Re: PATCH] tree-ssa-sink: Add heuristics for code sinking

[Ajit Agarwal <aagarwa1@linux.ibm.com>]

Hello Richard:

On 14/04/23 2:29 pm, Richard Biener wrote:
> On Fri, Apr 14, 2023 at 10:42 AM Ajit Agarwal via Gcc-patches
> <gcc-patches@gcc.gnu.org> wrote:
>>
>> Hello All:
>>
>> This patch add heuristics for code sinking opportunities.
>> Bootstrapped and regtested for powerpc64-linux-gnu.
>>
>> Thanks & Regards
>> Ajit
>>
>>         tree-ssa-sink: Add heuristics for code sinking.
>>
>>         Add following code sinking heuristics:
>>
>>         1. from code block dominates the call.
>>         2. To Code block have uses inside the function call.
>>         3. Loop headers.
>>         4. Sinking from code block after call increases register
>>         pressure.
>>         5. Sinking calls.
>>
>>         2023-04-14  Ajit Kumar Agarwal  <aagarwa1@linux.ibm.com>
>>
>> gcc/ChangeLog:
>>
>>         * tree-ssa-sink.cc (statement_sink_location): Add heuristics
>>         for code sinking.
>> ---
>>  gcc/tree-ssa-sink.cc | 33 +++++++++++++++++++++++++++++++++
>>  1 file changed, 33 insertions(+)
>>
>> diff --git a/gcc/tree-ssa-sink.cc b/gcc/tree-ssa-sink.cc
>> index 87b1d40c174..8de88b259a3 100644
>> --- a/gcc/tree-ssa-sink.cc
>> +++ b/gcc/tree-ssa-sink.cc
>> @@ -465,6 +465,39 @@ statement_sink_location (gimple *stmt, basic_block frombb,
>>           if (sinkbb == frombb)
>>             return false;
>>
>> +         auto_vec<basic_block> h;
>> +         h = get_all_dominated_blocks (CDI_DOMINATORS,
>> +                                       frombb);
>> +         bool is_call = false;
>> +         while (h.length ())
>> +           {
>> +             basic_block bb = h.pop ();
>> +
>> +             if (bb == frombb)
>> +               continue;
>> +
>> +             for (gimple_stmt_iterator gsi = gsi_last_bb (bb); !gsi_end_p (gsi);)
>> +               {
>> +                 gimple *stmt = gsi_stmt (gsi);
>> +
>> +                 if (is_gimple_call (stmt))
>> +                   {
>> +                     is_call = true;
>> +                     break;
>> +                   }
>> +
>> +                  if (!gsi_end_p (gsi))
>> +                    gsi_prev (&gsi);
>> +               }
>> +            }
>> +
>> +           if (!is_gimple_call (stmt)
>> +               && (gimple_bb (use) != frombb)
>> +               && !is_gimple_call (use)
>> +               && dominated_by_p (CDI_DOMINATORS, sinkbb, frombb)
>> +               && is_call)
>> +              return false;
>> +
> 
> Sorry, but this lacks a comment, it doesn't explain why the existing heuristics
> are not enough (select_best_block), it repeats dominance computing.
> 
> More so it lacks a testcase demonstrating the effect.
> 

Added testscases and comments in the code.
The heuristics are added to relieve from register pressure.

Thanks & Regards
Ajit

Here is the patch.

	tree-ssa-sink: Add heuristics for code sinking.

	Add following code sinking heuristics:

	1. from code block dominates the call.
	2. To Code block have uses inside the function call.
	3. Loop headers.
	4. Sinking from code block after call increases register
	pressure.
	5. Sinking calls.

	2023-04-14  Ajit Kumar Agarwal  <aagarwa1@linux.ibm.com>

gcc/ChangeLog:

	* tree-ssa-sink.cc (statement_sink_location): Add heuristics
	for code sinking.

gcc/testsuite/ChangeLog:

	* gcc.dg/tree-ssa/ssa-sink-20.c: New testcase.
	* gcc.dg/tree-ssa/ssa-sink-21.c: New testcase.
---
 gcc/testsuite/gcc.dg/tree-ssa/ssa-sink-20.c | 16 ++++++++++++++++
 gcc/testsuite/gcc.dg/tree-ssa/ssa-sink-21.c | 20 ++++++++++++++++++++
 gcc/tree-ssa-sink.cc                        |  6 ++++++
 3 files changed, 42 insertions(+)
 create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/ssa-sink-20.c
 create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/ssa-sink-21.c

diff --git a/gcc/testsuite/gcc.dg/tree-ssa/ssa-sink-20.c b/gcc/testsuite/gcc.dg/tree-ssa/ssa-sink-20.c
new file mode 100644
index 00000000000..ed2aefc01aa
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/ssa-sink-20.c
@@ -0,0 +1,16 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-sink -fdump-tree-optimized" } */
+
+void bar();
+int j;
+void foo(int a, int b, int c, int d, int e, int f)
+{
+  int l;
+  l = a + b + c + d +e + f;
+  if (a != 5)
+    {
+      bar();
+      j = l;
+    }
+}
+/* { dg-final { scan-tree-dump-times "Sunk statements: 1" 0 "sink1" } } */
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/ssa-sink-21.c b/gcc/testsuite/gcc.dg/tree-ssa/ssa-sink-21.c
new file mode 100644
index 00000000000..a39724df8ec
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/ssa-sink-21.c
@@ -0,0 +1,20 @@
+/* { dg-do compile } */ 
+/* { dg-options "-O2 -fdump-tree-sink-stats" } */
+
+void bar();
+int j, x;
+void foo(int a, int b, int c, int d, int e, int f)
+{
+  int l;
+  l = a + b + c + d +e + f;
+  if (a != 5)
+    {
+      bar();
+      if (b != 3)
+        x = 3;
+      else
+        x = 5;
+      j = l;
+    }
+}
+/* { dg-final { scan-tree-dump-times "Sunk statements: 1" 0 "sink1" } } */
diff --git a/gcc/tree-ssa-sink.cc b/gcc/tree-ssa-sink.cc
index 8de88b259a3..932fd71bec2 100644
--- a/gcc/tree-ssa-sink.cc
+++ b/gcc/tree-ssa-sink.cc
@@ -465,6 +465,12 @@ statement_sink_location (gimple *stmt, basic_block frombb,
 	  if (sinkbb == frombb)
 	    return false;
 
+	  /* The below heuristics describes the following.
+	     a) If the candidate to sink has call in the dominator basic
+	     basic blocks.
+	     b) statement to sink doesn't have  use in the call.
+	     c) candidate block  dominates sink block.
+	     In the above cases are true then don't do code sinking.  */
 	  auto_vec<basic_block> h;
 	  h = get_all_dominated_blocks (CDI_DOMINATORS,
 					frombb);
-- 
2.31.1


> Richard.
> 
>>           if (sinkbb == gimple_bb (use))
>>             *togsi = gsi_for_stmt (use);
>>           else
>> --
>> 2.31.1
>>