Re: adding -fnoalias ... would a patch be accepted ?

[torbenh <torbenh at gmx dot de>]

On Wed, Jan 06, 2010 at 04:25:59PM +0100, torbenh wrote:
> On Wed, Jan 06, 2010 at 02:27:15PM +0100, Richard Guenther wrote:
> > On Tue, Jan 5, 2010 at 5:39 PM, torbenh <torbenh@gmx.de> wrote:
> > > On Tue, Jan 05, 2010 at 04:27:33PM +0100, Richard Guenther wrote:
> > >> On Tue, Jan 5, 2010 at 4:03 PM, torbenh <torbenh@gmx.de> wrote:
> > >> > On Tue, Jan 05, 2010 at 02:46:30PM +0100, Richard Guenther wrote:
> > >> >> On Tue, Jan 5, 2010 at 2:40 PM, torbenh <torbenh@gmx.de> wrote:
> > >> >>
> > >> >> The -fno-alias-X things do not make much sense for user code (they
> > >> >> have been historically used from Frontends). ?If restrict doesn't work
> > >> >> for you (do you have a testcase that can reproduce your issue?)
> > >> >> then you probably need to wait for IPA pointer analysis to be
> > >> >> fixed in GCC 4.6.
> > >> >
> > >> > sorry... forget the attachment :S
> > >>
> > >> Yes, in this case you can fix it by making ramp static. ?Otherwise its
> > >> address may be takein in another translation unit. ?For Fortran we
> > >> have the DECL_RESTRICTED_P which we could expose to other
> > >> languages via an attribute. ?It tells that a decl is not aliased by
> > >> restrict qualified pointers, so
> > >>
> > >> struct Ramp {
> > >> ? ? float phase;
> > >> ? ? inline float process() { return phase++; }
> > >> } ramp __attribute__((restrict));
> > >>
> > >> void fill_buffer( float * __restrict buf, size_t nframes )
> > >> {
> > >> ? ? for( size_t i=0; i<nframes; i++ )
> > >> ? ? ? ? buf[i] = ramp.process();
> > >> }
> > >
> > > would that also work with this stuff:
> > >
> > >
> > > template<typename ... Args>
> > > class Mixer;
> > >
> > > template<typename T1, typename ... Args>
> > > class Mixer<T1, Args...> : public Block
> > > {
> > > ? ?private:
> > > ? ? ? ?T1 t1 __attribute__((restrict));
> > > ? ? ? ?Mixer<Args...> t2;
> > > ? ?public:
> > > ? ? ? ?inline float process() {
> > > ? ? ? ? ? ?return t1.process() + t2.process();
> > > ? ? ? ?}
> > > };
> > >
> > > template<typename T1, typename T2>
> > > class Mixer<T1,T2> : public Block
> > > {
> > > ? ?private:
> > > ? ? ? ?T1 t1 __attribute__((restrict));
> > > ? ? ? ?T2 t2 __attribute__((restrict));
> > > ? ?public:
> > > ? ? ? ?inline float process() {
> > > ? ? ? ? ? ?return t1.process() + t2.process();
> > > ? ? ? ?}
> > > };
> > >
> > > Mixer<Ramp,Ramp,Ramp,Ramp> mix __attribute__((restrict))
> 
> void fill_buffer( float * __restrict buf, size_t nframes )
> {
> ? ? for( size_t i=0; i<nframes; i++ )
> ? ? ? ? buf[i] = mix.process();
> }
> 
> there is your pointer :)
> 
> > >
> > > ?
> > 
> > I don't see a restrict qualified pointer here.  Note that the
> > restrict attribute would only disambiguate against those.
> > Also I think you need the restrict attribute on the Mixer
> > objects, not its members.
> 
> so the attribute would promote down to all member vars of 
> member objects ?
> 
> 
> > > i still dont understand whats the problem with -fnolias,
> > > as in attached patch.
> > 
> > The patch will miscompile everything.
> 
> point taken. 
> obviously reading code for a few hours without knowing enough about the
> code isnt enough :)
> 
> __attribute__((restrict)) is the better solution.
> although not portable to other compilers. 
> 
> but i need this kind of functionality now, to test my concepts. 
> thats why i am spending a bit time on this. 
> 
> when do you plan to add this feature ?
> since you know the code, there would be no point for me to tackle
> it if you do it soonish. 
> 
> (and you dont need to deal with dumb patches from me :)
> 
> 
> speaking of dumb patches:
> 
> would you care to comment this patch for gcc-4.4 ?
> this one seems to work for simple examples.

meh... i always forget attachments :(
> 
> -- 
> torben Hohn

-- 
torben Hohn

diff --git a/gcc/common.opt b/gcc/common.opt
index 023d773..e02db8a 100644
--- a/gcc/common.opt
+++ b/gcc/common.opt
@@ -790,6 +790,10 @@ fnon-call-exceptions
 Common Report Var(flag_non_call_exceptions) Optimization
 Support synchronous non-call exceptions
 
+fnoalias
+Common Report Var(flag_noalias) Optimization
+Assume no aliasing is happening
+
 fomit-frame-pointer
 Common Report Var(flag_omit_frame_pointer) Optimization
 When possible do not generate stack frames
diff --git a/gcc/tree-ssa-alias.c b/gcc/tree-ssa-alias.c
index 4dd4fb7..35b6a99 100644
--- a/gcc/tree-ssa-alias.c
+++ b/gcc/tree-ssa-alias.c
@@ -2430,8 +2430,9 @@ compute_flow_insensitive_aliasing (struct alias_info *ai)
 
       FOR_EACH_REFERENCED_VAR (var, rvi)
 	{
-	  if (var_ann (var)->is_heapvar)
-	    add_may_alias (tag, var);
+	  if (!flag_noalias)
+	    if (var_ann (var)->is_heapvar)
+	      add_may_alias (tag, var);
 	}
     }
 
@@ -2949,6 +2950,9 @@ may_alias_p (tree ptr, alias_set_type mem_alias_set,
       return false;
     }
 
+  if (flag_noalias)
+    return false;
+
   /* If -fargument-noalias-global is > 2, pointer arguments may
      not point to anything else.  */
   if (flag_argument_noalias > 2 && TREE_CODE (ptr) == PARM_DECL)