[Bug middle-end/65315] New: incorrect alignment of local variable with aligned attribute

[Bug middle-end/65315] New: incorrect alignment of local variable with aligned attribute

[sje at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65315

            Bug ID: 65315
           Summary: incorrect alignment of local variable with aligned
                    attribute
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: sje at gcc dot gnu.org

Created attachment 34953
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=34953&action=edit
Test case

When GCC has multiple local variables with different aligned attributes on
them, it creates an aligned block of space aligned to the least aligned
variable instead of the most aligned variable.

In the attached program, when run on MIPS, GCC is creating a 32 byte aligned
block of memory for foo1, foo3, and foo4 and a 128 byte aligned memory block
for foo2.  foo3 and foo4 should have 128 byte aligned memory.

I thought the problem was in stack_var_cmp where it checks the alignment of
two variables and returns '(int) largeb - (int) largea' to sort the variables
based on their alignment but when I swapped the two arguments I got an ICE
in expand_stack_vars [gcc_assert (large_base != NULL);].

It also tried changing:

large_align = stack_vars[stack_vars_sorted[0]].alignb * BITS_PER_UNIT;

to

large_align = stack_vars[stack_vars_sorted[n-1]].alignb * BITS_PER_UNIT;

but that caused the same ICE.

It may be that we need a loop to check the alignment of each variable.

[Bug middle-end/65315] incorrect alignment of local variable with aligned attribute

[sje at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65315

--- Comment #1 from Steve Ellcey <sje at gcc dot gnu.org> ---
It might be easier to see this bug if you apply this patch:

diff --git a/gcc/cfgexpand.c b/gcc/cfgexpand.c
index 7dfe1f6..7beb00e 100644
--- a/gcc/cfgexpand.c
+++ b/gcc/cfgexpand.c
@@ -973,6 +973,8 @@ expand_stack_vars (bool (*pred) (size_t), struct sta
ck_vars_data *data)
       i = stack_vars_sorted[si];
       alignb = stack_vars[i].alignb;

+      gcc_assert (alignb*BITS_PER_UNIT <= large_align);
+
       /* Stop when we get to the first decl with "small" alignment.  */
       if (alignb * BITS_PER_UNIT <= MAX_SUPPORTED_STACK_ALIGNMENT)
         break;


This way you get an ICE instead of just incorrectly aligned vectors.

[Bug middle-end/65315] incorrect alignment of local variable with aligned attribute

[sje at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65315

--- Comment #3 from Steve Ellcey <sje at gcc dot gnu.org> ---
Author: sje
Date: Thu Mar  5 16:34:03 2015
New Revision: 221219

URL: https://gcc.gnu.org/viewcvs?rev=221219&root=gcc&view=rev
Log:
2015-03-05  Steve Ellcey  <sellcey@imgtec.com>

    PR middle-end/65315
    * cfgexpand.c (expand_stack_vars): Update large_align to maximum
    needed alignment.

Modified:
    trunk/gcc/ChangeLog
    trunk/gcc/cfgexpand.c

[Bug middle-end/65315] incorrect alignment of local variable with aligned attribute

[sje at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65315

Steve Ellcey <sje at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |FIXED
   Target Milestone|---                         |5.0

--- Comment #4 from Steve Ellcey <sje at gcc dot gnu.org> ---
A patch for this was checked in before 5.0 branched.