[gcc r13-1794] tree-optimization/106397 - array prefetch and LC SSA

[Richard Biener <rguenth@gcc.gnu.org>]

https://gcc.gnu.org/g:b2e99bb6900f33f46a0f4ca6ae94b8a39b0b9bb1

commit r13-1794-gb2e99bb6900f33f46a0f4ca6ae94b8a39b0b9bb1
Author: Richard Biener <rguenther@suse.de>
Date:   Fri Jul 22 09:57:38 2022 +0200

    tree-optimization/106397 - array prefetch and LC SSA
    
    The following fixes maintaining LC SSA when array prefetch inserts
    mfence instructions on loop exits that do not use memory.  It also
    fixes the latent issue that it might split exit edges for this
    which will break LC SSA for non-virtuals as well.  It should also
    make the process cheaper by accumulating the required (LC) SSA
    update until the end of the pass.
    
            PR tree-optimization/106397
            * tree-ssa-loop-prefetch.cc (emit_mfence_after_loop): Do
            not update SSA form here.
            (mark_nontemporal_stores): Return whether we marked any
            non-temporal stores and inserted mfence.
            (loop_prefetch_arrays): Note when we need to update SSA.
            (tree_ssa_prefetch_arrays): Perform required (LC) SSA update
            at the end of the pass.
    
            * gcc.dg/pr106397.c: New testcase.

Diff:
---
 gcc/testsuite/gcc.dg/pr106397.c | 17 +++++++++++++++++
 gcc/tree-ssa-loop-prefetch.cc   | 27 +++++++++++++++++----------
 2 files changed, 34 insertions(+), 10 deletions(-)

diff --git a/gcc/testsuite/gcc.dg/pr106397.c b/gcc/testsuite/gcc.dg/pr106397.c
new file mode 100644
index 00000000000..a6b2e913346
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/pr106397.c
@@ -0,0 +1,17 @@
+/* { dg-do compile } */
+/* { dg-options "-O3 -fprefetch-loop-arrays --param l2-cache-size=0 --param prefetch-latency=3 -fprefetch-loop-arrays" } */
+
+int
+bar (void)
+{
+  /* No return statement. */
+}
+
+__attribute__ ((simd)) int
+foo (void)
+{
+  if (bar ())
+    return 0;
+
+  __builtin_unreachable ();
+}
diff --git a/gcc/tree-ssa-loop-prefetch.cc b/gcc/tree-ssa-loop-prefetch.cc
index 8f190ae469b..b6690b0e805 100644
--- a/gcc/tree-ssa-loop-prefetch.cc
+++ b/gcc/tree-ssa-loop-prefetch.cc
@@ -1308,8 +1308,6 @@ emit_mfence_after_loop (class loop *loop)
 
       gsi_insert_before (&bsi, call, GSI_NEW_STMT);
     }
-
-  update_ssa (TODO_update_ssa_only_virtuals);
 }
 
 /* Returns true if we can use storent in loop, false otherwise.  */
@@ -1340,23 +1338,27 @@ may_use_storent_in_loop_p (class loop *loop)
 }
 
 /* Marks nontemporal stores in LOOP.  GROUPS contains the description of memory
-   references in the loop.  */
+   references in the loop.  Returns whether we inserted any mfence call.  */
 
-static void
+static bool
 mark_nontemporal_stores (class loop *loop, struct mem_ref_group *groups)
 {
   struct mem_ref *ref;
   bool any = false;
 
   if (!may_use_storent_in_loop_p (loop))
-    return;
+    return false;
 
   for (; groups; groups = groups->next)
     for (ref = groups->refs; ref; ref = ref->next)
       any |= mark_nontemporal_store (ref);
 
   if (any && FENCE_FOLLOWING_MOVNT != NULL_TREE)
-    emit_mfence_after_loop (loop);
+    {
+      emit_mfence_after_loop (loop);
+      return true;
+    }
+  return false;
 }
 
 /* Determines whether we can profitably unroll LOOP FACTOR times, and if
@@ -1874,10 +1876,11 @@ insn_to_prefetch_ratio_too_small_p (unsigned ninsns, unsigned prefetch_count,
 
 
 /* Issue prefetch instructions for array references in LOOP.  Returns
-   true if the LOOP was unrolled.  */
+   true if the LOOP was unrolled and updates NEED_LC_SSA_UPDATE if we need
+   to update SSA for virtual operands and LC SSA for a split edge.  */
 
 static bool
-loop_prefetch_arrays (class loop *loop)
+loop_prefetch_arrays (class loop *loop, bool &need_lc_ssa_update)
 {
   struct mem_ref_group *refs;
   unsigned ahead, ninsns, time, unroll_factor;
@@ -1952,7 +1955,7 @@ loop_prefetch_arrays (class loop *loop)
 					  unroll_factor))
     goto fail;
 
-  mark_nontemporal_stores (loop, refs);
+  need_lc_ssa_update |= mark_nontemporal_stores (loop, refs);
 
   /* Step 4: what to prefetch?  */
   if (!schedule_prefetches (refs, unroll_factor, ahead))
@@ -1980,6 +1983,7 @@ unsigned int
 tree_ssa_prefetch_arrays (void)
 {
   bool unrolled = false;
+  bool need_lc_ssa_update = false;
   int todo_flags = 0;
 
   if (!targetm.have_prefetch ()
@@ -2028,12 +2032,15 @@ tree_ssa_prefetch_arrays (void)
       if (dump_file && (dump_flags & TDF_DETAILS))
 	fprintf (dump_file, "Processing loop %d:\n", loop->num);
 
-      unrolled |= loop_prefetch_arrays (loop);
+      unrolled |= loop_prefetch_arrays (loop, need_lc_ssa_update);
 
       if (dump_file && (dump_flags & TDF_DETAILS))
 	fprintf (dump_file, "\n\n");
     }
 
+  if (need_lc_ssa_update)
+    rewrite_into_loop_closed_ssa (NULL, TODO_update_ssa_only_virtuals);
+
   if (unrolled)
     {
       scev_reset ();