On Fri, 2014-09-05 at 12:45 -0600, Jeff Law wrote:
> On 09/04/14 14:04, David Malcolm wrote:
> > On Tue, 2014-09-02 at 19:50 +0200, Andi Kleen wrote:
> >>> I suspect the bulk of them currently are coming from the safe_as_a
> >>> <rtx_insn *> calls within NEXT_INSN and PREV_INSN; do you happen to have
> >>> information handy on that?
> >>
> >> Yes that's right:
> >>
> >> - 1.03% lto1 [.] bool
> >> is_a_helper<rtx_insn*>::test<rtx_def>(rtx_def*)
> >> ▒
> >> - bool is_a_helper<rtx_insn*>::test<rtx_def>(rtx_def*)
> >> ▒
> >> - 92.20% bool is_a<rtx_insn*, rtx_def>(rtx_def*)
> >> ▒
> >> - 98.53% rtx_insn* safe_as_a<rtx_insn*, rtx_def>(rtx_def*)
> >> ▒
> >> - 73.28% NEXT_INSN(rtx_insn const*)
> >> ▒
> >
> > The is_a_helper for rtx_insn * is non-trivial, so it may be worth
> > avoiding it, even when inlined.
> >
> > The attached patch rewrites the inline NEXT_INSN/PREV_INSN to avoid
> > doing the safe_as_a, instead tightening up the interface so that one can
> > only set them to an insn, and introducing a new XINSN access macro and
> > corresponding rt_insn member of the union.
> >
> > Bootstrapped on x86_64-unknown-linux-gnu (Fedora 20), and has been
> > rebuilt as part of a config-list.mk build for all working configurations
> > (albeit with other patches for the latter case).
> >
> > OK for trunk?
> So is this just to deal with the overhead in the safe_as_a helper until
> we can strengthen more code? And is that overhead significant in an
> optimized build?
>
> Would an alternate approach be to make the checking in safe_as_a
> conditionalized on ENABLE_CHECKING?
I wrote the attached patch to do that, but I don't yet have numbers to
back it up.
Bootstrapped with current settings, and smoketested with
--enable-checking=release (both on Fedora 20 x86_64).
>From 24b98616c4425a8ef380d2d5fef00f82af9df985 Mon Sep 17 00:00:00 2001
From: David Malcolm <dmalc...@redhat.com>
Date: Fri, 29 Aug 2014 17:04:49 -0400
Subject: [PATCH] Ensure that safe_as_a can have no performance overhead in a
release build
gcc/
* is-a.h (safe_as_a): Eliminate the conditional from the
!defined(ENABLE_CHECKING) case, to ensure there can be no
performance degradation of the release build.
---
gcc/is-a.h | 4 ++++
1 file changed, 4 insertions(+)
diff --git a/gcc/is-a.h b/gcc/is-a.h
index 176066b..f0c05b0 100644
--- a/gcc/is-a.h
+++ b/gcc/is-a.h
@@ -200,6 +200,7 @@ template <typename T, typename U>
inline T
safe_as_a (U *p)
{
+#ifdef ENABLE_CHECKING
if (p)
{
gcc_checking_assert (is_a <T> (p));
@@ -207,6 +208,9 @@ safe_as_a (U *p)
}
else
return NULL;
+#else
+ return is_a_helper <T>::cast (p);
+#endif
}
/* A generic checked conversion from a base type U to a derived type T. See
--
1.8.5.3
