[Bug middle-end/85720] bad codegen for looped assignment of primitives at -O2

[redbeard0531 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85720

--- Comment #4 from Mathias Stearn  ---
(In reply to Marc Glisse from comment #3)
> Again, you are ignoring aliasing issues (just like in your other PR the
> function copy isn't equivalent to memmove). Does adding __restrict change
> the result? Also, B[i]=B[i]+1 doesn't look like a memset...

Sorry, I typoed. It was supposed to be B[i] = A[i] + 1. That still does
basically the same thing though: https://godbolt.org/g/dtmU5t. Good point about
aliasing though. I guess the right code gen in that case would actually be
something that detected the overlap and did the right calls to memset to only
set each byte once. Or just do the simple thing:

if (b > a && b < a + n) {
  memset(b, 1, n);
  memset(a, 0, n);
} else {
  memset(a, 0, n);
  memset(b, 1, n);
}

Yes, __restrict helps, but that isn't part of standard c++, and it seems like
it never will be.

[Bug middle-end/85720] bad codegen for looped assignment of primitives at -O2

[glisse at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85720

--- Comment #3 from Marc Glisse  ---
(In reply to Mathias Stearn from comment #2)
> Hmm. Taking the example from the -ftree-loop-distribute-patterns
> documentation, it still seems to generate poor code, this time at both -O2
> and -O3: https://godbolt.org/g/EsQDj8
> 
> Why isn't that transformed to memset(A, 0, N); memset(B, 1, N); ? This feels
> similar to https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85721. Should I make
> a new ticket with this example?

Again, you are ignoring aliasing issues (just like in your other PR the
function copy isn't equivalent to memmove). Does adding __restrict change the
result? Also, B[i]=B[i]+1 doesn't look like a memset...

[Bug middle-end/85720] bad codegen for looped assignment of primitives at -O2

[redbeard0531 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85720

--- Comment #2 from Mathias Stearn  ---
Hmm. Taking the example from the -ftree-loop-distribute-patterns documentation,
it still seems to generate poor code, this time at both -O2 and -O3:
https://godbolt.org/g/EsQDj8

Why isn't that transformed to memset(A, 0, N); memset(B, 1, N); ? This feels
similar to https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85721. Should I make a
new ticket with this example?