From: Andrew Pinski <andrew.pinski@oss.qualcomm.com>
Date: Fri, 31 Oct 2025 00:46:54 +0000 (-0700)
Subject: sccp: Fix ICE during folding after proping const in some cases [PR122497]
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=7b9d32aa2ffcb5b02643b280c8404020c1e9d6b2;p=thirdparty%2Fgcc.git

sccp: Fix ICE during folding after proping const in some cases [PR122497]

In this case we originally had:
```
  # g_4.3_21 = PHI <g_4.3_12(3)>
  _1 = g_4.3_21 + 2;
  _2 = g_2[_1][g_4.3_21];
```

SCCP figures out g_4.3_12/g_4.3_21 is 1.
final_value_replacement_loop would remove the phi defining _12.
do the constant prop of the const 1 but that would ICE as we try
to fold the reference `g_2[_1][g_4.3_21]` but that would try to see
the range of `_1` but since `_1` refers back to _21 there would be an
ICE as the phi is already been removed.

The obvious fix is to move the constant prop until after the new statement
for _21 is created.

This fixes the change done by r14-6010-g2dde9f326ded84 and r14-6114-gde0ab339a79535.

This does not fix gcc.dg/graphite/pr82819.c nor tr2/dynamic_bitset/pr92059.cc though;
I will look into those issues in a few.

Pushed as obvious after bootstrap/test.

	PR tree-optimization/122497

gcc/ChangeLog:

	* tree-scalar-evolution.cc (final_value_replacement_loop): Call replace_uses_by
	only after the replacement statement was created.

gcc/testsuite/ChangeLog:

	* gcc.dg/torture/pr122497-1.c: New test.

Signed-off-by: Andrew Pinski <andrew.pinski@oss.qualcomm.com>
---

diff --git a/gcc/testsuite/gcc.dg/torture/pr122497-1.c b/gcc/testsuite/gcc.dg/torture/pr122497-1.c
new file mode 100644
index 00000000000..8b027ca3aaa
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/torture/pr122497-1.c
@@ -0,0 +1,13 @@
+/* { dg-do compile } */
+/* PR tree-optimization/122497 */
+
+/* This was ICEing during SCCP
+   trying to simplify a reference back to the phi
+   which was removed.  */
+
+char g_2[1][2];
+int g_4, g_5;
+void main() {
+  for (; g_4; g_4 -= 1)
+    g_5 = g_2[g_4 + 2][g_4];
+}
diff --git a/gcc/tree-scalar-evolution.cc b/gcc/tree-scalar-evolution.cc
index ecdef7529a6..7907893b916 100644
--- a/gcc/tree-scalar-evolution.cc
+++ b/gcc/tree-scalar-evolution.cc
@@ -3949,11 +3949,6 @@ final_value_replacement_loop (class loop *loop)
       auto loc = gimple_phi_arg_location (phi, exit->dest_idx);
       remove_phi_node (&psi, false);
 
-      /* Propagate constants immediately, but leave an unused initialization
-	 around to avoid invalidating the SCEV cache.  */
-      if (CONSTANT_CLASS_P (def) && !SSA_NAME_OCCURS_IN_ABNORMAL_PHI (rslt))
-	replace_uses_by (rslt, def);
-
       /* Create the replacement statements.  */
       gimple_seq stmts;
       def = force_gimple_operand (def, &stmts, false, NULL_TREE);
@@ -3961,6 +3956,11 @@ final_value_replacement_loop (class loop *loop)
       gimple_set_location (ass, loc);
       gimple_seq_add_stmt (&stmts, ass);
 
+      /* Propagate constants immediately, but leave an unused initialization
+	 around to avoid invalidating the SCEV cache.  */
+      if (CONSTANT_CLASS_P (def) && !SSA_NAME_OCCURS_IN_ABNORMAL_PHI (rslt))
+	replace_uses_by (rslt, def);
+
       /* If def's type has undefined overflow and there were folded
 	 casts, rewrite all stmts added for def into arithmetics
 	 with defined overflow behavior.  */