[gcc r11-10463] Fix problematic interaction between bitfields, unions, SSO and SRA

[Eric Botcazou <ebotcazou@gcc.gnu.org>]

https://gcc.gnu.org/g:01e80c4c630a5a6286a521b047d0ef80631c892c

commit r11-10463-g01e80c4c630a5a6286a521b047d0ef80631c892c
Author: Eric Botcazou <ebotcazou@adacore.com>
Date:   Wed Jan 11 15:58:47 2023 +0100

    Fix problematic interaction between bitfields, unions, SSO and SRA
    
    The handling of bitfields by the SRA pass is peculiar and this must be taken
    into account to support the scalar_storage_order attribute.
    
    gcc/
            PR tree-optimization/108199
            * tree-sra.c (sra_modify_expr): Deal with reverse storage order
            for bit-field references.
    
    gcc/testsuite/
            * gcc.dg/sso-17.c: New test.

Diff:
---
 gcc/testsuite/gcc.dg/sso-17.c | 52 +++++++++++++++++++++++++++++++++++++++++++
 gcc/tree-sra.c                | 18 ++++++++++++++-
 2 files changed, 69 insertions(+), 1 deletion(-)

diff --git a/gcc/testsuite/gcc.dg/sso-17.c b/gcc/testsuite/gcc.dg/sso-17.c
new file mode 100644
index 00000000000..67e2d3793fd
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/sso-17.c
@@ -0,0 +1,52 @@
+/* { dg-do run } */
+/* { dg-options "-O2" } */
+
+#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
+#define REV_ENDIANNESS __attribute__((scalar_storage_order("big-endian")))
+#else
+#define REV_ENDIANNESS __attribute__((scalar_storage_order("little-endian")))
+#endif
+
+typedef unsigned long long u64;
+
+union DST {
+  u64 val;
+
+  struct {
+    u64 x : 1;
+    u64 y : 1;
+    u64 r: 62;
+  } REV_ENDIANNESS;
+} REV_ENDIANNESS;
+
+
+struct SRC {
+  u64 a;
+} REV_ENDIANNESS;
+
+[[gnu::noipa]]
+void foo () {__builtin_abort();}
+
+[[gnu::noinline]]
+int bar(struct SRC *src)
+{
+  union DST dst;
+  
+  dst.val = src->a;
+
+  if (dst.y) {
+    foo();
+  }
+  return 0;
+}
+
+int main(void)
+{
+#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
+  struct SRC t = {-1ull & (~(0x01ull<<62))};
+#else
+  struct SRC t = {-1ull & (~(0x01ull<<1))};
+#endif
+  bar(&t);
+  return 0;
+}
diff --git a/gcc/tree-sra.c b/gcc/tree-sra.c
index dea9cf3235c..5a1fc81d301 100644
--- a/gcc/tree-sra.c
+++ b/gcc/tree-sra.c
@@ -3828,7 +3828,23 @@ sra_modify_expr (tree *expr, gimple_stmt_iterator *gsi, bool write)
 	    }
 	}
       else
-	*expr = repl;
+	{
+	  /* If we are going to replace a scalar field in a structure with
+	     reverse storage order by a stand-alone scalar, we are going to
+	     effectively byte-swap the scalar and we also need to byte-swap
+	     the portion of it represented by the bit-field.  */
+	  if (bfr && REF_REVERSE_STORAGE_ORDER (bfr))
+	    {
+	      REF_REVERSE_STORAGE_ORDER (bfr) = 0;
+	      TREE_OPERAND (bfr, 2)
+		= size_binop (MINUS_EXPR, TYPE_SIZE (TREE_TYPE (repl)),
+			      size_binop (PLUS_EXPR, TREE_OPERAND (bfr, 1),
+						     TREE_OPERAND (bfr, 2)));
+	    }
+
+	  *expr = repl;
+	}
+
       sra_stats.exprs++;
     }
   else if (write && access->grp_to_be_debug_replaced)