Re: [NEWS/RFA] Re: [gdbserver] x86 agent expression bytecode compiler (speed up conditional tracepoints)

[Pedro Alves <pedro@codesourcery.com>]

On Wednesday 16 June 2010 20:12:44, Stan Shebs wrote:
> Doug Evans wrote:
> > On Mon, Jun 14, 2010 at 3:19 PM, Pedro Alves <pedro@codesourcery.com> wrote:
> >   
> >> Thanks.  I've checked the whole thing in.
> >>     
> >
> > I'm getting build failures that go away when compiling linux-x86-low.c with -g.
> >
> > gcc is optimizing out the skipped over stuff I believe.
> >   
> 
> Hmm, this particular trickery was my idea, but it always seemed 
> vulnerable to ever-smarter optimization.  Does anybody have any better 
> strategy?  Declaring the asm volatile didn't work, because the compiler 
> was whacking everything around it.
> 
> The other approach is to build up instructions from bitfields, which is 
> reliable but needs a lot of setup and helper routines.

Quick thought: can we stick a couple of __attribute__((used))'s in the
macro, so the compiler doesn optimized things away, thinking they're
unused (given the uses are behind asm)?

> 
> Stan
> >
> > /* Our general strategy for emitting code is to avoid specifying raw
> >    bytes whenever possible, and instead copy a block of inline asm
> >    that is embedded in the function.  This is a little messy, because
> >    we need to keep the compiler from discarding what looks like dead
> >    code, plus suppress various warnings.  */
> >
> > #define EMIT_ASM(NAME,INSNS)                                            \
> >   { extern unsigned char start_ ## NAME, end_ ## NAME;                  \
> >     add_insns (&start_ ## NAME, &end_ ## NAME - &start_ ## NAME);       \
> >     if (always_true ())                                         \
> >       goto skipover ## NAME;                                            \
> >     __asm__ ("start_" #NAME ":\n\t" INSNS "\n\tend_" #NAME ":\n\t");    \
> >     skipover ## NAME:                                                   \
> >     ; }
> >
> >   
> 
> 


-- 
Pedro Alves