Re: debug evaluator is faster than regular one?

[Keisuke Nishida <kxn30 at po dot cwru dot edu>]

cwitty@newtonlabs.com (Carl R. Witty) writes:

> The Hugs implementation of Haskell uses a bytecode interpreter that
> uses "computed goto" when compiled with gcc and "switch" otherwise.
> It does this with a few lines of preprocessor hacks; the main body of
> the bytecode interpreter is shared between the two compilation
> variants.  You might want to look how they do it.

Ok, this is how they does:

--- aclocal.m4 ----
dnl check for gcc's "labels as values" feature
AC_DEFUN(AC_C_LABELS_AS_VALUES,
[AC_CACHE_CHECK([labels as values], ac_cv_labels_as_values,
[AC_TRY_COMPILE([
int foo(int);
int foo(i)
int i; { 
static void *label[] = { &&l1, &&l2 };
goto *label[i];
l1: return 1;
l2: return 2;
}
],
[int i;], 
ac_cv_labels_as_values=yes,
ac_cv_labels_as_values=no)])
if test "$ac_cv_labels_as_values" = yes; then
AC_DEFINE(HAVE_LABELS_AS_VALUES)
fi
])
-----------------------

---- configure.in ----
AC_C_LABELS_AS_VALUES    dnl can we use gcc's "labels as values" feature?
----------------------

---- config.h.in ----
/* Define if compiler supports gcc's "labels as values" (aka computed goto)
 * feature (which is used to speed up instruction dispatch in the interpreter).
 * Here's what typical code looks like:
 *
 * void *label[] = { &&l1, &&l2 };
 * ...
 * goto *label[i];
 * l1: ...
 * l2: ...
 * ...
 */
#define HAVE_LABELS_AS_VALUES 0
------------------

---- machine.c ----
#if     !DEBUG_CODE && HAVE_LABELS_AS_VALUES
#define Ins(x)          &&l##x
static  void *labs[] = { INSTRLIST };
#undef  Ins
#define Case(x)         l##x
#define Continue        goto *labs[(pc++)->instr]
#define Dispatch        Continue;
#define EndDispatch
#else
#if DEBUG_CODE
#define Dispatch        for (;;) {                                        \
			    if (debugCode) {                              \
				Printf("%*s0x%04X: ", root, "", pc-memory);  \
				dissInstr(pc-memory);                     \
			    }                                             \
			    switch((pc++)->instr) {
#else
#define Dispatch        for (;;) { switch((pc++)->instr) {
#endif
#define Case(x)         case x
#define Continue        continue
#define EndDispatch     default : internal("illegal instruction"); \
				  break;                           \
			}}
#endif
------------------

I'll copy this configuration test.

Thanks,
Keisuke Nishida