https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99919
Aldy Hernandez <aldyh at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |aldyh at gcc dot gnu.org
--- Comment #3 from Aldy Hernandez <aldyh at gcc dot gnu.org> ---
(In reply to Martin Sebor from comment #1)
> The IL below shows there is enough information in the IL for the warning to
> determine that x_5 is never read from.
>
> void g (struct B b)
> {
> _Bool b$j;
> _Bool b$i;
> _Bool x;
> unsigned char _1;
> unsigned char _2;
> unsigned char _3;
> unsigned char _4;
>
> <bb 2> [local count: 1073741824]:
> b$i_11 = b.i;
> _1 = VIEW_CONVERT_EXPR<unsigned char>(b);
> _2 = _1 & 1;
> if (_2 != 0)
> goto <bb 4>; [50.00%]
> else
> goto <bb 3>; [50.00%]
>
> <bb 3> [local count: 536870912]:
>
> <bb 4> [local count: 1073741824]:
> # x_5 = PHI <x_8(D)(2), b$i_11(3)>
> # b$j_10 = PHI <0(2), b$i_11(3)>
> b.j = b$j_10;
> _3 = VIEW_CONVERT_EXPR<unsigned char>(b);
> _4 = _3 & 2;
> if (_4 != 0)
> goto <bb 5>; [50.00%]
> else
> goto <bb 6>; [50.00%]
>
> <bb 5> [local count: 536870913]:
> z = x_5;
>
> <bb 6> [local count: 1073741824]:
> return;
>
> }
The warning on the above IL seems legit.
x_5 is initialized from b$i_11 when b & 1 == 0, but the read from x_5 happens
when b & 2 != 0. So the set and the read are predicated on two different
things.
Maybe I'm missing some subtle bit field thing. Does setting b.j somehow alter
b.i?
