On Fri, 9 Dec 2016, Jakub Jelinek wrote:
This patch fixes the recently added A < 0 ? C : 0 for power of 2 C
optimization. The if (!TYPE_UNSIGNED (TREE_TYPE (@0))) part is just
for safety, I'd hope that unsigned < 0 is folded into 0 earlier, but just
in case this is handled first.
The issue which breaks the testcase is that the patch actually assumed
that C has type narrower or as wide as A, which generally doesn't have to be
the case. If the type of C is narrower or as wide as A's type, then the
shift count is necessarily non-negative, but if A is narrower than C, we
might need a left shift instead of right shift (and in that case have to
convert A to C's type first, then left shift so that the MSB of A is moved
to the right position and finally and it.
Do you ever actually need a left shift? The conversion is a sign extension
in that case (you require that A is signed), so it already has the
appropriate 1 or 0 in the right place.
--
Marc Glisse
