On 9/9/21 2:52 PM, Michael Matz wrote:
Hello,
On Thu, 9 Sep 2021, Aldy Hernandez wrote:
The ldist-22 regression is interesting though:
void foo ()
{
int i;
<bb 2> :
goto <bb 6>; [INV]
<bb 3> :
a[i_1] = 0;
if (i_1 > 100)
goto <bb 4>; [INV]
else
goto <bb 5>; [INV]
<bb 4> :
b[i_1] = i_1;
<bb 5> :
i_8 = i_1 + 1;
<bb 6> :
# i_1 = PHI <0(2), i_8(5)>
if (i_1 <= 1023)
goto <bb 3>; [INV]
else
goto <bb 7>; [INV]
Here there's no simple latch block to start with (the backedge comes
directly out of the loop exit block). So my suggested improvement
(testing if the latch was empty and only then reject the thread), would
solve this.
Well, there's the thing. Loop discovery is marking BB5 as the latch, so
it's not getting threaded:
Checking profitability of path (backwards): bb:6 (2 insns) bb:5 (latch)
Would it be crazy to suggest that we disable threading through latches
altogether,
I think it wouldn't be crazy, but we can do a bit better as suggested
above (only reject empty latches, and reject it only for the threaders
coming before the loop optims).
BTW, I'm not sure your check for the non-last position makes a difference:
diff --git a/gcc/tree-ssa-threadbackward.c b/gcc/tree-ssa-threadbackward.c
index 449232c7715..528a753b886 100644
--- a/gcc/tree-ssa-threadbackward.c
+++ b/gcc/tree-ssa-threadbackward.c
@@ -600,6 +600,7 @@ back_threader_profitability::profitable_path_p (const
vec<basic_block> &m_path,
loop_p loop = m_path[0]->loop_father;
bool path_crosses_loops = false;
bool threaded_through_latch = false;
+ bool latch_within_path = false;
bool multiway_branch_in_path = false;
bool threaded_multiway_branch = false;
bool contains_hot_bb = false;
@@ -725,7 +726,13 @@ back_threader_profitability::profitable_path_p (const
vec<basic_block> &m_path,
the last entry in the array when determining if we thread
through the loop latch. */
if (loop->latch == bb)
- threaded_through_latch = true;
+ {
+ threaded_through_latch = true;
+ if (j != 0)
+ latch_within_path = true;
+ if (dump_file && (dump_flags & TDF_DETAILS))
+ fprintf (dump_file, " (latch)");
+ }
}
If the last position being considered is a simple latch, it only has a
simple outgoing jump. There's no need to thread that. You need a block
with >= 2 succ edges to thread anything.
Aldy
