On 02/06/18 04:34, Ian Romanick wrote:
On 05/31/2018 10:37 PM, Timothy Arceri wrote:
This pass detects potential loop terminators and moves intructions
from the non breaking branch after the if.
This enables both the new opt_if_simplification() pass and loop
unrolling to potentially progress further.
Unexpectedly this change speed up shader-db run times by ~3%
Ivy Bridge shader-db results (all changes in dolphin/ubershaders):
I was going to look at the changes in the generated code, but the
smallest of these shaders is > 2300 instructions.
Do you have any idea if any piglit or CTS tests hit this optimization path?
I have a feeling they don't. I'm probably going to create a loop
unrolling piglit test for it though.
I was looking at a vkpipeline-db shader when I wrote this. It was a
shader from Prey running on DXVK.
One tiny nit below...
total instructions in shared programs: 9995662 -> 9995338 (-0.00%)
instructions in affected programs: 87845 -> 87521 (-0.37%)
helped: 27
HURT: 0
total cycles in shared programs: 230931495 -> 230925015 (-0.00%)
cycles in affected programs: 56391385 -> 56384905 (-0.01%)
helped: 27
HURT: 0
---
src/compiler/nir/nir_opt_if.c | 68 +++++++++++++++++++++++++++++++++++
1 file changed, 68 insertions(+)
diff --git a/src/compiler/nir/nir_opt_if.c b/src/compiler/nir/nir_opt_if.c
index b03657a4244..1edeefdd5d1 100644
--- a/src/compiler/nir/nir_opt_if.c
+++ b/src/compiler/nir/nir_opt_if.c
@@ -24,6 +24,7 @@
#include "nir.h"
#include "nir/nir_builder.h"
#include "nir_control_flow.h"
+#include "nir_loop_analyze.h"
/**
* This optimization detects if statements at the tops of loops where the
@@ -283,6 +284,72 @@ opt_if_simplification(nir_builder *b, nir_if *nif)
return true;
}
+/**
+ * This optimization simplifies potential loop terminators which then allows
+ * other passes such as opt_if_simplification() and loop unrolling to progress
+ * further:
+ *
+ * if (cond) {
+ * ... then block instructions ...
+ * } else {
+ * ...
+ * break;
+ * }
+ *
+ * into:
+ *
+ * if (cond) {
+ * } else {
+ * ...
+ * break;
+ * }
+ * ... then block instructions ...
+ */
+static bool
+opt_if_loop_terminator(nir_if *nif)
+{
+ nir_block *break_blk = NULL;
+ nir_block *continue_from_blk = NULL;
+ bool continue_from_then = true;
+
+ nir_block *last_then = nir_if_last_then_block(nif);
+ nir_block *last_else = nir_if_last_else_block(nif);
+
+ if (nir_block_ends_in_break(last_then)) {
+ break_blk = last_then;
+ continue_from_blk = last_else;
+ continue_from_then = false;
+ } else if (nir_block_ends_in_break(last_else)) {
+ break_blk = last_else;
+ continue_from_blk = last_then;
+ }
+
+ /* Continue if the if contained no jumps at all */
In prose, I like to use if-statement because I think it reads more
easily. "Continue if the if-statement..."
+ if (!break_blk)
+ return false;
+
+ /* If the continue from block is empty then return as there is nothing to
+ * move.
+ */
+ nir_block *first_continue_from_blk = continue_from_then ?
+ nir_if_first_then_block(nif) :
+ nir_if_first_else_block(nif);
+ if (is_block_empty(first_continue_from_blk))
+ return false;
+
+ if (!nir_is_trivial_loop_if(nif, break_blk))
+ return false;
+
+ /* Finally, move the continue from branch after the if. */
+ nir_cf_list tmp;
+ nir_cf_extract(&tmp, nir_before_block(first_continue_from_blk),
+ nir_after_block(continue_from_blk));
+ nir_cf_reinsert(&tmp, nir_after_cf_node(&nif->cf_node));
+ nir_cf_delete(&tmp);
+
+ return true;
+}
+
static bool
opt_if_cf_list(nir_builder *b, struct exec_list *cf_list)
{
@@ -296,6 +363,7 @@ opt_if_cf_list(nir_builder *b, struct exec_list *cf_list)
nir_if *nif = nir_cf_node_as_if(cf_node);
progress |= opt_if_cf_list(b, &nif->then_list);
progress |= opt_if_cf_list(b, &nif->else_list);
+ progress |= opt_if_loop_terminator(nif);
progress |= opt_if_simplification(b, nif);
break;
}
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