[Bug c++/106654] [C++23] P1774 - Portable assumptions

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106654

--- Comment #14 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Created attachment 53675
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=53675&action=edit
gcc13-pr106654-gimple-wip.patch

My current WIP patch for the handling of more complex assumptions.

My current testcase is for -O2:
int
foo (int x)
{
  [[assume (x == 42)]];
  return x;
}

int
bar (int x)
{
  [[assume (++x == 43)]];
  return x;
}

int
baz (int x)
{
  [[assume (({ int z = ++x; if (z == 51) return -1; if (z == 53) goto lab1; z
== 43; }))]];
lab1:
  return x;
}

Before IPA, that is roughly how I'd like it to look like.

What still needs to be done is:
1) mark the assumption functions somehow and arrange for not being
   actually expanded into RTL, just the bodies kept around for optimization
   of other functions
2) for LTO ideally ensure they are duplicated in every partition that
   refers to them
3) teach ranger to get something useful ouf ot it
4) perhaps for later, decide what to do e.g. for SRA, so that passing an
   argument by value to .ASSUME doesn't prevent its SRA optimization
5) guess see if one can somehow bypass fab pass (-fdisable-tree-fab doesn't
seem
   to work) and if yes, don't ICE if .ASSUME call makes it down to expansion,
   instead fold it away right before or during expansion such that args
   don't need to be evaluated)

Anyway, I must be doing something wrong, because on the above testcase I get
ICE in:
during RTL pass: expand
a.C: In function ‘_Z3bari._assume.0’:
a.C:11:3: internal compiler error: in adjust_one_expanded_partition_var, at
cfgexpand.cc:1577
   11 |   [[assume (++x == 43)]];
      |   ^~~~~~~~~~~~~~~~~~~~~~
0x99d1ba adjust_one_expanded_partition_var
../../gcc/cfgexpand.cc:1577
0x9af44c execute
../../gcc/cfgexpand.cc:6737
where the name in question is default def for the argument - x_1(D):
bool _Z3bari._assume.0 (int x)
{
  bool _2;

  <bb 2> [local count: 1073741824]:
  _2 = x_1(D) == 42;
  return _2;

}