupid (probably) idea wrt dyngen & gcc 3.4 & 4.0
Date: Sun, 8 May 2005 21:25:42 -0300


  inc ax
  dec ax
  inc ax
  dec ax

which is the same size as 4 nops (on x86 assembly), has a net result
of doing nothing (caveat interrupts/preemption), and is *absolutely
illogical* to find in any machine-generated code...

There must be some way to generate similar code on other supported
platforms (I mean: code that's extremely unlikely to be generated by a
machine but could be used as a sentinel code sequence to dyngen), but
feasibility considerations apart, I don't really think this is the
most elegant solution...

2005/5/8, Sebastian Kaliszewski <address@hidden>:
> Hello!
> As I understand the problem with dyngen & GCC 3.4 and newer is that even
> when using the following marcro (line 158 of dynget-exec.h) in op_*
> functions
> #define FORCE_RET() asm volatile ("");
> GCC still puts multiple exit points of a function.
> But did anyone try the following one:
> #define FORCE_RET() asm volatile ("" : : : "memory" );
> This tells GCC that that asm block clobbers arbitrary memory. If it doesnt
> help, then maybe putting few instructions will help (increasing the weight
> of the code thus convincing optimiser not to multiplicate the asm block)?
> #define FORCE_RET() asm volatile ("nop; nop; nop; nop" : : : "memory" );
> or
> #define FORCE_RET() asm volatile ("ret; ret; ret; ret" : : : "memory" );
> Then if the above fails, then simply search the binary code for such block
> of fout instructions (in case of nops it'd be 0x90909090, in case of ret's
> -- don't remember). It's rather impossible, that such immediate value would
> apear inside op_* code, so the only real possibility is FORCE_RET()
> occurence (Ther is also slim possibility that such code would be an align
> fill block -- but AFAIR gcc is instructed ther not to align code and AFAIR
> gcc would use block of 4 one byte nops -- it will use longer nops in such
> cases). So then replacing such nops with jumps to end inside blocks is
> trivial.
> What do you think?
> rgds
> Sebastian Kaliszewski

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