early-remat: Resync with new DF postorders [PR109940]

When I wrote early-remat, the DF_FORWARD block order was a postorder
of a reverse/backward walk (i.e. of the inverted cfg), rather than a
reverse postorder of a forward walk.  A postorder of a backward walk
lacked the important property that dominators come before the blocks
they dominate; instead it ensures that postdominators come after
the blocks that they postdominate.

The DF_BACKWARD block order was similarly a postorder of a forward
walk.  Since early-remat wanted a standard postorder and reverse
postorder with normal dominator properties, it used the DF_BACKWARD
order instead of the DF_FORWARD order.

g:53dddbfeb213ac4ec39f fixed the DF orders so that DF_FORWARD was
an RPO of a forward walk and so that DF_BACKWARD was an RPO of a
backward walk.  This meant that iterating backwards over the
DF_BACKWARD order had the exact problem that the original DF_FORWARD
order had, triggering a flurry of ICEs for SVE.

This fixes the build with SVE enabled.  It also fixes an ICE
in g++.target/aarch64/sve/pr99766.C with normal builds.  I've
included the test from the PR as well, for extra coverage.

gcc/
	PR rtl-optimization/109940
	* early-remat.cc (postorder_index): Rename to...
	(rpo_index): ...this.
	(compare_candidates): Sort by decreasing rpo_index rather than
	increasing postorder_index.
	(early_remat::sort_candidates): Calculate the forward RPO from
	DF_FORWARD.
	(early_remat::local_phase): Follow forward RPO using DF_FORWARD,
	rather than DF_BACKWARD in reverse.

gcc/testsuite/
	* gcc.dg/torture/pr109940.c: New test.

2 files changed, 32 insertions, 14 deletions

diff --git a/gcc/early-remat.cc b/gcc/early-remat.cc
index b76771e..1ee63c7 100644
--- a/gcc/early-remat.cc
+++ b/gcc/early-remat.cc
@@ -1010,8 +1010,8 @@ early_remat::init_block_info (void)
   m_block_info.safe_grow_cleared (n_blocks, true);
 }
 
-/* Maps basic block indices to their position in the post order.  */
-static unsigned int *postorder_index;
+/* Maps basic block indices to their position in the forward RPO.  */
+static unsigned int *rpo_index;
 
 /* Order remat_candidates X_IN and Y_IN according to the cfg postorder.  */
 
@@ -1024,7 +1024,7 @@ compare_candidates (const void *x_in, const void *y_in)
   basic_block y_bb = BLOCK_FOR_INSN (y->insn);
   if (x_bb != y_bb)
     /* Make X and Y follow block postorder.  */
-    return postorder_index[x_bb->index] - postorder_index[y_bb->index];
+    return rpo_index[y_bb->index] - rpo_index[x_bb->index];
 
   /* Make X and Y follow a backward traversal of the containing block.  */
   return DF_INSN_LUID (y->insn) - DF_INSN_LUID (x->insn);
@@ -1051,15 +1051,15 @@ early_remat::sort_candidates (void)
   /* Create a mapping from block numbers to their position in the
      postorder.  */
   unsigned int n_blocks = last_basic_block_for_fn (m_fn);
-  int *postorder = df_get_postorder (DF_BACKWARD);
-  unsigned int postorder_len = df_get_n_blocks (DF_BACKWARD);
-  postorder_index = new unsigned int[n_blocks];
-  for (unsigned int i = 0; i < postorder_len; ++i)
-    postorder_index[postorder[i]] = i;
+  int *rpo = df_get_postorder (DF_FORWARD);
+  unsigned int rpo_len = df_get_n_blocks (DF_FORWARD);
+  rpo_index = new unsigned int[n_blocks];
+  for (unsigned int i = 0; i < rpo_len; ++i)
+    rpo_index[rpo[i]] = i;
 
   m_candidates.qsort (compare_candidates);
 
-  delete[] postorder_index;
+  delete[] rpo_index;
 }
 
 /* Commit to the current candidate indices and initialize cross-references.  */
@@ -2097,11 +2097,11 @@ early_remat::local_phase (void)
   if (dump_file)
     fprintf (dump_file, "\n;; Local phase:\n");
 
-  int *postorder = df_get_postorder (DF_BACKWARD);
-  unsigned int postorder_len = df_get_n_blocks (DF_BACKWARD);
-  for (unsigned int i = postorder_len; i-- > 0; )
-    if (postorder[i] >= NUM_FIXED_BLOCKS)
-      process_block (BASIC_BLOCK_FOR_FN (m_fn, postorder[i]));
+  int *rpo = df_get_postorder (DF_FORWARD);
+  unsigned int rpo_len = df_get_n_blocks (DF_FORWARD);
+  for (unsigned int i = 0; i < rpo_len; ++i)
+    if (rpo[i] >= NUM_FIXED_BLOCKS)
+      process_block (BASIC_BLOCK_FOR_FN (m_fn, rpo[i]));
 }
 
 /* Return true if available values survive across edge E.  */
diff --git a/gcc/testsuite/gcc.dg/torture/pr109940.c b/gcc/testsuite/gcc.dg/torture/pr109940.c
new file mode 100644
index 0000000..2336470
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/torture/pr109940.c
@@ -0,0 +1,18 @@
+/* { dg-additional-options "-march=armv9-a" { target aarch64*-*-* } } */
+
+int a;
+int *b;
+void
+c (int *d) { *d = a; }
+
+int
+e(int d, int f) {
+  if (d <= 1)
+    return 1;
+  int g = d / 2;
+  for (int h = 0; h < g; h++)
+    if (f == (long int)b > b[h])
+      c(&b[h]);
+  e(g, f);
+  e(g, f);
+}