[PATCH] ext-dce: Avoid multi-register paradoxical subregs for reload [PR122438]

ext-dce can turn an extension into a paradoxical subreg when the
upper bits are dead.  With classic reload, this can fail if the modes
use different numbers of hard registers.  On m68k, SI-to-DI creates an
invalid overlap and an ICE in postreload.

Skip the rewrite in that case.  This change also fixes PR119953
on MSP430.  LRA and same-register-count cases are unchanged.

Cross-tested on GCC trunk:

1) m68k-linux-gnu: the unpatched compiler reproduces PR122438.  The
gcc.target/m68k suite changes from 128 PASS / 9 FAIL / 1 unsupported
to 129 PASS / 9 FAIL / 1 unsupported with the new test.

2) msp430-elf: the new PR119953 test passes.

3) alpha-linux-gnu and vax-netbsdelf: the tested same-register-count
extension cases produce identical assembly with and without the patch.

Also tested on GCC 16.1.1 for m68k-linux-gnu.  The unpatched compiler
reproduces PR122438 while the new test passes with the patch.

Assisted-by: Codex gpt-5.6-terra

gcc/ChangeLog:

	PR rtl-optimization/122438
	PR target/119953
	* ext-dce.cc: Include regs.h.
	(ext_dce_try_optimize_extension): Avoid multi-register
	paradoxical subregs with classic reload.

gcc/testsuite/ChangeLog:

	PR rtl-optimization/122438
	PR target/119953
	* gcc.target/m68k/pr122438.c: New test.
	* g++.target/msp430/pr119953.C: New test.

Signed-off-by: Karl Mehltretter <[email protected]>
---
OK for trunk?

 gcc/ext-dce.cc                             |  8 +++++
 gcc/testsuite/g++.target/msp430/pr119953.C | 40 ++++++++++++++++++++++
 gcc/testsuite/gcc.target/m68k/pr122438.c   | 23 +++++++++++++
 3 files changed, 71 insertions(+)
 create mode 100644 gcc/testsuite/g++.target/msp430/pr119953.C
 create mode 100644 gcc/testsuite/gcc.target/m68k/pr122438.c

diff --git a/gcc/ext-dce.cc b/gcc/ext-dce.cc
index 9e4b0a429b..8e0c5ab9db 100644
--- a/gcc/ext-dce.cc
+++ b/gcc/ext-dce.cc
@@ -26,6 +26,7 @@ along with GCC; see the file COPYING3.  If not see
 #include "memmodel.h"
 #include "insn-config.h"
 #include "emit-rtl.h"
+#include "regs.h"
 #include "expr.h"
 #include "recog.h"
 #include "cfganal.h"
@@ -533,6 +534,13 @@ ext_dce_try_optimize_extension (rtx_insn *insn, rtx set)
       return;
     }
 
+  /* Avoid known classic-reload problems with multi-register paradoxical
+     subregs (PR122438, PR119953).  */
+  if (!targetm.lra_p ()
+      && (hard_regno_nregs (0, GET_MODE (src))
+	  != hard_regno_nregs (0, GET_MODE (inner))))
+    return;
+
   /* Avoid (subreg (mem)) and other constructs which may be valid RTL, but
      not useful for this optimization.  */
   if (!(REG_P (inner) || (SUBREG_P (inner) && REG_P (SUBREG_REG (inner)))))
diff --git a/gcc/testsuite/g++.target/msp430/pr119953.C b/gcc/testsuite/g++.target/msp430/pr119953.C
new file mode 100644
index 0000000000..fd2562eff9
--- /dev/null
+++ b/gcc/testsuite/g++.target/msp430/pr119953.C
@@ -0,0 +1,40 @@
+/* PR target/119953 */
+/* { dg-do compile } */
+/* { dg-options "-O2" } */
+
+bool equals_tmp;
+int native ();
+
+struct path
+{
+  class iterator;
+  iterator begin ();
+  iterator end ();
+} p, hash_path;
+
+struct path::iterator
+{
+  path operator* () { return p; }
+  void operator++ () {}
+  friend bool operator!= (iterator lhs, iterator rhs)
+  {
+    return lhs.equals (rhs);
+  }
+  bool equals (iterator rhs)
+  {
+    if (equals_tmp)
+      return cur == rhs.cur;
+    return at_end == rhs.at_end;
+  }
+  char cur;
+  bool at_end;
+};
+
+int
+hash_value ()
+{
+  int seed;
+  for (auto x : hash_path)
+    seed ^= native () + 0x9e3779b9 + (seed << 6) + (seed >> 2);
+  return seed;
+}
diff --git a/gcc/testsuite/gcc.target/m68k/pr122438.c b/gcc/testsuite/gcc.target/m68k/pr122438.c
new file mode 100644
index 0000000000..353bcc1b8f
--- /dev/null
+++ b/gcc/testsuite/gcc.target/m68k/pr122438.c
@@ -0,0 +1,23 @@
+/* PR rtl-optimization/122438 */
+/* { dg-do compile } */
+/* { dg-options "-O2 -w" } */
+
+long describeCell_nLocal;
+char describeCell_a;
+char describeCell_zDesc[];
+int strlen (char *);
+void describeContent (char *, char *);
+void localPayload (void);
+
+void
+describeCell (char cType)
+{
+  long long nDesc;
+  if (cType)
+    nDesc += nDesc;
+  localPayload ();
+  if (describeCell_nLocal)
+    nDesc += strlen (&describeCell_zDesc[nDesc]);
+  if (cType)
+    describeContent (&describeCell_a, &describeCell_zDesc[nDesc]);
+}
-- 
2.53.0