[Bug rtl-optimization/25654] New: [4.0/4.1/4.2 Regression] RTL alias analysis unprepared to handle stack slot sharing

[Bug rtl-optimization/25654] New: [4.0/4.1/4.2 Regression] RTL alias analysis unprepared to handle stack slot sharing

[steven at gcc dot gnu dot org]

In the following C code, the order of loads and stores is messed up, leading to
wrong code:

extern void abort (void) __attribute__((noreturn));

union setconflict
{
  short a[20];
  int b[10];
};

int
main ()
{
  int sum = 0;
  {
    union setconflict a;
    short *c;
    c = a.a;
    asm ("": "=r" (c):"0" (c));
    *c = 0;
    asm ("": "=r" (c):"0" (c));
    sum += *c;
  }
  {
    union setconflict a;
    int *c;
    c = a.b;
    asm ("": "=r" (c):"0" (c));
    *c = 1;
    asm ("": "=r" (c):"0" (c));
    sum += *c;
  }

  printf ("%d\n",sum);
  if (sum != 1)
    abort();
  return 0;
}



        .file   "t.c"
# GNU C version 4.2.0 20060101 (experimental) (x86_64-unknown-linux-gnu)
#       compiled by GNU C version 4.0.2 20050901 (prerelease) (SUSE Linux).
# GGC heuristics: --param ggc-min-expand=30 --param ggc-min-heapsize=4096
# options passed:  -iprefix -isystem -m32 -march=pentiumpro -auxbase -O2
# -fdump-tree-vars -fomit-frame-pointer -fverbose-asm
# options enabled:  -falign-loops -fargument-alias -fbranch-count-reg
# -fcaller-saves -fcommon -fcprop-registers -fcrossjumping
# -fcse-follow-jumps -fcse-skip-blocks -fdefer-pop
# -fdelete-null-pointer-checks -fearly-inlining
# -feliminate-unused-debug-types -fexpensive-optimizations -ffunction-cse
# -fgcse -fgcse-lm -fguess-branch-probability -fident -fif-conversion
# -fif-conversion2 -finline-functions-called-once -fipa-pure-const
# -fipa-reference -fipa-type-escape -fivopts -fkeep-static-consts
# -fleading-underscore -floop-optimize -floop-optimize2 -fmath-errno
# -fmerge-constants -fomit-frame-pointer -foptimize-register-move
# -foptimize-sibling-calls -fpcc-struct-return -fpeephole -fpeephole2
# -fregmove -freorder-blocks -freorder-functions -frerun-cse-after-loop
# -frerun-loop-opt -fsched-interblock -fsched-spec
# -fsched-stalled-insns-dep -fschedule-insns2 -fshow-column
# -fsplit-ivs-in-unroller -fstrength-reduce -fstrict-aliasing
# -fthread-jumps -ftrapping-math -ftree-ccp -ftree-ch -ftree-copy-prop
# -ftree-copyrename -ftree-dce -ftree-dominator-opts -ftree-dse -ftree-fre
# -ftree-loop-im -ftree-loop-ivcanon -ftree-loop-optimize -ftree-lrs
# -ftree-pre -ftree-salias -ftree-sink -ftree-sra -ftree-store-ccp
# -ftree-store-copy-prop -ftree-ter -ftree-vect-loop-version -ftree-vrp
# -funit-at-a-time -fverbose-asm -fzero-initialized-in-bss -m32 -m80387
# -m96bit-long-double -maccumulate-outgoing-args -malign-stringops
# -mfancy-math-387 -mfp-ret-in-387 -mieee-fp -mno-red-zone -mpush-args
# -mtls-direct-seg-refs

# Compiler executable checksum: 6d90f1c30ff8027bc6976ab2dbfe2320

        .section        .rodata.str1.1,"aMS",@progbits,1
.LC0:
        .string "%d\n"
        .text
        .p2align 4,,15
.globl main
        .type   main, @function
main:
        leal    4(%esp), %ecx   #,
        andl    $-16, %esp      #,
        pushl   -4(%ecx)        #
        subl    $76, %esp       #,
        leal    28(%esp), %eax  #, tmp64
        movl    %ecx, 68(%esp)  #,
        movl    %eax, %edx      # tmp64, c
        movl    %ebx, 72(%esp)  #,
        movw    $0, (%edx)      #,* c
        movl    $1, (%eax)      #,* c
        movswl  (%edx),%ebx     #* c, sum
        movl    (%eax), %edx    #* c,
        movl    $.LC0, (%esp)   #,
        addl    %edx, %ebx      #, sum
        movl    %ebx, 4(%esp)   # sum,
        call    printf  #
        decl    %ebx    # sum
        jne     .L6     #,
        movl    68(%esp), %ecx  #,
        xorl    %eax, %eax      # <result>
        movl    72(%esp), %ebx  #,
        addl    $76, %esp       #,
        leal    -4(%ecx), %esp  #,
        ret
.L6:
        call    abort   #
        .size   main, .-main
        .ident  "GCC: (GNU) 4.2.0 20060101 (experimental)"
        .section        .note.GNU-stack,"",@progbits

See also the thread on the gcc@ mailing list starting here:
http://gcc.gnu.org/ml/gcc/2006-01/msg00008.html.


-- 
           Summary: [4.0/4.1/4.2 Regression] RTL alias analysis unprepared
                    to handle stack slot sharing
           Product: gcc
           Version: 4.2.0
            Status: UNCONFIRMED
          Keywords: wrong-code, alias
          Severity: major
          Priority: P3
         Component: rtl-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: steven at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25654

[Bug rtl-optimization/25654] [4.0/4.1/4.2 Regression] RTL alias analysis unprepared to handle stack slot sharing

[pinskia at gcc dot gnu dot org]

------- Comment #1 from pinskia at gcc dot gnu dot org  2006-01-03 23:23 -------
Confirmed.


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |pinskia at gcc dot gnu dot
                   |                            |org
           Severity|major                       |critical
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|0                           |1
      Known to fail|                            |4.0.3 4.1.0 4.2.0
      Known to work|                            |3.4.4
   Last reconfirmed|0000-00-00 00:00:00         |2006-01-03 23:23:36
               date|                            |
   Target Milestone|---                         |4.0.3


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25654

[Bug rtl-optimization/25654] [4.0/4.1/4.2 Regression] RTL alias analysis unprepared to handle stack slot sharing

[steven at gcc dot gnu dot org]

-- 

steven at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Priority|P3                          |P1


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25654

[Bug rtl-optimization/25654] [4.0/4.1/4.2 Regression] RTL alias analysis unprepared to handle stack slot sharing

[steven at gcc dot gnu dot org]

------- Comment #2 from steven at gcc dot gnu dot org  2006-01-13 21:30 -------
I wonder if this problem can also be triggered without using two variables of
the same union type.  There is code in add_alias_set_conflicts to avoid the
situation we're running into:

static void
add_alias_set_conflicts (void)
{
  size_t i, j, n = stack_vars_num;

  for (i = 0; i < n; ++i)
    {
      tree type_i = TREE_TYPE (stack_vars[i].decl);
      bool aggr_i = AGGREGATE_TYPE_P (type_i);

      for (j = 0; j < i; ++j)
        {
          tree type_j = TREE_TYPE (stack_vars[j].decl);
          bool aggr_j = AGGREGATE_TYPE_P (type_j);
          if (aggr_i != aggr_j || !objects_must_conflict_p (type_i, type_j))
            add_stack_var_conflict (i, j);
        }
    }
}

but if you have two stack variables of the same union type, aggr_i == aggr_j,
and type_i == type_j so objects_must_conflict_p returns true.  So perhaps if
type_i and type_j are unions and type_i == type_j, we should also do an
add_stack_var_conflict...?


-- 

steven at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |hubicka at gcc dot gnu dot
                   |                            |org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25654

[Bug rtl-optimization/25654] [4.0/4.1/4.2 Regression] RTL alias analysis unprepared to handle stack slot sharing

[rguenth at gcc dot gnu dot org]

------- Comment #3 from rguenth at gcc dot gnu dot org  2006-01-13 22:04 -------
I think we need to add a conflict if type_i and type_j are or contain a union
of the same type.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25654

[Bug rtl-optimization/25654] [4.0/4.1/4.2 Regression] RTL alias analysis unprepared to handle stack slot sharing

[rguenth at gcc dot gnu dot org]

------- Comment #4 from rguenth at gcc dot gnu dot org  2006-01-13 22:11 -------
Like consider the case of

void foo(void)
{
 struct A { union U u; } a; struct B { union U u; } b;

}

with a same-type union wrapped in different structs.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25654

[Bug rtl-optimization/25654] [4.0/4.1/4.2 Regression] RTL alias analysis unprepared to handle stack slot sharing

[steven at gcc dot gnu dot org]

------- Comment #5 from steven at gcc dot gnu dot org  2006-01-14 00:10 -------
Hmm, not sure...  Consider this modified test case:

nion setconflict
{
  short a[20];
  int b[10];
};

int
foo (void)
{
  int sum = 0;
  {
    struct A { union setconflict u; } a;
    short *c;
    c = a.u.a;
    asm ("": "=r" (c):"0" (c));
    *c = 2;
    asm ("": "=r" (c):"0" (c));
    sum += *c;
  }
  {
    struct B { union setconflict u; } a;
    int *c;
    c = a.u.b;
    asm ("": "=r" (c):"0" (c));
    *c = 1;
    asm ("": "=r" (c):"0" (c));
    sum += *c;
  }

  return sum;
}


The two objects called a are put into different partitions because
objects_must_conflict_p (type_i, type_j) now says the types A and B don't have
to conflict, and we get:

;; Function foo (foo)

Partition 0: size 40 align 4
        a, offset 0
Partition 1: size 40 align 4
        a, offset 0

This happens because the alias sets for A and B are different.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25654

[Bug rtl-optimization/25654] [4.0/4.1/4.2 Regression] RTL alias analysis unprepared to handle stack slot sharing

[rguenth at gcc dot gnu dot org]

------- Comment #6 from rguenth at gcc dot gnu dot org  2006-01-16 11:23 -------
Created an attachment (id=10651)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=10651&action=view)
patch

Testing the attached patch.


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         AssignedTo|unassigned at gcc dot gnu   |rguenth at gcc dot gnu dot
                   |dot org                     |org
             Status|NEW                         |ASSIGNED


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25654

[Bug rtl-optimization/25654] [4.0/4.1/4.2 Regression] RTL alias analysis unprepared to handle stack slot sharing

[hubicka at gcc dot gnu dot org]

------- Comment #7 from hubicka at gcc dot gnu dot org  2006-01-16 14:56 -------
These two testcases seems to still fail for me even with the patch. (I use  b.c
-mpreferred-stack-boundary=2 -S -march=i686 -frename-registers)
extern void abort (void) __attribute__((noreturn));

struct setconflict
{
  char a[36];
  int b;
};

int
main ()
{
  int sum = 0;
  {
    struct setconflict a;
    short *c;
    c = (void *)a.a;
    asm ("": "=r" (c):"0" (c));
    *c = 0;
    asm ("": "=r" (c):"0" (c));
    sum += *c;
  }
  {
    struct setconflict a;
    int *c;
    c = (void *)a.a;
    asm ("": "=r" (c):"0" (c));
    *c = 1;
    asm ("": "=r" (c):"0" (c));
    sum += *c;
  }

  printf ("%d\n",sum);
  if (sum != 1)
    abort();
  return 0;
}

extern void abort (void) __attribute__((noreturn));

struct wrapper {
union setconflict
{
  short a[20];
  int b[10];
} a;
};

int
main ()
{
  int sum = 0;
  {
    struct wrapper a;
    short *c;
    c = a.a.a;
    asm ("": "=r" (c):"0" (c));
    *c = 0;
    asm ("": "=r" (c):"0" (c));
    sum += *c;
  }
  {
    struct wrapper a;
    int *c;
    c = a.a.b;
    asm ("": "=r" (c):"0" (c));
    *c = 1;
    asm ("": "=r" (c):"0" (c));
    sum += *c;
  }

  printf ("%d\n",sum);
  if (sum != 1)
    abort();
  return 0;
}

So looking for unions is IMO not strong enought test.  Not sure what proper
solution shall be :(
Honza


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25654

[Bug rtl-optimization/25654] [4.0/4.1/4.2 Regression] RTL alias analysis unprepared to handle stack slot sharing

[rguenth at gcc dot gnu dot org]

------- Comment #8 from rguenth at gcc dot gnu dot org  2006-01-16 15:03 -------
Your char testcase is invalid - it violates type based aliasing rules as you
access memory of type char as int and short.  Is it solved with
-fno-strict-aliasing?


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25654

[Bug rtl-optimization/25654] [4.0/4.1/4.2 Regression] RTL alias analysis unprepared to handle stack slot sharing

[rguenth at gcc dot gnu dot org]

------- Comment #9 from rguenth at gcc dot gnu dot org  2006-01-16 15:38 -------
It is solved (all are) with -fno-strict-aliasing.  Whether the failure mode is
that of an aliasing problem or not, is another question.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25654

[Bug rtl-optimization/25654] [4.0/4.1/4.2 Regression] RTL alias analysis unprepared to handle stack slot sharing

[hubicka at gcc dot gnu dot org]

------- Comment #10 from hubicka at gcc dot gnu dot org  2006-01-19 00:14 -------
My understanding of C type based aliasing rules always was that char, as an
exception, might alias with everything.  Perhaps I lived in false belief for a
while, but at least -Wstrict-aliasing seems to think so:
ibm:~ # more t.c
char a[10];
short b[10];
main()
{
  *(int *)a=5;
  *(int *)b=5;
}
ibm:~ # gcc -O2 -Wstrict-aliasing t.c
t.c: In function main:
t.c:6: warning: dereferencing type-punned pointer will break strict-aliasing
rules


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25654

[Bug rtl-optimization/25654] [4.0/4.1/4.2 Regression] RTL alias analysis unprepared to handle stack slot sharing

[pinskia at gcc dot gnu dot org]

------- Comment #11 from pinskia at gcc dot gnu dot org  2006-01-19 00:19 -------
(In reply to comment #10)
> My understanding of C type based aliasing rules always was that char, as an
> exception, might alias with everything.  Perhaps I lived in false belief for a
> while, but at least -Wstrict-aliasing seems to think so:

The aliasing rules are asymmetrical.

See http://gcc.gnu.org/ml/gcc-patches/2005-11/msg00980.html


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25654

[Bug rtl-optimization/25654] [4.0/4.1/4.2 Regression] RTL alias analysis unprepared to handle stack slot sharing

[hubicka at gcc dot gnu dot org]

------- Comment #12 from hubicka at gcc dot gnu dot org  2006-01-19 00:38 -------
Right, forgot about that...  At the moment I can't think of testcase that would
break the transitivity without use of unions...

Honza


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25654

[Bug rtl-optimization/25654] [4.0/4.1/4.2 Regression] RTL alias analysis unprepared to handle stack slot sharing

[mmitchel at gcc dot gnu dot org]

------- Comment #13 from mmitchel at gcc dot gnu dot org  2006-01-20 22:37 -------
RTH's comments are here:

http://gcc.gnu.org/ml/gcc-patches/2006-01/msg01390.html


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25654

[Bug rtl-optimization/25654] [4.0/4.1/4.2 Regression] RTL alias analysis unprepared to handle stack slot sharing

[rguenth at gcc dot gnu dot org]

------- Comment #14 from rguenth at gcc dot gnu dot org  2006-01-23 09:47 -------
Subject: Bug 25654

Author: rguenth
Date: Mon Jan 23 09:47:01 2006
New Revision: 110109

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=110109
Log:
2006-01-23  Steven Bosscher  <stevenb.gcc@gmail.com>
        Jan Hubicka  <jh@suse.cz>
        Richard Guenther  <rguenther@suse.de>

        PR rtl-optimization/25654
        * cfgexpand.c (aggregate_contains_union_type): New function.
        (add_alias_set_conflicts): Call it.  Make sure to add conflicts
        for structure variables that contain a union type.

        * gcc.dg/torture/pr25654.c: New testcase.
        * gcc.target/i386/pr25654.c: Likewise.

Added:
    trunk/gcc/testsuite/gcc.dg/torture/pr25654.c
    trunk/gcc/testsuite/gcc.target/i386/pr25654.c
Modified:
    trunk/gcc/ChangeLog
    trunk/gcc/cfgexpand.c
    trunk/gcc/testsuite/ChangeLog


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25654

[Bug rtl-optimization/25654] [4.0/4.1/4.2 Regression] RTL alias analysis unprepared to handle stack slot sharing

[rguenth at gcc dot gnu dot org]

------- Comment #15 from rguenth at gcc dot gnu dot org  2006-01-23 09:50 -------
Subject: Bug 25654

Author: rguenth
Date: Mon Jan 23 09:50:07 2006
New Revision: 110110

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=110110
Log:
2006-01-23  Steven Bosscher  <stevenb.gcc@gmail.com>
        Jan Hubicka  <jh@suse.cz>
        Richard Guenther  <rguenther@suse.de>

        PR rtl-optimization/25654
        * cfgexpand.c (aggregate_contains_union_type): New function.
        (add_alias_set_conflicts): Call it.  Make sure to add conflicts
        for structure variables that contain a union type.

        * gcc.dg/torture/pr25654.c: New testcase.
        * gcc.target/i386/pr25654.c: Likewise.

Added:
    branches/gcc-4_1-branch/gcc/testsuite/gcc.dg/torture/pr25654.c
    branches/gcc-4_1-branch/gcc/testsuite/gcc.target/i386/pr25654.c
Modified:
    branches/gcc-4_1-branch/gcc/ChangeLog
    branches/gcc-4_1-branch/gcc/cfgexpand.c
    branches/gcc-4_1-branch/gcc/testsuite/ChangeLog


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25654

[Bug rtl-optimization/25654] [4.0/4.1/4.2 Regression] RTL alias analysis unprepared to handle stack slot sharing

[rguenth at gcc dot gnu dot org]

------- Comment #16 from rguenth at gcc dot gnu dot org  2006-01-23 09:54 -------
Subject: Bug 25654

Author: rguenth
Date: Mon Jan 23 09:54:12 2006
New Revision: 110111

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=110111
Log:
2006-01-23  Steven Bosscher  <stevenb.gcc@gmail.com>
        Jan Hubicka  <jh@suse.cz>
        Richard Guenther  <rguenther@suse.de>

        PR rtl-optimization/25654
        * cfgexpand.c (aggregate_contains_union_type): New function.
        (add_alias_set_conflicts): Call it.  Make sure to add conflicts
        for structure variables that contain a union type.

        * gcc.dg/torture/pr25654.c: New testcase.
        * gcc.target/i386/pr25654.c: Likewise.

Added:
    branches/gcc-4_0-branch/gcc/testsuite/gcc.dg/torture/pr25654.c
    branches/gcc-4_0-branch/gcc/testsuite/gcc.target/i386/pr25654.c
Modified:
    branches/gcc-4_0-branch/gcc/ChangeLog
    branches/gcc-4_0-branch/gcc/cfgexpand.c
    branches/gcc-4_0-branch/gcc/testsuite/ChangeLog


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25654

[Bug rtl-optimization/25654] [4.0/4.1/4.2 Regression] RTL alias analysis unprepared to handle stack slot sharing

[rguenth at gcc dot gnu dot org]

------- Comment #17 from rguenth at gcc dot gnu dot org  2006-01-23 09:54 -------
Fixed.


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|                            |FIXED


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25654