This was another PR caused by the way that
vect_determine_precisions_from_range handles shifts. We tried to
narrow 32768 >> x to a 16-bit shift based on range information for
the inputs and outputs, with vect_recog_over_widening_pattern
(after PR110828) adjusting the shift amount. But this doesn't
work for the case where x is in [16, 31], since then 32-bit
32768 >> x is a well-defined zero, whereas no well-defined
16-bit 32768 >> y will produce 0.
We could perhaps generate x < 16 ? 32768 >> x : 0 instead,
but since vect_determine_precisions_from_range was never really
supposed to rely on fix-ups, it seems better to fix that instead.
The patch also makes the code more selective about which codes
can be narrowed based on input and output ranges. This showed
that vect_truncatable_operation_p was missing cases for
BIT_NOT_EXPR (equivalent to BIT_XOR_EXPR of -1) and NEGATE_EXPR
(equivalent to BIT_NOT_EXPR followed by a PLUS_EXPR of 1).
pr113281-1.c is the original testcase. pr113281-[23].c failed
before the patch due to overly optimistic narrowing. pr113281-[45].c
previously passed and are meant to protect against accidental
optimisation regressions.
gcc/
PR target/113281
* tree-vect-patterns.cc (vect_recog_over_widening_pattern): Remove
workaround for right shifts.
(vect_truncatable_operation_p): Handle NEGATE_EXPR and BIT_NOT_EXPR.
(vect_determine_precisions_from_range): Be more selective about
which codes can be narrowed based on their input and output ranges.
For shifts, require at least one more bit of precision than the
maximum shift amount.