This patch adds comments to describe each use of ira_loop_border_costs.
I think this highlights that move_spill_restore was using the wrong cost
in one case, which came from tranposing [0] and [1] in the original
(pre-ira_loop_border_costs) ira_memory_move_cost expressions. The
difference would only be noticeable on targets that distinguish between
load and store costs.
gcc/
PR rtl-optimization/98782
* ira-color.c (color_pass): Add comments to describe the spill costs.
(move_spill_restore): Likewise. Fix reversed calculation.
---
gcc/ira-color.c | 28 +++-
1 file changed, 27 insertions(+), 1 deletion(-)
diff --git a/gcc/ira-color.c b/gcc/ira-color.c
index 66c11710b97..e7433312675 100644
--- a/gcc/ira-color.c
+++ b/gcc/ira-color.c
@@ -3479,6 +3479,13 @@ color_pass (ira_loop_tree_node_t loop_tree_node)
}
else if (hard_regno < 0)
{
+ /* If we allocate a register to SUBLOOP_ALLOCNO, we'll need
+to load the register on entry to the subloop and store
+the register back on exit from the subloop. This incurs
+a fixed cost for all registers. Since UPDATED_MEMORY_COST
+is (and should only be) used relative to the register costs
+for the same allocno, we can subtract this shared register
+cost from the memory cost. */
ira_loop_border_costs border_costs (subloop_allocno);
ALLOCNO_UPDATED_MEMORY_COST (subloop_allocno)
-= border_costs.spill_outside_loop_cost ();
@@ -3503,6 +3510,9 @@ color_pass (ira_loop_tree_node_t loop_tree_node)
> ALLOCNO_UPDATED_HARD_REG_COSTS (subloop_allocno)[index])
ALLOCNO_UPDATED_CLASS_COST (subloop_allocno)
= ALLOCNO_UPDATED_HARD_REG_COSTS (subloop_allocno)[index];
+ /* If we spill SUBLOOP_ALLOCNO, we'll need to store HARD_REGNO
+on entry to the subloop and restore HARD_REGNO on exit from
+the subloop. */
ALLOCNO_UPDATED_MEMORY_COST (subloop_allocno)
+= border_costs.spill_inside_loop_cost ();
}
@@ -3601,9 +3611,17 @@ move_spill_restore (void)
: ALLOCNO_HARD_REG_COSTS (subloop_allocno)[index]));
ira_loop_border_costs border_costs (subloop_allocno);
if ((hard_regno2 = ALLOCNO_HARD_REGNO (subloop_allocno)) < 0)
- cost -= border_costs.spill_outside_loop_cost ();
+ /* The register was spilled in the subloop. If we spill
+ it in the outer loop too then we'll no longer need to
+ save the register on entry to the subloop and restore
+ the register on exit from the subloop. */
+ cost -= border_costs.spill_inside_loop_cost ();
else
{
+ /* The register was also allocated in the subloop. If we
+spill it in the outer loop then we'll need to load the
+register on entry to the subloop and store the register
+back on exit from the subloop. */
cost += border_costs.spill_outside_loop_cost ();
if (hard_regno2 != hard_regno)
cost -= border_costs.move_between_loops_cost ();
@@ -3615,9 +3633,17 @@ move_spill_restore (void)
ira_assert (rclass == ALLOCNO_CLASS (parent_allocno));
ira_loop_border_costs border_costs (a);
if ((hard_regno2 = ALLOCNO_HARD_REGNO (parent_allocno)) < 0)
+ /* The register was spilled in the parent loop. If we spill
+ it in this loop too then we'll no longer need to load the
+ register on entry to this loop and save the register back
+ on exit from this loop. */
cost -= border_costs.spill_outside_loop_cost ();
else
{
+ /* The register was also allocated in the parent loop.
+If we spill it in this loop then we'll need to save
+the register on entry to this loop and restore the
+register on exit from this loop. */
cost += border_costs.spill_inside_loop_cost ();
if (hard_regno2 != hard_regno)
cost -= border_costs.move_between_loops_cost ();
--
2.25.1
