ia32: Fix alignment of _Atomic fields [PR65146]

For _Atomic fields, lowering the alignment of long long or double etc.
fields on ia32 is undesirable, because then one really can't perform atomic
operations on those using cmpxchg8b.

The following patch stops lowering the alignment in fields for _Atomic
types (the x86_field_alignment change) and for -mpreferred-stack-boundary=2
also ensures we don't misalign _Atomic long long etc. automatic variables
(the ix86_{local,minimum}_alignment changes).
Not sure about iamcu_alignment change, I know next to nothing about IA MCU,
but unless it doesn't have cmpxchg8b instruction, it would surprise me if we
don't want to do it as well.
clang apparently doesn't lower the field alignment for _Atomic.

2020-08-27  Jakub Jelinek  <jakub@redhat.com>

	PR target/65146
	* config/i386/i386.c (iamcu_alignment): Don't decrease alignment
	for TYPE_ATOMIC types.
	(ix86_local_alignment): Likewise.
	(ix86_minimum_alignment): Likewise.
	(x86_field_alignment): Likewise, and emit a -Wpsabi diagnostic
	for it.

	* gcc.target/i386/pr65146.c: New test.

2 files changed, 42 insertions, 5 deletions

diff --git a/gcc/config/i386/i386.c b/gcc/config/i386/i386.c
index 37e77ea..a15807d 100644
--- a/gcc/config/i386/i386.c
+++ b/gcc/config/i386/i386.c
@@ -16487,7 +16487,11 @@ iamcu_alignment (tree type, int align)
 
   /* Intel MCU psABI specifies scalar types > 4 bytes aligned to 4
      bytes.  */
-  mode = TYPE_MODE (strip_array_types (type));
+  type = strip_array_types (type);
+  if (TYPE_ATOMIC (type))
+    return align;
+
+  mode = TYPE_MODE (type);
   switch (GET_MODE_CLASS (mode))
     {
     case MODE_INT:
@@ -16644,7 +16648,8 @@ ix86_local_alignment (tree exp, machine_mode mode,
       && align == 64
       && ix86_preferred_stack_boundary < 64
       && (mode == DImode || (type && TYPE_MODE (type) == DImode))
-      && (!type || !TYPE_USER_ALIGN (type))
+      && (!type || (!TYPE_USER_ALIGN (type)
+		    && !TYPE_ATOMIC (strip_array_types (type))))
       && (!decl || !DECL_USER_ALIGN (decl)))
     align = 32;
 
@@ -16757,7 +16762,8 @@ ix86_minimum_alignment (tree exp, machine_mode mode,
   /* Don't do dynamic stack realignment for long long objects with
      -mpreferred-stack-boundary=2.  */
   if ((mode == DImode || (type && TYPE_MODE (type) == DImode))
-      && (!type || !TYPE_USER_ALIGN (type))
+      && (!type || (!TYPE_USER_ALIGN (type)
+		    && !TYPE_ATOMIC (strip_array_types (type))))
       && (!decl || !DECL_USER_ALIGN (decl)))
     {
       gcc_checking_assert (!TARGET_STV);
@@ -20293,11 +20299,30 @@ x86_field_alignment (tree type, int computed)
     return computed;
   if (TARGET_IAMCU)
     return iamcu_alignment (type, computed);
-  mode = TYPE_MODE (strip_array_types (type));
+  type = strip_array_types (type);
+  mode = TYPE_MODE (type);
   if (mode == DFmode || mode == DCmode
       || GET_MODE_CLASS (mode) == MODE_INT
       || GET_MODE_CLASS (mode) == MODE_COMPLEX_INT)
-    return MIN (32, computed);
+    {
+      if (TYPE_ATOMIC (type) && computed > 32)
+	{
+	  static bool warned;
+
+	  if (!warned && warn_psabi)
+	    {
+	      const char *url
+		= CHANGES_ROOT_URL "gcc-11/changes.html#ia32_atomic";
+
+	      warned = true;
+	      inform (input_location, "the alignment of %<_Atomic %T%> "
+				      "fields changed in %{GCC 11.1%}",
+		      TYPE_MAIN_VARIANT (type), url);
+	    }
+	}
+      else
+      return MIN (32, computed);
+    }
   return computed;
 }
 
diff --git a/gcc/testsuite/gcc.target/i386/pr65146.c b/gcc/testsuite/gcc.target/i386/pr65146.c
new file mode 100644
index 0000000..96284e0
--- /dev/null
+++ b/gcc/testsuite/gcc.target/i386/pr65146.c
@@ -0,0 +1,12 @@
+/* PR target/65146 */
+/* { dg-do compile } */
+/* { dg-options "-Wno-psabi" } */
+
+struct A { char a; _Atomic long long b; };
+struct B { char a; _Atomic double b; };
+struct C { char a; _Atomic long long b[2]; };
+struct D { char a; _Atomic double b[2]; };
+extern int a[__builtin_offsetof (struct A, b) == 8 ? 1 : -1];
+extern int b[__builtin_offsetof (struct B, b) == 8 ? 1 : -1];
+extern int c[__builtin_offsetof (struct C, b) == 8 ? 1 : -1];
+extern int d[__builtin_offsetof (struct D, b) == 8 ? 1 : -1];