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tree-optimization/115841 - reduction epilogue placement issue

Message ID 20240716100959.F2833136E5@imap1.dmz-prg2.suse.org
State New
Headers show
Series tree-optimization/115841 - reduction epilogue placement issue | expand

Commit Message

Richard Biener July 16, 2024, 10:09 a.m. UTC 
When emitting the compensation to the vectorized main loop for
a vector reduction value to be re-used in the vectorized epilogue
we fail to place it in the correct block when the main loop is
known to be entered (no loop_vinfo->main_loop_edge) but the
epilogue is not (a loop_vinfo->skip_this_loop_edge).  The code
currently disregards this situation.

With the recent znver4 cost fix I couldn't trigger this situation
with the testcase but I adjusted it so it could eventually trigger
on other targets.

Bootstrap and regtest running on x86_64-unknown-linux-gnu.

	PR tree-optimization/115841
	* tree-vect-loop.cc (vect_transform_cycle_phi): Correctly
	place the partial vector reduction for the accumulator
	re-use when the main loop cannot be skipped but the
	epilogue can.

	* gcc.dg/vect/pr115841.c: New testcase.
---
 gcc/testsuite/gcc.dg/vect/pr115841.c | 42 ++++++++++++++++++++++++++++
 gcc/tree-vect-loop.cc                |  7 +++--
 2 files changed, 46 insertions(+), 3 deletions(-)
 create mode 100644 gcc/testsuite/gcc.dg/vect/pr115841.c
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Patch

diff --git a/gcc/testsuite/gcc.dg/vect/pr115841.c b/gcc/testsuite/gcc.dg/vect/pr115841.c
new file mode 100644
index 00000000000..aa5c66004a0
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/vect/pr115841.c
@@ -0,0 +1,42 @@ 
+/* { dg-do compile } */
+/* { dg-additional-options "-Ofast -fcommon -fvect-cost-model=dynamic --param vect-partial-vector-usage=1" } */
+/* { dg-additional-options "-mavx512vl" { target avx512vl } } */
+
+/* To trigger the bug costing needs to determine that aligning the A170
+   accesses with a prologue is good and there should be a vectorized
+   epilogue with a smaller vector size, re-using the vector accumulator
+   from the vectorized main loop that's statically known to execute
+   but the epilogue loop is not.  */
+
+static unsigned char xl[192];
+unsigned char A170[192*3];
+
+void jerate (unsigned char *, unsigned char *);
+float foo (unsigned n)
+{
+  jerate (xl, A170);
+
+  unsigned i = 32;
+  int kr = 1;
+  float sfn11s = 0.f;
+  float sfn12s = 0.f;
+  do
+    {
+      int krm1 = kr - 1;
+      long j = krm1;
+      float a = (*(float(*)[n])A170)[j];
+      float b = (*(float(*)[n])xl)[j];
+      float c = a * b;
+      float d = c * 6.93149983882904052734375e-1f;
+      float e = (*(float(*)[n])A170)[j+48];
+      float f = (*(float(*)[n])A170)[j+96];
+      float g = d * e;
+      sfn11s = sfn11s + g;
+      float h = f * d;
+      sfn12s = sfn12s + h;
+      kr++;
+    }
+  while (--i != 0);
+  float tem = sfn11s + sfn12s;
+  return tem;
+}
diff --git a/gcc/tree-vect-loop.cc b/gcc/tree-vect-loop.cc
index a64b5082bd1..b8124a32128 100644
--- a/gcc/tree-vect-loop.cc
+++ b/gcc/tree-vect-loop.cc
@@ -9026,14 +9026,15 @@  vect_transform_cycle_phi (loop_vec_info loop_vinfo,
 	  /* And the reduction could be carried out using a different sign.  */
 	  if (!useless_type_conversion_p (vectype_out, TREE_TYPE (def)))
 	    def = gimple_convert (&stmts, vectype_out, def);
-	  if (loop_vinfo->main_loop_edge)
+	  edge e;
+	  if ((e = loop_vinfo->main_loop_edge)
+	      || (e = loop_vinfo->skip_this_loop_edge))
 	    {
 	      /* While we'd like to insert on the edge this will split
 		 blocks and disturb bookkeeping, we also will eventually
 		 need this on the skip edge.  Rely on sinking to
 		 fixup optimal placement and insert in the pred.  */
-	      gimple_stmt_iterator gsi
-		= gsi_last_bb (loop_vinfo->main_loop_edge->src);
+	      gimple_stmt_iterator gsi = gsi_last_bb (e->src);
 	      /* Insert before a cond that eventually skips the
 		 epilogue.  */
 	      if (!gsi_end_p (gsi) && stmt_ends_bb_p (gsi_stmt (gsi)))