Hi!
Our rotate expansion if rotate optab isn't present for the respective
mode looks unsafe for rotations by variable count if that count could
be 0, because then it invokes right or left shift by bitsize.
While on most targets the hw will probably do the right thing
(it is fine if x << 32 will either yield x or 0, in both cases
that ored together with x >> 0 aka x will still yield x), perhaps gcc
could try to optimize based on the fact that undefined behavior can't
happen, so IMHO it is better to generate a safer sequence.
Ok for trunk?
2013-05-10 Jakub Jelinek <ja...@redhat.com>
* expmed.c (expand_shift_1): For rotations by const0_rtx just
return shifted. Use (-op1) & (prec - 1) as other_amount
instead of prec - op1.
--- gcc/expmed.c.jj 2013-05-07 10:26:46.000000000 +0200
+++ gcc/expmed.c 2013-05-10 11:54:34.659987038 +0200
@@ -2166,7 +2166,8 @@ expand_shift_1 (enum tree_code code, enu
{
/* If we have been unable to open-code this by a rotation,
do it as the IOR of two shifts. I.e., to rotate A
- by N bits, compute (A << N) | ((unsigned) A >> (C - N))
+ by N bits, compute
+ (A << N) | ((unsigned) A >> ((-N) & (C - 1)))
where C is the bitsize of A.
It is theoretically possible that the target machine might
@@ -2181,14 +2182,22 @@ expand_shift_1 (enum tree_code code, enu
rtx temp1;
new_amount = op1;
- if (CONST_INT_P (op1))
+ if (op1 == const0_rtx)
+ return shifted;
+ else if (CONST_INT_P (op1))
other_amount = GEN_INT (GET_MODE_BITSIZE (mode)
- INTVAL (op1));
else
- other_amount
- = simplify_gen_binary (MINUS, GET_MODE (op1),
- GEN_INT (GET_MODE_PRECISION (mode)),
- op1);
+ {
+ other_amount
+ = simplify_gen_unary (NEG, GET_MODE (op1),
+ op1, GET_MODE (op1));
+ other_amount
+ = simplify_gen_binary (AND, GET_MODE (op1),
+ other_amount,
+ GEN_INT (GET_MODE_PRECISION (mode)
+ - 1));
+ }
shifted = force_reg (mode, shifted);
Jakub
