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

Pedro Alves
2010-06-16T19:16:32Z

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