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From: "amacleod at redhat dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/104530] [12 Regression] Dead Code Elimination
 Regression at -O3 (trunk vs. 11.2.0)
Date: Tue, 15 Feb 2022 23:14:44 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D104530

--- Comment #2 from Andrew Macleod <amacleod at redhat dot com> ---
mmm. tricky.=20=20

Imports: b.0_1  d.3_7
Exports: b.0_1  _2  _3  d.3_7  _8
         _2 : b.0_1(I)
         _3 : b.0_1(I)  _2
         _8 : b.0_1(I)  _2  _3  d.3_7(I)

    <bb 2> :
    b.0_1 =3D b;
    _2 =3D b.0_1 =3D=3D 0B;
    _3 =3D (int) _2;
    c =3D _3;
    _5 =3D *b.0_1;        <<-- from this point b.0_1 is [+1, +INF]
    a =3D _5;
    d.3_7 =3D d;
    _8 =3D _3 % d.3_7;
    if (_8 !=3D 0)
      goto <bb 6>; [INV]
    else
      goto <bb 5>; [INV]

This is a bit at conflict with the "lets be safe" approach without pre-scan=
ning
the block for lack of throwing stmts.   Although that's really just a half
measure since you can construct more complicated blocks with a mix.

Im experimenting with marking values as stale for all exports of the block
which are dependant on the value we set to non null. ie, _2, _3 and _8=20

Then when we ask for a range of that name AFTER the de-reference, it will
recompute it.  The use of _3 in computing _8 is then "stale", and will be
re-evaluated.

Simple in concept, but I'm encountering an interaction with how we currently
set the global values for the end of the pass, and although I can recompute=
 _2
fine, it is also setting the global value of _2 to [0,0], which is not what=
 we
want.  Works for this case, but returns us to the same situation we had bef=
ore
if there is a call in the block.

Im continuing to look at it.  Easy fix in the next release since we are goi=
ng
to change the way we export globals outside the pass... but we shall see ab=
out
this release whether I can keep it contained.=