From 8408d34570c9fe9f3d22a25a76df2a4c64f08477 Mon Sep 17 00:00:00 2001
From: Jakub Jelinek <jakub@redhat.com>
Date: Fri, 23 Jul 2021 19:55:16 +0200
Subject: [PATCH] expmed: Fix store_integral_bit_field [PR101562]

Our documentation says that paradoxical subregs shouldn't appear
in strict_low_part:
'(strict_low_part (subreg:M (reg:N R) 0))'
     This expression code is used in only one context: as the
     destination operand of a 'set' expression.  In addition, the
     operand of this expression must be a non-paradoxical 'subreg'
     expression.
but on the testcase below that triggers UB at runtime
store_integral_bit_field emits exactly that.

The following patch fixes it by ensuring the requirement is satisfied.

2021-07-23  Jakub Jelinek  <jakub@redhat.com>

	PR rtl-optimization/101562
	* expmed.c (store_integral_bit_field): Only use movstrict_optab
	if the operand isn't paradoxical.

	* gcc.c-torture/compile/pr101562.c: New test.
---
 gcc/expmed.c                                  |  5 ++++-
 .../gcc.c-torture/compile/pr101562.c          | 21 +++++++++++++++++++
 2 files changed, 25 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/gcc.c-torture/compile/pr101562.c

diff --git a/gcc/expmed.c b/gcc/expmed.c
index 1fb63170be9..3143f38e057 100644
--- a/gcc/expmed.c
+++ b/gcc/expmed.c
@@ -921,7 +921,10 @@ store_integral_bit_field (rtx op0, opt_scalar_int_mode op0_mode,
 	}
 
       subreg_off = bitnum / BITS_PER_UNIT;
-      if (validate_subreg (fieldmode, GET_MODE (arg0), arg0, subreg_off))
+      if (validate_subreg (fieldmode, GET_MODE (arg0), arg0, subreg_off)
+	  /* STRICT_LOW_PART must have a non-paradoxical subreg as
+	     operand.  */
+	  && !paradoxical_subreg_p (fieldmode, GET_MODE (arg0)))
 	{
 	  arg0 = gen_rtx_SUBREG (fieldmode, arg0, subreg_off);
 
diff --git a/gcc/testsuite/gcc.c-torture/compile/pr101562.c b/gcc/testsuite/gcc.c-torture/compile/pr101562.c
new file mode 100644
index 00000000000..ea4a5f7103a
--- /dev/null
+++ b/gcc/testsuite/gcc.c-torture/compile/pr101562.c
@@ -0,0 +1,21 @@
+/* PR rtl-optimization/101562 */
+
+struct S { char c; };
+void baz (struct S a, struct S b);
+
+void
+foo (void)
+{
+  struct S x[1];
+  *(short *)&x[0] = 256;
+  baz (x[0], x[1]);
+}
+
+void
+bar (void)
+{
+  struct S x[1];
+  x[0].c = 0;
+  x[1].c = 1;
+  baz (x[0], x[1]);
+}