For very small loops (< 6 insns), it would be fine to unroll 4
times to use cache line better. Like below loops:
`while (i) a[--i] = NULL; while (p < e) *d++ = *p++;`
And for very complex loops which may cause negative impacts:
branch-miss or cache-miss. Like below loop: there are calls,
early exits and branches in loop.
```
for (int i = 0; i < n; i++) {
int e = a[I];
....
if (function_call(e)) break;
....
}
```
This patch enhances RTL unroll for small loops and prevent to
unroll complex loops.
gcc/ChangeLog
2020-07-03 Jiufu Guo <guoji...@linux.ibm.com>
* config/rs6000/rs6000.c (rs6000_loop_unroll_adjust): Refine hook.
(rs6000_complex_loop_p): New function.
(num_loop_calls): New function.
---
gcc/config/rs6000/rs6000.c | 46 +++++++++++++++++++++++++++++++++-----
1 file changed, 40 insertions(+), 6 deletions(-)
diff --git a/gcc/config/rs6000/rs6000.c b/gcc/config/rs6000/rs6000.c
index 58f5d780603..a4874fa0efc 100644
--- a/gcc/config/rs6000/rs6000.c
+++ b/gcc/config/rs6000/rs6000.c
@@ -5130,22 +5130,56 @@ rs6000_destroy_cost_data (void *data)
free (data);
}
+/* Count the number of call insns in LOOP. */
+static unsigned int
+num_loop_calls (struct loop *loop)
+{
+ basic_block *bbs;
+ rtx_insn *insn;
+ unsigned int i;
+ unsigned int call_ins_num = 0;
+
+ bbs = get_loop_body (loop);
+ for (i = 0; i < loop->num_nodes; i++)
+ FOR_BB_INSNS (bbs[i], insn)
+ if (CALL_P (insn))
+ call_ins_num++;
+
+ free (bbs);
+
+ return call_ins_num;
+}
+
+/* Return true if LOOP is too complex to be unrolled. */
+static bool
+rs6000_complex_loop_p (struct loop *loop)
+{
+ unsigned call_num;
+
+ return loop->ninsns > 10
+ && (call_num = num_loop_calls (loop)) > 0
+ && (call_num + num_loop_branches (loop)) * 5 > loop->ninsns
+ && !single_exit (loop);
+}
+
/* Implement targetm.loop_unroll_adjust. */
static unsigned
rs6000_loop_unroll_adjust (unsigned nunroll, struct loop *loop)
{
- if (unroll_only_small_loops)
+ if (unroll_only_small_loops)
{
- /* TODO: This is hardcoded to 10 right now. It can be refined, for
- example we may want to unroll very small loops more times (4 perhaps).
- We also should use a PARAM for this. */
+ if (loop->ninsns <= 6)
+ return MIN (4, nunroll);
if (loop->ninsns <= 10)
return MIN (2, nunroll);
- else
- return 0;
+
+ return 0;
}
+ if (rs6000_complex_loop_p (loop))
+ return 0;
+
return nunroll;
}
--
2.25.1
