[Bug c/48197] possible wrong code bug at -O0

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

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48197

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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                 CC|                            |jakub at gcc dot gnu.org

--- Comment #7 from Jakub Jelinek <jakub at gcc dot gnu.org> 2011-03-18 20:54:07 UTC ---
I think the bug is in shorten_compare.
We are called for GT_EXPR with op0 0LL and op1 (unsigned) (short) ((short) 0 ^
(short) y)
with long long result type. ^ above is BIT_XOR_EXPR with short type (shortened
earlier, but that's fine, (unsigned) (short) ((short) 0 ^ (short y)) is still
(unsigned) (short) y, i.e. sign extending y from 16 bits to 32 bits.  In the
above the (short) cast doesn't really exist, it is simply a NOP_EXPR with
unsigned int type of BIT_XOR_EXPR with short int type.
get_narrower gives us 0LL (no change, unsignedp0 set to 0) and the short int
BIT_XOR_EXPR (again, with unsignedp1 set to 0).  That is IMHO also fine,
unsignedp1 says that the BIT_XOR_EXPR needs to be sign extended to the type.

But in the end shorten_compare says that the comparison just should be done in
short int, which is wrong.