diff options
Diffstat (limited to 'gcc')
| -rw-r--r-- | gcc/ChangeLog | 4 | ||||
| -rw-r--r-- | gcc/doloop.c | 27 |
2 files changed, 25 insertions, 6 deletions
diff --git a/gcc/ChangeLog b/gcc/ChangeLog index 32f3649..00f7de4 100644 --- a/gcc/ChangeLog +++ b/gcc/ChangeLog @@ -1,3 +1,7 @@ +2001-11-29 Zoltan Hidvegi <hzoli@hzoli.2y.net> + + * doloop.c (doloop_valid_p): Check for LTU and GTU as well. + 2001-11-29 Aldy Hernandez <aldyh@redhat.com> * config/rs6000/rs6000.c (function_arg): Unnamed vector arguments diff --git a/gcc/doloop.c b/gcc/doloop.c index cf50d68..f6b765a 100644 --- a/gcc/doloop.c +++ b/gcc/doloop.c @@ -353,23 +353,38 @@ doloop_valid_p (loop, jump_insn) && ((loop_info->comparison_code == LEU && INTVAL (loop_info->increment) > 0) || (loop_info->comparison_code == GEU - && INTVAL (loop_info->increment) < 0))) + && INTVAL (loop_info->increment) < 0) + || (loop_info->comparison_code == LTU + && INTVAL (loop_info->increment) > 1) + || (loop_info->comparison_code == GTU + && INTVAL (loop_info->increment) < -1))) { /* If the comparison is LEU and the comparison value is UINT_MAX then the loop will not terminate. Similarly, if the comparison code is GEU and the initial value is 0, the loop will not terminate. - Note that with LE and GE, the loop behaviour can be - implementation dependent if an overflow occurs, say between - INT_MAX and INT_MAX + 1. We thus don't have to worry about - these two cases. + If the absolute increment is not 1, the loop can be infinite + even with LTU/GTU, e.g. for (i = 3; i > 0; i -= 2) + + Note that with LE and GE, the loop behaviour is undefined + (C++ standard section 5 clause 5) if an overflow occurs, say + between INT_MAX and INT_MAX + 1. We thus don't have to worry + about these two cases. ??? We could compute these conditions at run-time and have a additional jump around the loop to ensure an infinite loop. However, it is very unlikely that this is the intended behaviour of the loop and checking for these rare boundary - conditions would pessimize all other code. */ + conditions would pessimize all other code. + + If the loop is executed only a few times an extra check to + restart the loop could use up most of the benefits of using a + count register loop. Note however, that normally, this + restart branch would never execute, so it could be predicted + well by the CPU. We should generate the pessimistic code by + default, and have an option, e.g. -funsafe-loops that would + enable count-register loops in this case. */ if (loop_dump_stream) fprintf (loop_dump_stream, "Doloop: Possible infinite iteration case ignored.\n"); |