On Fri, 6 Oct 2006, Frank Ch. Eigler wrote:
> Hi -
>
> On Fri, Oct 06, 2006 at 01:33:11AM -0400, Steven Rostedt wrote:
> > Coming into this really late, and I'm still behind in reading this and
> > related threads, but I want to throw this idea out, and it's getting
> > late.
> > [...]
> > #define MARK(label, var) \
> > asm ("debug_" #label ":\n" \
> > ".section .data\n" \
> > #label "_" #var ": xor %0,%0\n" \
> > ".previous" : : "r"(var))
> > [...]
> > $ gcc -O2 -o mark mark.c
> > $ ./mark
> > func y is in reg B at 0x80483ce
> > [...]
>
> Clever.
>
> > Now the question is, isn't MARK() in this code a non intrusive marker?
>
> Not quite. The assembly code forces gcc to materialize the data that
> it might already have inlined, and to borrow a register for the
> duration. It's still a neat idea though.
Thanks!
You're right, it is intrusive in a way that it does modify the way gcc can
optimize that section of code. But what I like about this idea, is that
it allows for us to tell gcc that we want this variable inside a register,
and then gcc can do that for us and still optimize around that. We put no
more constraints on the code, except that we want some value in a
register at some given point of execution. This should only be done for
local variables that are not easily captured in a probe.
Of course with i386's limit on registers, it can put a little strain if we
want more than one variable. But x86_64 will soon be the norm, and the
added registers should help out a lot.
-- Steve
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