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Re: [RFA] [tree-optimization/80576] Handle non-constant sizes in DSE

On Fri, 16 Aug 2019, Jeff Law wrote:

On 8/16/19 12:09 PM, Marc Glisse wrote:
On Fri, 16 Aug 2019, Jeff Law wrote:

This patch improves our ability to detect dead stores by handling cases
where the size memcpy, memset, strncpy, etc call is not constant.  This
addresses some, but not all, of the issues in 80576.

The key here is when the size is not constant we can make conservative
decisions that still give us a chance to analyze the code for dead

Remember that for dead store elimination, we're trying to prove that
given two stores, the second store overwrites (partially or fully) the
same memory locations as the first store.  That makes the first store
either partially or fully dead.

When we encounter the first store, we set up a bitmap of bytes written
by that store (live_bytes).  We then look at subsequent stores and clear
the appropriate entries in the bitmap.

If the first store has a nonconstant length argument we can use the
range of the length argument (max) and the size of the destination
object to make a conservative estimation of how many bytes are written.

For the second store the conservative thing to do for a non-constant
length is to use the minimum of the range of the length argument.

So I guess it won't handle things like

void f(char*p,int n){

where we know nothing about the length, except that it is the same? Or
do you look at symbolic ranges?

So handling this was slightly uglier than I'd hoped.  I mis-remembered
what we had in an ao_ref.  We have a way to describe constant sizes and
to mark that something was non-constant (size of -1), but not what the
non-constant value was.

I looked at two approaches.  One created a dse_ref structure that had an
embedded ao_ref.  That creates a fair amount of churn.  But it certainly
looks do-able.

The second derived a dse_ref from an ao_ref which allows the vast
majority of the code in tree-ssa-dse.c to just work as-is.  With that in
place it was fairly simply to initialize the new field and check it in a
couple places.  Resulting in:

; Function f (f, funcdef_no=0, decl_uid=1909, cgraph_uid=1, symbol_order=0)

  Deleted dead call: __builtin_memset (p_5(D), 3, _1);

f (char * p, int n)
  long unsigned int _1;

;;   basic block 2, loop depth 0, maybe hot
;;    prev block 0, next block 1, flags: (NEW, VISITED)
  _1 = (long unsigned int) n_3(D);
  __builtin_memset (p_5(D), 7, _1);
;;    succ:       EXIT (EXECUTABLE)


Not sure how useful this will be in practice though.

Oh, I wasn't expecting you to handle it right now...

I've seen this kind of thing (not necessarily with memset, memcpy was probably involved as well) a few times, enough that it immediately came to mind when I saw the title of your message. There are probably traces in bugzilla, although I don't think there is a convenient search query to find them. I found PR 79716 that seems related at least, but not what I was looking for.

Marc Glisse

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