c++: Fix up -fstrong-eval-order handling of call arguments [PR70796]

For -fstrong-eval-order (default for C++17 and later) we make sure to
gimplify arguments in the right order, but as the following testcase
shows that is not enough.
The problem is that some lvalues can satisfy the is_gimple_val / fb_rvalue
predicate used by gimplify_arg for is_gimple_reg_type typed expressions,
or is_gimple_lvalue / fb_either used for other types.
E.g. in foo we have:
  C::C (&p,  ++i,  ++i)
before gimplification where i is an automatic int variable and without this
patch gimplify that as:
  i = i + 1;
  i = i + 1;
  C::C (&p, i, i);
which means that the ctor is called with the original i value incremented
by 2 in both arguments, while because the call is CALL_EXPR_ORDERED_ARGS
the first argument should be different.  Similarly in qux we have:
  B::B (&p, TARGET_EXPR <D.2274, *(const struct A &) A::operator++ (&i)>,
        TARGET_EXPR <D.2275, *(const struct A &) A::operator++ (&i)>)
and gimplify it as:
      _1 = A::operator++ (&i);
      _2 = A::operator++ (&i);
      B::B (&p, MEM[(const struct A &)_1], MEM[(const struct A &)_2]);
but because A::operator++ returns the passed in argument, again we have
the same value in both cases due to gimplify_arg doing:
      /* Also strip a TARGET_EXPR that would force an extra copy.  */
      if (TREE_CODE (*arg_p) == TARGET_EXPR)
        {
          tree init = TARGET_EXPR_INITIAL (*arg_p);
          if (init
              && !VOID_TYPE_P (TREE_TYPE (init)))
            *arg_p = init;
        }
which is perfectly fine optimization for calls with unordered arguments,
but breaks the ordered ones.
Lastly, in corge, we have before gimplification:
  D::foo (NON_LVALUE_EXPR <p>, 3,  ++p)
and gimplify it as
  p = p + 4;
  D::foo (p, 3, p);
which is again wrong, because the this argument isn't before the
side-effects but after it.
The following patch adds cp_gimplify_arg wrapper, which if ordered
and is_gimple_reg_type forces non-SSA_NAME is_gimple_variable
result into a temporary, and if ordered, not is_gimple_reg_type
and argument is TARGET_EXPR bypasses the gimplify_arg optimization.
So, in foo with this patch we gimplify it as:
  i = i + 1;
  i.0_1 = i;
  i = i + 1;
  C::C (&p, i.0_1, i);
in qux as:
      _1 = A::operator++ (&i);
      D.2312 = MEM[(const struct A &)_1];
      _2 = A::operator++ (&i);
      B::B (&p, D.2312, MEM[(const struct A &)_2]);
where D.2312 is a temporary and in corge as:
  p.9_1 = p;
  p = p + 4;
  D::foo (p.9_1, 3, p);
The is_gimple_reg_type forcing into a temporary should be really cheap
(I think even at -O0 it should be optimized if there is no modification in
between), the aggregate copies might be more expensive but I think e.g. SRA
or FRE should be able to deal with those if there are no intervening
changes.  But still, the patch tries to avoid those when it is cheaply
provable that nothing bad happens (if no argument following it in the
strong evaluation order doesn't have TREE_SIDE_EFFECTS, then even VAR_DECLs
etc. shouldn't be modified after it).  There is also an optimization to
avoid doing that for this or for arguments with reference types as nothing
can modify the parameter values during evaluation of other argument's
side-effects.

I've tried if e.g.
  int i = 1;
  return i << ++i;
doesn't suffer from this problem as well, but it doesn't, the FE uses
  SAVE_EXPR <i>, SAVE_EXPR <i> << ++i;
in that case which gimplifies the way we want (temporary in the first
operand).

2021-11-19  Jakub Jelinek  <jakub@redhat.com>

	PR c++/70796
	* cp-gimplify.c (cp_gimplify_arg): New function.
	(cp_gimplify_expr): Use cp_gimplify_arg instead of gimplify_arg,
	pass true as last argument to it if there are any following
	arguments in strong evaluation order with side-effects.

	* g++.dg/cpp1z/eval-order11.C: New test.

(cherry picked from commit a84177aff7ca86f501d6aa5ef407fac5e71f56fb)

diff --git a/gcc/cp/cp-gimplify.c b/gcc/cp/cp-gimplify.c
index 741a69b66ac9ad93da276201415ef2456ff2c2a4..9d87ff42b2f3a2622b74577a771321680ee83366 100644 (file)
--- a/gcc/cp/cp-gimplify.c
+++ b/gcc/cp/cp-gimplify.c
@@ -674,6 +674,47 @@ gimplify_to_rvalue (tree *expr_p, gimple_seq *pre_p, gimple_seq *post_p,
   return t;
 }
 
+/* Like gimplify_arg, but if ORDERED is set (which should be set if
+   any of the arguments this argument is sequenced before has
+   TREE_SIDE_EFFECTS set, make sure expressions with is_gimple_reg_type type
+   are gimplified into SSA_NAME or a fresh temporary and for
+   non-is_gimple_reg_type we don't optimize away TARGET_EXPRs.  */
+
+static enum gimplify_status
+cp_gimplify_arg (tree *arg_p, gimple_seq *pre_p, location_t call_location,
+                bool ordered)
+{
+  enum gimplify_status t;
+  if (ordered
+      && !is_gimple_reg_type (TREE_TYPE (*arg_p))
+      && TREE_CODE (*arg_p) == TARGET_EXPR)
+    {
+      /* gimplify_arg would strip away the TARGET_EXPR, but
+        that can mean we don't copy the argument and some following
+        argument with side-effect could modify it.  */
+      protected_set_expr_location (*arg_p, call_location);
+      return gimplify_expr (arg_p, pre_p, NULL, is_gimple_lvalue, fb_either);
+    }
+  else
+    {
+      t = gimplify_arg (arg_p, pre_p, call_location);
+      if (t == GS_ERROR)
+       return GS_ERROR;
+      else if (ordered
+              && is_gimple_reg_type (TREE_TYPE (*arg_p))
+              && is_gimple_variable (*arg_p)
+              && TREE_CODE (*arg_p) != SSA_NAME
+              /* No need to force references into register, references
+                 can't be modified.  */
+              && !TYPE_REF_P (TREE_TYPE (*arg_p))
+              /* And this can't be modified either.  */
+              && *arg_p != current_class_ptr)
+       *arg_p = get_initialized_tmp_var (*arg_p, pre_p);
+      return t;
+    }
+
+}
+
 /* Do C++-specific gimplification.  Args are as for gimplify_expr.  */
 
 int
@@ -888,7 +929,8 @@ cp_gimplify_expr (tree *expr_p, gimple_seq *pre_p, gimple_seq *post_p)
          gcc_assert (call_expr_nargs (*expr_p) == 2);
          gcc_assert (!CALL_EXPR_ORDERED_ARGS (*expr_p));
          enum gimplify_status t
-           = gimplify_arg (&CALL_EXPR_ARG (*expr_p, 1), pre_p, loc);
+           = cp_gimplify_arg (&CALL_EXPR_ARG (*expr_p, 1), pre_p, loc,
+                              TREE_SIDE_EFFECTS (CALL_EXPR_ARG (*expr_p, 0)));
          if (t == GS_ERROR)
            ret = GS_ERROR;
        }
@@ -897,10 +939,18 @@ cp_gimplify_expr (tree *expr_p, gimple_seq *pre_p, gimple_seq *post_p)
          /* Leave the last argument for gimplify_call_expr, to avoid problems
             with __builtin_va_arg_pack().  */
          int nargs = call_expr_nargs (*expr_p) - 1;
+         int last_side_effects_arg = -1;
+         for (int i = nargs; i > 0; --i)
+           if (TREE_SIDE_EFFECTS (CALL_EXPR_ARG (*expr_p, i)))
+             {
+               last_side_effects_arg = i;
+               break;
+             }
          for (int i = 0; i < nargs; ++i)
            {
              enum gimplify_status t
-               = gimplify_arg (&CALL_EXPR_ARG (*expr_p, i), pre_p, loc);
+               = cp_gimplify_arg (&CALL_EXPR_ARG (*expr_p, i), pre_p, loc,
+                                  i < last_side_effects_arg);
              if (t == GS_ERROR)
                ret = GS_ERROR;
            }
@@ -914,8 +964,17 @@ cp_gimplify_expr (tree *expr_p, gimple_seq *pre_p, gimple_seq *post_p)
            fntype = TREE_TYPE (fntype);
          if (TREE_CODE (fntype) == METHOD_TYPE)
            {
+             int nargs = call_expr_nargs (*expr_p);
+             bool side_effects = false;
+             for (int i = 1; i < nargs; ++i)
+               if (TREE_SIDE_EFFECTS (CALL_EXPR_ARG (*expr_p, i)))
+                 {
+                   side_effects = true;
+                   break;
+                 }
              enum gimplify_status t
-               = gimplify_arg (&CALL_EXPR_ARG (*expr_p, 0), pre_p, loc);
+               = cp_gimplify_arg (&CALL_EXPR_ARG (*expr_p, 0), pre_p, loc,
+                                  side_effects);
              if (t == GS_ERROR)
                ret = GS_ERROR;
            }

diff --git a/gcc/testsuite/g++.dg/cpp1z/eval-order11.C b/gcc/testsuite/g++.dg/cpp1z/eval-order11.C
new file mode 100644 (file)
index 0000000..19a24c8
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/eval-order11.C
@@ -0,0 +1,89 @@
+// PR c++/70796
+// { dg-do run { target c++11 } }
+// { dg-options "-fstrong-eval-order" { target c++14_down } }
+
+struct A
+{
+  int x = 0;
+  A & operator ++ () { ++x; return *this; }
+};
+struct B
+{
+  A first, second;
+  B (A x, A y) : first{x}, second{y} {}
+};
+struct C
+{
+  int first, second;
+  C (int x, int y) : first{x}, second{y} {}
+};
+struct D
+{
+  int d;
+  void foo (int x, D *y)
+  {
+    if (y != this + 1)
+      __builtin_abort ();
+    d = x;
+  }
+};
+D d[2] = { { 1 }, { 2 } };
+
+void
+foo ()
+{
+  int i = 0;
+  C p{++i, ++i};
+  if (p.first != 1 || p.second != 2)
+    __builtin_abort ();
+}
+
+void
+bar ()
+{
+  int i = 0;
+  C p{++i, ++i};
+  if (p.first != 1 || p.second != 2)
+    __builtin_abort ();
+  int &j = i;
+  C q{++j, ++j};
+  if (q.first != 3 || q.second != 4)
+    __builtin_abort ();
+}
+
+void
+baz ()
+{
+  int i = 0;
+  C p{(int &) ++i, (int &) ++i};
+  if (p.first != 1 || p.second != 2)
+    __builtin_abort ();
+}
+
+void
+qux ()
+{
+  A i;
+  B p{++i, ++i};
+  if (p.first.x != 1 || p.second.x != 2)
+    __builtin_abort ();
+}
+
+void
+corge ()
+{
+  D *p = &d[0];
+  p->foo (3, ++p);
+  if (d[0].d != 3 || d[1].d != 2)
+    __builtin_abort ();
+}
+
+int
+main ()
+{
+  bar ();
+  baz ();
+  foo ();
+  qux ();
+  corge ();
+}