[Bug tree-optimization/78810] New: [7 Regression] Optimized out volatile memory access

[marxin at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78810

            Bug ID: 78810
           Summary: [7 Regression] Optimized out volatile memory access
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Keywords: wrong-code
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: marxin at gcc dot gnu.org
  Target Milestone: ---

Starting from r239778, Ruby 2.3 package can't be compiled.

$ cat /tmp/chunk.cpp 
class NODE;
typedef unsigned long VALUE;

struct parser_params
{
};

struct parser_params *get_parser();
NODE *yycompile(parser_params *parser, VALUE a, VALUE b);

NODE*
rb_parser_compile_file_path(VALUE vparser, VALUE fname, VALUE file, int start)
{
    struct parser_params *parser;
    parser = get_parser(); 

    NODE *node = yycompile(parser, fname, start);
    (*({volatile VALUE *rb_gc_guarded_ptr = (&(vparser));
rb_gc_guarded_ptr;}));

    return node;
}

$ ./xgcc -B. /tmp/chunk.cpp -c -O2 -fdump-tree-optimized=/dev/stdout

;; Function NODE* rb_parser_compile_file_path(VALUE, VALUE, VALUE, int)
(_Z27rb_parser_compile_file_pathmmmi, funcdef_no=0, decl_uid=2287,
cgraph_uid=0, symbol_order=0)

NODE* rb_parser_compile_file_path(VALUE, VALUE, VALUE, int) (VALUE vparser,
VALUE fname, VALUE file, int start)
{
  long unsigned int _1;
  struct parser_params * _4;
  struct NODE * _8;

  <bb 2> [100.0%]:
  _4 = get_parser ();
  _1 = (long unsigned int) start_5(D);
  _8 = yycompile (_4, fname_6(D), _1); [tail call]
  return _8;

}

Testing following patch:

diff --git a/gcc/tree-ssa.c b/gcc/tree-ssa.c
index 62eea8bb8a4..b92513fcf23 100644
--- a/gcc/tree-ssa.c
+++ b/gcc/tree-ssa.c
@@ -1385,7 +1385,8 @@ non_rewritable_mem_ref_base (tree ref)
       if (! DECL_P (decl))
        return NULL_TREE;
       if (! is_gimple_reg_type (TREE_TYPE (base))
-         || VOID_TYPE_P (TREE_TYPE (base)))
+         || VOID_TYPE_P (TREE_TYPE (base))
+         || TREE_THIS_VOLATILE (decl) != TREE_THIS_VOLATILE (base))
        return decl;
       if ((TREE_CODE (TREE_TYPE (decl)) == VECTOR_TYPE
           || TREE_CODE (TREE_TYPE (decl)) == COMPLEX_TYPE)