[Bug target/113847] [14 Regression] 10% slowdown of 462.libquantum on AMD Ryzen 7700X and Ryzen 7900X

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113847

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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                 CC|                            |hubicka at gcc dot gnu.org

--- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> ---
Hmm, the important one is actually MEM[ptr + CST] vs MEM[ptr].component.  But
those are not semantically equivalent, even when the same TBAA type is in
effect.

  _31 = MEM <int> [(struct quantum_reg *)reg_3(D)];
  _33 = MEM <int> [(struct quantum_reg *)reg_3(D) + 8B];
  _34 = MEM <struct quantum_reg_node *> [(struct quantum_reg *)reg_3(D) + 16B];
  _35 = MEM <int *> [(struct quantum_reg *)reg_3(D) + 24B];
  out = quantum_state_collapse.isra (pos_1(D), result_22, _31, _32, _33, _34,
_35); [return slot optimization]

this is from inlined quantum_state_collapse where IPA SRA is eventually
applied producing the above.

That we do produce those might hint at that we can't really assume the
dynamic type quantum_reg is at offset 8 but that was the original intent.
What we are left with is the special-case where typeof (MEM[ptr + CST])
== typeof (alias-pointed-to-type) (with CST == 0).  For any other case
what we know is only that the access MEM[ptr + CST] is to somewhere
inside an object of dynamic type quantum_reg?

I'm not sure that's not less than we make use of in the alias-oracle,
esp. aliasing_component_refs_walk and friends?  We might be fine in
practice for "bare" MEM_REFs like the above, but if we ever fold only
part of the access path into the constant offset funny things may happen?

So I think IPA SRA does wrong here (and maybe GCC in other places as well),
possibly only pessimizing and possibly creating latent wrong-code.
Note quantum_state_collapse has

  reg$size_62 = reg.size;
  reg$node_75 = reg.node;
...

pre-IPA.

Honza, any opinion?