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

Pedro Alves
2010-06-20T22:27:03Z

On Thursday 17 June 2010 00:57:52, Ian Lance Taylor wrote:
> Doug Evans <dje@google.com> writes:
>
> > This worked with the gcc4.4.0 variant I was using.
> >
> > Ian: Is there a Right way to do this?
>
> I don't know about a right way, but since this code is both gcc and
> x86 specific, I think I would just write something along the lines of
>
> #define EMIT_ASM(NAME, INSNS) \
> do \
> { \
> extern unsigned char start_ ## NAME, end_ ## NAME; \
> add_insns (&start_ ## NAME, &end_ ## NAME - &start_ ## NAME); \
> asm ("jmp end_" NAME "\; start_" NAME ":\;" INSNS "\; end_" NAME ":"); \
> } while (0)

Thanks, works, nice and simple. I've applied the patch below.

--
Pedro Alves

2010-06-20 Ian Lance Taylor <iant@google.com>
Pedro Alves <pedro@codesourcery.com>

* linux-x86-low.c (always_true): Delete.
(EMIT_ASM, EMIT_ASM32): Use an uncondition asm jmp instead of
trying to fool the compiler with always_true.

---
gdb/gdbserver/linux-x86-low.c | 46 +++++++++++++++++++-----------------------
1 file changed, 21 insertions(+), 25 deletions(-)

Index: src/gdb/gdbserver/linux-x86-low.c
===================================================================
--- src.orig/gdb/gdbserver/linux-x86-low.c 2010-06-20 22:43:08.000000000 +0100
+++ src/gdb/gdbserver/linux-x86-low.c 2010-06-20 22:59:05.000000000 +0100
@@ -1484,41 +1484,37 @@ add_insns (unsigned char *start, int len
current_insn_ptr = buildaddr;
}

-/* A function used to trick optimizers. */
-
-int
-always_true (void)
-{
- return 1;
-}
-
/* 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: \
- ; }
-
+#define EMIT_ASM(NAME, INSNS) \
+ do \
+ { \
+ extern unsigned char start_ ## NAME, end_ ## NAME; \
+ add_insns (&start_ ## NAME, &end_ ## NAME - &start_ ## NAME); \
+ __asm__ ("jmp end_" #NAME "\n" \
+ "\t" "start_" #NAME ":" \
+ "\t" INSNS "\n" \
+ "\t" "end_" #NAME ":"); \
+ } while (0)

#ifdef __x86_64__

#define EMIT_ASM32(NAME,INSNS) \
- { extern unsigned char start_ ## NAME, end_ ## NAME; \
- add_insns (&start_ ## NAME, &end_ ## NAME - &start_ ## NAME); \
- if (always_true ()) \
- goto skipover ## NAME; \
- __asm__ (".code32\n\tstart_" #NAME ":\n\t" INSNS "\n\tend_" #NAME ":\n" \
- "\t.code64\n\t"); \
- skipover ## NAME: \
- ; }
+ do \
+ { \
+ extern unsigned char start_ ## NAME, end_ ## NAME; \
+ add_insns (&start_ ## NAME, &end_ ## NAME - &start_ ## NAME); \
+ __asm__ (".code32\n" \
+ "\t" "jmp end_" #NAME "\n" \
+ "\t" "start_" #NAME ":\n" \
+ "\t" INSNS "\n" \
+ "\t" "end_" #NAME ":\n" \
+ ".code64\n"); \
+ } while (0)

#else