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.
On Mon, 2006-09-18 at 13:28 -0700, Vara Prasad wrote:
Alan Cox wrote:
This still doesn't solve the problem of compiler optimizing such that a
variable i would like to read in my probe not being available at the
probe point.
Then what we really need by the sound of it is enough gcc smarts to do
something of the form
.section "debugbits"
.asciiz 'hook_sched'
.dword l1 # Address to probe
.word 1 # Argument count
.dword gcc_magic_whatregister("next"); [ reg num or memory ]
.dword gcc_magic_whataddress("next"); [ address if exists]
Can gcc do any of that for us today ?
No, gcc doesn't do that today.
---- cut here ----
#include <stdio.h>
#define MARK(label, var) \
asm ("debug_" #label ":\n" \
".section .data\n" \
#label "_" #var ": xor %0,%0\n" \
".previous" : : "r"(var))
That's a nice idea. As Frank pointed out, it does force things into
register. You could use "rm" as a constraint, so you can also get the
location wherever it exists. It will still force gcc into keeping the
value around at all, but presumably if its interesting for a mark, its
interesting to keep:
asm volatile("..." \
#label "_" #var ": mov %0,%%eax\n" \
".previous" : : "rm" (var))
and, aside from the naming issues, it could be a general expression
rather than a specific variable.
Of course, this requires a more complex addressing mode decoder, but it
does give gcc more flexibility. And in principle this is all redundant,
since DWARF should be able to encode all this too, and if you make use
of the variable as an asm argument, gcc really should be outputting the
debug info about it.
J
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