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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug middle-end/114347] wrong constant folding when casting __bf16
 to int
Date: Fri, 15 Mar 2024 09:03:16 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D114347

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |wrong-code
                 CC|                            |jsm28 at gcc dot gnu.org

--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
I suspect we fail to round at parsing time.  I see we get to see

 <excess_precision_expr 0x7ffff6bb6500
    type <real_type 0x7ffff6a33888 __bf16 BF
        size <integer_cst 0x7ffff6a2d108 constant 16>
        unit-size <integer_cst 0x7ffff6a2d120 constant 2>
        align:16 warn_if_not_align:0 symtab:0 alias-set -1 canonical-type
0x7ffff6a33888 precision:16>

    arg:0 <real_cst 0x7ffff6be06f0
        type <real_type 0x7ffff6a332a0 float SF
            size <integer_cst 0x7ffff6a2d198 constant 32>
            unit-size <integer_cst 0x7ffff6a2d1b0 constant 4>
            align:32 warn_if_not_align:0 symtab:0 alias-set -1 canonical-ty=
pe
0x7ffff6a332a0 precision:32
            pointer_to_this <pointer_type 0x7ffff6a33930>>
        constant 2.57e+2>

and then upon processing the (int) cast simply strip the excess_precision_e=
xpr.

#0  convert (type=3D<integer_type 0x7ffff6a2b5e8 int>,=20
    expr=3D<real_cst 0x7ffff6be0930>)
    at /home/rguenther/src/trunk/gcc/c/c-convert.cc:253
#1  0x0000000000d8e9e2 in build_c_cast (loc=3D263296,=20
    type=3D<integer_type 0x7ffff6a2b5e8 int>, expr=3D<real_cst 0x7ffff6be09=
30>)
    at /home/rguenther/src/trunk/gcc/c/c-typeck.cc:6387
#2  0x0000000000d8f01c in c_cast_expr (loc=3D263296, type_name=3D0x46c6760,=
=20
    expr=3D<excess_precision_expr 0x7ffff6bb6640>)
    at /home/rguenther/src/trunk/gcc/c/c-typeck.cc:6461

/* Build an expression representing a cast to type TYPE of expression EXPR.
   LOC is the location of the cast-- typically the open paren of the cast. =
 */

tree
build_c_cast (location_t loc, tree type, tree expr)
{
  tree value;

  bool int_operands =3D EXPR_INT_CONST_OPERANDS (expr);

  if (TREE_CODE (expr) =3D=3D EXCESS_PRECISION_EXPR)
    expr =3D TREE_OPERAND (expr, 0);


But maybe that's correct behavior here?  Or we shouldn't have an
excess_precision_expr in the first place for BF16?=