diff options
| -rw-r--r-- | gcc/unroll.c | 29 |
1 files changed, 24 insertions, 5 deletions
diff --git a/gcc/unroll.c b/gcc/unroll.c index c6de53b..72f1f04 100644 --- a/gcc/unroll.c +++ b/gcc/unroll.c @@ -431,15 +431,34 @@ unroll_loop (loop_end, insn_count, loop_start, end_insert_before, if (unroll_type == UNROLL_COMPLETELY || unroll_type == UNROLL_MODULO) { - /* Loops of these types should never start with a jump down to - the exit condition test. For now, check for this case just to - be sure. UNROLL_NAIVE loops can be of this form, this case is - handled below. */ + /* Loops of these types can start with jump down to the exit condition + in rare circumstances. + + Consider a pair of nested loops where the inner loop is part + of the exit code for the outer loop. + + In this case jump.c will not duplicate the exit test for the outer + loop, so it will start with a jump to the exit code. + + Then consider if the inner loop turns out to iterate once and + only once. We will end up deleting the jumps associated with + the inner loop. However, the loop notes are not removed from + the instruction stream. + + And finally assume that we can compute the number of iterations + for the outer loop. + + In this case unroll may want to unroll the outer loop even though + it starts with a jump to the outer loop's exit code. + + We could try to optimize this case, but it hardly seems worth it. + Just return without unrolling the loop in such cases. */ + insn = loop_start; while (GET_CODE (insn) != CODE_LABEL && GET_CODE (insn) != JUMP_INSN) insn = NEXT_INSN (insn); if (GET_CODE (insn) == JUMP_INSN) - abort (); + return; } if (unroll_type == UNROLL_COMPLETELY) |