Re: [REPOST] Invalid Code when reading from unaligned zero-sized array

[Richard Biener <richard dot guenther at gmail dot com>]

On Tue, Dec 3, 2013 at 2:10 PM, Richard Biener
<richard.guenther@gmail.com> wrote:
> On Tue, Dec 3, 2013 at 1:48 PM, Bernd Edlinger
> <bernd.edlinger@hotmail.de> wrote:
>> Hi Jeff,
>>
>> please find attached the patch (incl. test cases) for the unaligned read BUG that I found while investigating
>> on PR#57748: http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57748
>>
>> one test case is this one:
>>
>> pr57748-3.c:
>> /* PR middle-end/57748 */
>> /* { dg-do run } */
>> /* wrong code in expand_expr_real_1.  */
>>
>> #include <stdlib.h>
>>
>> extern void abort (void);
>>
>> typedef long long V
>>   __attribute__ ((vector_size (2 * sizeof (long long)), may_alias));
>>
>> typedef struct S { V a; V b[0]; } P __attribute__((aligned (1)));
>>
>> struct __attribute__((packed)) T { char c; P s; };
>>
>> void __attribute__((noinline, noclone))
>> check (P *p)
>> {
>>   if (p->b[0][0] != 3 || p->b[0][1] != 4)
>>     abort ();
>> }
>>
>> void __attribute__((noinline, noclone))
>> foo (struct T *t)
>> {
>>   V a = { 3, 4 };
>>   t->s.b[0] = a;
>> }
>>
>> int
>> main ()
>> {
>>   struct T *t = (struct T *) calloc (128, 1);
>>
>>   foo (t);
>>   check (&t->s);
>>
>>   free (t);
>>   return 0;
>> }
>>
>>
>> and the other one is
>> pr57748-4.c:
>> /* PR middle-end/57748 */
>> /* { dg-do run } */
>> /* wrong code in expand_expr_real_1.  */
>>
>> #include <stdlib.h>
>>
>> extern void abort (void);
>>
>> typedef long long V
>>   __attribute__ ((vector_size (2 * sizeof (long long)), may_alias));
>>
>> typedef struct S { V b[1]; } P __attribute__((aligned (1)));
>>
>> struct __attribute__((packed)) T { char c; P s; };
>>
>> void __attribute__((noinline, noclone))
>> check (P *p)
>> {
>>   if (p->b[1][0] != 3 || p->b[1][1] != 4)
>>     abort ();
>> }
>>
>> void __attribute__((noinline, noclone))
>> foo (struct T *t)
>> {
>>   V a = { 3, 4 };
>>   t->s.b[1] = a;
>> }
>>
>> int
>> main ()
>> {
>>   struct T *t = (struct T *) calloc (128, 1);
>>
>>   foo (t);
>>   check (&t->s);
>>
>>   free (t);
>>   return 0;
>> }
>>
>>
>> The patch does add a boolean "expand_reference" parameter to expand_expr_real and
>> expand_expr_real_1. I pass true when I intend to use the returned memory context
>> as an array reference, instead of a value. At places where mis-aligned values are extracted,
>> I do not return a register with the extracted mis-aligned value if expand_reference is true.
>> When I have a VIEW_CONVERT_EXPR I pay attention to pass down the outer "expand_reference"
>> to the inner expand_expr_real call. Expand_reference, is pretty much similar to the
>> expand_modifier "EXPAND_MEMORY".
>>
>> Boot-strapped and regression-tested on X86_64-pc-linux-gnu (many times).
>>
>> Ok for trunk?
>
> It still feels like papering over the underlying issue.  Let me have a
> second (or third?) look.

Few comments on your patch.

@@ -9520,6 +9526,7 @@ expand_expr_real_1 (tree exp, rtx target, enum mac
        align = get_object_alignment (exp);
        if (modifier != EXPAND_WRITE
            && modifier != EXPAND_MEMORY
+           && !expand_reference
            && mode != BLKmode
            && align < GET_MODE_ALIGNMENT (mode)
            /* If the target does not have special handling for unaligned

(TARGET_MEM_REF), expand_reference should never be true here,
there may be no component-refs around TARGET_MEM_REFs.

You miss adjusting the VIEW_CONVERT_EXPR path?  (line-numbers
are off a lot in your patch, context doesn't help very much :/  Does not
seem to be against 4.8 either ...)

Index: gcc/cfgexpand.c
===================================================================
--- gcc/cfgexpand.c     (revision 204411)
+++ gcc/cfgexpand.c     (working copy)
@@ -2189,7 +2189,7 @@ expand_call_stmt (gimple stmt)
   if (lhs)
     expand_assignment (lhs, exp, false);
   else
-    expand_expr_real_1 (exp, const0_rtx, VOIDmode, EXPAND_NORMAL, NULL);
+    expand_expr_real_1 (exp, const0_rtx, VOIDmode, EXPAND_NORMAL, NULL, false);

   mark_transaction_restart_calls (stmt);
 }

this should use

  expand_expr (exp, const0_rtx, VOIDmode, EXPAND_NORMAL);

anyway.

@@ -10286,7 +10297,10 @@ expand_expr_real_1 (tree exp, rtx target, enum mac
              op0 = copy_rtx (op0);
              set_mem_align (op0, MAX (MEM_ALIGN (op0), TYPE_ALIGN (type)));
            }
-         else if (mode != BLKmode
+         else if (modifier != EXPAND_WRITE
+                  && modifier != EXPAND_MEMORY
+                  && !expand_reference
+                  && mode != BLKmode
                   && MEM_ALIGN (op0) < GET_MODE_ALIGNMENT (mode)
                   /* If the target does have special handling for unaligned
                      loads of mode then use them.  */

@@ -10307,6 +10321,9 @@ expand_expr_real_1 (tree exp, rtx target, enum mac
              return reg;
            }
          else if (STRICT_ALIGNMENT
+                  && modifier != EXPAND_WRITE
+                  && modifier != EXPAND_MEMORY
+                  && !expand_reference
                   && mode != BLKmode
                   && MEM_ALIGN (op0) < GET_MODE_ALIGNMENT (mode))
            {

why the unrelated change to add the modifier checks?  Looks like both
if cases are close by and factoring out common code like

   else if (!expand_reference
             && mode != BLKmode
             && MEM_ALIGN (...
     {
        if (...)
        else if (STRICT_ALIGNMENT)

would be better, also matching the other similar occurances.

Looking for some more time your patch may be indeed the easiest
without big re-factoring.

Richard.

> Richard.
>
>>
>> Thanks
>> Bernd.