Re: [PATCH v4] Fix ICE when mixing VLAs and statement expressions [PR91038]

[Jason Merrill via Gcc-patches]

On 10/17/21 09:52, Uecker, Martin wrote:


Here is the 4th version of the patch. I tried to implement
Jason's suggestion and this also fixes the problem. But
I am not sure I understand the condition on
the TREE_SIDE_EFFECTS ...

Checking TREE_SIDE_EFFECTS filters out many trivial cases that we don't 
need to worry about.  I think we also want to check 
variably_modified_type_p, which ought to avoid the OMP problem below.

And there is now another problem:

c_finish_omp_for in c-family/c-omp.c does not seem
to understand the expressions anymore and I get a test
failure in

testsuite/c-c++-common/gomp/for-5.c

where I now get an "invalid increment expression"
instead of the expected error.

(bootstrapping and all other tests work fine)


Martin




Fix ICE when mixing VLAs and statement expressions [PR91038]

When returning VM-types from statement expressions, this can
lead to an ICE when declarations from the statement expression
are referred to later. Most of these issues can be addressed by
gimplifying the base expression earlier in gimplify_compound_lval.
>
Another issue is fixed by adding SAVE_EXPRs in pointer_int_sum
in the FE to force a correct order of evaluation. This fixes
PR91038 and some of the test cases from PR29970 (structs with
VLA members need further work).

     
     2021-08-01  Martin Uecker  <muec...@gwdg.de>
     
    2021-08-01  Martin Uecker  <muec...@gwdg.de>
     
     gcc/
         PR c/91038
         PR c/29970
      
    * gimplify.c (gimplify_var_or_parm_decl): Update comment.
         (gimplify_compound_lval):
Gimplify base expression first.
         (gimplify_target_expr): Add comment.
         * c-family/c-
common.c (pointer_int_sum): Wrap pointer
        operand in SAVE_EXPR and also it to the integer
argument.
     
     gcc/testsuite/
         PR c/91038
         PR c/29970
         * gcc.dg/vla-stexp-3.c:
New test.
         * gcc.dg/vla-stexp-4.c: New test.
         * gcc.dg/vla-stexp-5.c: New test.
         *
gcc.dg/vla-stexp-6.c: New test.
         * gcc.dg/vla-stexp-7.c: New test.
         * gcc.dg/vla-stexp-
8.c: New test.
         * gcc.dg/vla-stexp-9.c: New test.


diff --git a/gcc/c-family/c-common.c b/gcc/c-family/c-common.c
index 9d19e352725..522085664f5 100644
--- a/gcc/c-family/c-common.c
+++ b/gcc/c-family/c-common.c
@@ -3348,6 +3348,16 @@ pointer_int_sum (location_t loc, enum tree_code 
resultcode,
      intop = convert (c_common_type_for_size (TYPE_PRECISION (sizetype),
                                             TYPE_UNSIGNED (sizetype)), intop);
  
+  /* Wrap the pointer expression in a SAVE_EXPR to make sure it
+   * is evaluated first because the size expression may depend on it
+   * for VM types.
+   */

We usually don't give the trailing */ its own line.

+  if (TREE_SIDE_EFFECTS (size_exp))
+    {
+        ptrop = build1_loc (loc, SAVE_EXPR, TREE_TYPE (ptrop), ptrop);

Why not use the save_expr function?

+        intop = build2 (COMPOUND_EXPR, TREE_TYPE (intop), ptrop, intop);
+    }
+
    /* Replace the integer argument with a suitable product by the object size.
       Do this multiplication as signed, then convert to the appropriate type
       for the pointer operation and disregard an overflow that occurred only
diff --git a/gcc/gimplify.c b/gcc/gimplify.c
index d8e4b139349..be5b00b6716 100644
--- a/gcc/gimplify.c
+++ b/gcc/gimplify.c
@@ -2958,7 +2958,10 @@ gimplify_var_or_parm_decl (tree *expr_p)
       declaration, for which we've already issued an error.  It would
       be really nice if the front end wouldn't leak these at all.
       Currently the only known culprit is C++ destructors, as seen
-     in g++.old-deja/g++.jason/binding.C.  */
+     in g++.old-deja/g++.jason/binding.C.
+     Another possible culpit are size expressions for variably modified
+     types which are lost in the FE or not gimplified correctly.
+  */

As above.

    if (VAR_P (decl)
        && !DECL_SEEN_IN_BIND_EXPR_P (decl)
        && !TREE_STATIC (decl) && !DECL_EXTERNAL (decl)
@@ -3103,16 +3106,22 @@ gimplify_compound_lval (tree *expr_p, gimple_seq 
*pre_p, gimple_seq *post_p,
       expression until we deal with any variable bounds, sizes, or
       positions in order to deal with PLACEHOLDER_EXPRs.
  
-     So we do this in three steps.  First we deal with the annotations
-     for any variables in the components, then we gimplify the base,
-     then we gimplify any indices, from left to right.  */
+     The base expression may contain a statement expression that
+     has declarations used in size expressions, so has to be
+     gimplified before gimplifying the size expressions.
+
+     So we do this in three steps.  First we deal with variable
+     bounds, sizes, and positions, then we gimplify the base,
+     then we deal with the annotations for any variables in the
+     components and any indices, from left to right.  */
+
    for (i = expr_stack.length () - 1; i >= 0; i--)
      {
        tree t = expr_stack[i];
  
        if (TREE_CODE (t) == ARRAY_REF || TREE_CODE (t) == ARRAY_RANGE_REF)
        {
-         /* Gimplify the low bound and element type size and put them into
+         /* Deal with the low bound and element type size and put them into
             the ARRAY_REF.  If these values are set, they have already been
             gimplified.  */
          if (TREE_OPERAND (t, 2) == NULL_TREE)
@@ -3121,18 +3130,8 @@ gimplify_compound_lval (tree *expr_p, gimple_seq *pre_p, 
gimple_seq *post_p,
              if (!is_gimple_min_invariant (low))
                {
                  TREE_OPERAND (t, 2) = low;
-                 tret = gimplify_expr (&TREE_OPERAND (t, 2), pre_p,
-                                       post_p, is_gimple_reg,
-                                       fb_rvalue);
-                 ret = MIN (ret, tret);
                }
            }
-         else
-           {
-             tret = gimplify_expr (&TREE_OPERAND (t, 2), pre_p, post_p,
-                                   is_gimple_reg, fb_rvalue);
-             ret = MIN (ret, tret);
-           }
  
  	  if (TREE_OPERAND (t, 3) == NULL_TREE)
            {
@@ -3149,18 +3148,8 @@ gimplify_compound_lval (tree *expr_p, gimple_seq *pre_p, 
gimple_seq *post_p,
                                              elmt_size, factor);
  
  		  TREE_OPERAND (t, 3) = elmt_size;
-                 tret = gimplify_expr (&TREE_OPERAND (t, 3), pre_p,
-                                       post_p, is_gimple_reg,
-                                       fb_rvalue);
-                 ret = MIN (ret, tret);
                }
            }
-         else
-           {
-             tret = gimplify_expr (&TREE_OPERAND (t, 3), pre_p, post_p,
-                                   is_gimple_reg, fb_rvalue);
-             ret = MIN (ret, tret);
-           }
        }
        else if (TREE_CODE (t) == COMPONENT_REF)
        {
@@ -3180,18 +3169,8 @@ gimplify_compound_lval (tree *expr_p, gimple_seq *pre_p, 
gimple_seq *post_p,
                                           offset, factor);
  
  		  TREE_OPERAND (t, 2) = offset;
-                 tret = gimplify_expr (&TREE_OPERAND (t, 2), pre_p,
-                                       post_p, is_gimple_reg,
-                                       fb_rvalue);
-                 ret = MIN (ret, tret);
                }
            }
-         else
-           {
-             tret = gimplify_expr (&TREE_OPERAND (t, 2), pre_p, post_p,
-                                   is_gimple_reg, fb_rvalue);
-             ret = MIN (ret, tret);
-           }
        }
      }
  
@@ -3202,21 +3181,34 @@ gimplify_compound_lval (tree *expr_p, gimple_seq *pre_p, gimple_seq *post_p,
                        fallback | fb_lvalue);
    ret = MIN (ret, tret);
  
-  /* And finally, the indices and operands of ARRAY_REF.  During this
-     loop we also remove any useless conversions.  */
+  /* Step 3: gimplify size expressions and the indices and operands of
+     ARRAY_REF.  During this loop we also remove any useless conversions.  */
+
    for (; expr_stack.length () > 0; )
      {
        tree t = expr_stack.pop ();
  
        if (TREE_CODE (t) == ARRAY_REF || TREE_CODE (t) == ARRAY_RANGE_REF)
        {
+         /* Gimplify the low bound and element type size. */
+         tret = gimplify_expr (&TREE_OPERAND (t, 2), pre_p, post_p,
+                               is_gimple_reg, fb_rvalue);
+         ret = MIN (ret, tret);
+
+         tret = gimplify_expr (&TREE_OPERAND (t, 3), pre_p, post_p,
+                               is_gimple_reg, fb_rvalue);
+         ret = MIN (ret, tret);
+
          /* Gimplify the dimension.  */
-         if (!is_gimple_min_invariant (TREE_OPERAND (t, 1)))
-           {
-             tret = gimplify_expr (&TREE_OPERAND (t, 1), pre_p, post_p,
-                                   is_gimple_val, fb_rvalue);
-             ret = MIN (ret, tret);
-           }
+         tret = gimplify_expr (&TREE_OPERAND (t, 1), pre_p, post_p,
+                               is_gimple_val, fb_rvalue);
+         ret = MIN (ret, tret);
+       }
+      else if (TREE_CODE (t) == COMPONENT_REF)
+       {
+         tret = gimplify_expr (&TREE_OPERAND (t, 2), pre_p, post_p,
+                               is_gimple_reg, fb_rvalue);
+         ret = MIN (ret, tret);
        }
  
        STRIP_USELESS_TYPE_CONVERSION (TREE_OPERAND (t, 0));
@@ -6914,6 +6906,8 @@ gimplify_target_expr (tree *expr_p, gimple_seq *pre_p, 
gimple_seq *post_p)
        {
          if (!TYPE_SIZES_GIMPLIFIED (TREE_TYPE (temp)))
            gimplify_type_sizes (TREE_TYPE (temp), pre_p);
+         /* FIXME: this is correct only when the size of the type does
+            not depend on expressions evaluated in init. */
          gimplify_vla_decl (temp, pre_p);
        }
        else