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

[Pedro Alves <pedro@codesourcery.com>]

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