Il 27/04/2012 13:16, Richard Guenther ha scritto:
> In optabs.c we compare the CTZ_DEFINED_VALUE_AT_ZERO against two,
> is != 0 really what you want here? The docs suggest to me
> that as you are using the optab below you should compare against two, too.
Interesting, first time I hear about this...
... except that no target sets the macros to 2, and all of them could
(as far as I could see). Looks like the code trumps the documentation;
how does this look?
2012-04-27 Paolo Bonzini <bonz...@gnu.org>
* optabs.c (expand_ffs): Check CTZ_DEFINED_VALUE_AT_ZERO
against 1.
* doc/tm.texi (Misc): Invert meaning of 1 and 2 for
CLZ_DEFINED_VALUE_AT_ZERO and CTZ_DEFINED_VALUE_AT_ZERO.
Index: optabs.c
===================================================================
--- optabs.c (revisione 186903)
+++ optabs.c (copia locale)
@@ -2764,7 +2764,7 @@ expand_ffs
if (!temp)
goto fail;
- defined_at_zero = (CTZ_DEFINED_VALUE_AT_ZERO (mode, val) == 2);
+ defined_at_zero = (CTZ_DEFINED_VALUE_AT_ZERO (mode, val) == 1);
}
else if (optab_handler (clz_optab, mode) != CODE_FOR_nothing)
{
@@ -2773,7 +2773,7 @@ expand_ffs
if (!temp)
goto fail;
- if (CLZ_DEFINED_VALUE_AT_ZERO (mode, val) == 2)
+ if (CLZ_DEFINED_VALUE_AT_ZERO (mode, val) == 1)
{
defined_at_zero = true;
val = (GET_MODE_PRECISION (mode) - 1) - val;
Index: doc/tm.texi
===================================================================
--- doc/tm.texi (revisione 186903)
+++ doc/tm.texi (copia locale)
@@ -10640,9 +10640,9 @@
for @code{clz} or @code{ctz} with a zero operand.
A result of @code{0} indicates the value is undefined.
If the value is defined for only the RTL expression, the macro should
-evaluate to @code{1}; if the value applies also to the corresponding optab
+evaluate to @code{2}; if the value applies also to the corresponding optab
entry (which is normally the case if it expands directly into
-the corresponding RTL), then the macro should evaluate to @code{2}.
+the corresponding RTL), then the macro should evaluate to @code{1}.
In the cases where the value is defined, @var{value} should be set to
this value.
plus tweaking this patch to reject 2 as well.
> What about cost considerations? We only seem to have the general
> "branches are expensive" metric - but ctz/clz may be prohibitely expensive
> themselves, no?
Yeah, that's a general problem with this kind of tricks. In general
however clz/ctz is getting less and less expensive, so I don't think
it is worrisome (at least at the beginning of stage 1). We can add
rtx_costs checks later.
Among architectures I know, only i386 has an expensive bsf/bsr but
it also has sete/setne which GCC will use instead of this trick.
Looking at rtx_costs, nothing seems to mark clz/ctz as prohibitively
expensive (Xtensa does, but only in the case when the optab handler
will not exist). I realize though that this is not a particularly
good statistic, since the compiler would not generate them out of
its hat until now.
Paolo
