Re: [PATCH] Gracefully handle right shifts larger than the precision.

[Aldy Hernandez via Gcc-patches]

On 10/19/20 1:01 PM, Jakub Jelinek wrote:

On Mon, Oct 19, 2020 at 12:55:38PM +0200, Aldy Hernandez wrote:

Shifts are undefined already when the shift count is equal to the precision.
And arbitrarily choosing one of the values as the shift count seems risky to
me (and even when used, the most natural would be truncating the excess bits
rather than saturating), can't it just punt (return VARYING) for the UB
shifts?

Good point.  It looks cleaner too.

How about this?

diff --git a/gcc/range-op.cc b/gcc/range-op.cc
index 30d2a4d3987..0ab5e62be7f 100644
--- a/gcc/range-op.cc
+++ b/gcc/range-op.cc
@@ -1579,6 +1579,9 @@ operator_lshift::op1_range (irange &r,
        wide_int shift = wi::to_wide (shift_amount);
        if (wi::lt_p (shift, 0, SIGNED))
         return false;
+      if (wi::gt_p (shift, wi::uhwi (TYPE_PRECISION (type),
+                                    TYPE_PRECISION (op2.type ()))))
+       return false;

If you use wi::ge_p instead, then LGTM.

Thanks.

Pushed.