[Bug tree-optimization/57742] memset(malloc(n),0,n) -> calloc(n,1)

["glisse at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57742

--- Comment #4 from Marc Glisse <glisse at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #2)
> (In reply to Marc Glisse from comment #1)
> > This is a very limited version of this optimization. It is in
> > simplify_builtin_call, so only triggers if malloc/calloc is
> > SSA_NAME_DEF_STMT(gimple_vuse(memset_stmt)). However, generalizing it means
> > we would need plenty of tests protecting against cases where the
> > transformation would be wrong. Note that this transforms:
> > p=malloc(n);
> > if(cond)memset(p,0,n);
> > into:
> > p=calloc(n,1);
> > cond;
> > which is good if cond is p!=0 but may not always be so great otherwise.
> 
> ;)  post-dominator tests (or simply tests whether both calls are in the
> same basic-block ...).

Same basic block is quite limited, and for the condition below we don't
directly have post-domination, we would need post-domination between the bbs
with gimple_cond and malloc, and the bb of memset with the landing block of the
gimple_cond. But even finding the gimple_cond in: malloc; loop; cond; loop;
memset; can be hard. I guess I'll have to limit my expectations a bit...

> Also you can transform
> 
> p = malloc (n);
> if (p)
>   memset (p, 0, n);
> 
> which might be a common-enough case to optimize for.

Yes, that's the goal.

> dereferencing a double wouldn't have a VDEF (unless you store a double).

I do want to be able to store in between, so I think I have to walk the vdef
chain. But as soon as I do that, I need to make sure that the writes are to
places that can't alias, which complicates things a lot (and it can get a bit
expensive in a function with many memset). Consider this program:

#include <vector>
void f(void*p,int n){ new(p)std::vector<int>(n,0); }

With -O3, we end up with:

  _27 = operator new (_26);
  MEM[(struct _Vector_base *)p_4(D)]._M_impl._M_start = _27;
  MEM[(struct _Vector_base *)p_4(D)]._M_impl._M_finish = _27;
  _16 = _27 + _26;
  MEM[(struct _Vector_base *)p_4(D)]._M_impl._M_end_of_storage = _16;
  __builtin_memset (_27, 0, _26);

which has memory stores between the allocation and memset. That's exactly the
type of code where I'd want the optimization to apply. Joost's example has the
same pattern: malloc, test for 0, several unrelated memory stores, memset.

(how to handle the fact that we have operator new and not malloc is a different
issue, I am thinking of having a mode/flag where we promise not to replace
operator new so it can be inlined, which will include an if(p!=0) test)

It would be great (in particular for application-specific plugins) to have an
easy way to say things like: this is the next read/write use of this memory
region (but other memory regions may be used in between), and it isn't
post-dominated only because of this gimple_cond, etc. It's almost noon, too
late to be dreaming ;-)