[Bug tree-optimization/45704] New: [4.5 Regression] load byte instruction is used for volatile int

[Bug tree-optimization/45704] New: [4.5 Regression] load byte instruction is used for volatile int

[anemo at mba dot ocn dot ne dot jp]

A cast to volatile int pointer is ignored on gcc 4.5 with this test case.

struct st {
        int ptr;
};

int foo(struct st *st)
{
        int v = *(volatile int *)&st->ptr;
        return v & 0xff;
}

mipsel-linux-gcc-4.5.1 -O2 foo.c -S output:
        lbu     $2,0($4)
        j       $31
        nop

It seems the cast are ignored and "load int and mask" was optimized to "load
byte".
gcc 4.4.4 works fine.
mipsel-linux-gcc-4.4.4 -O2 foo.c -S output:
        lw      $2,0($4)
        j       $31
        andi    $2,$2,0x00ff

git-bisect tell me 0d9f1189f3df5ce5c0efc3ecadc7c0a4f75b202d is the first bad
commit.
URL: http://gcc.gnu.org/viewcvs?view=revision&revision=156571
The commit is fix for Bug 42956.

The PR 42956 bugzilla shows same fix was applied to both 4.5.0 and 4.4.4,
but they behave differently on this test case.

Comparing patches for 4.4 branch and 4.5, I see the latter contains two more
changes for gimplify.c:
A STRIP_NOP line and lines starting with "/* *(foo *)&complexfoo => __real__
complexfoo */" comment.

I do not know gcc internal at all but it seems reverting this change fixes this
problem.

-  STRIP_USELESS_TYPE_CONVERSION (sub);
+  STRIP_NOPS (sub);

Hope this helps.


-- 
           Summary: [4.5 Regression] load byte instruction is used for
                    volatile int
           Product: gcc
           Version: 4.5.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: anemo at mba dot ocn dot ne dot jp


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45704

[Bug tree-optimization/45704] [4.5 Regression] load byte instruction is used for volatile int

[rguenth at gcc dot gnu dot org]

------- Comment #1 from rguenth at gcc dot gnu dot org  2010-09-17 15:45 -------
Confirmed.  Mine.


-- 

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|UNCONFIRMED                 |ASSIGNED
     Ever Confirmed|0                           |1
   Last reconfirmed|0000-00-00 00:00:00         |2010-09-17 15:45:01
               date|                            |
   Target Milestone|---                         |4.5.2


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45704

[Bug tree-optimization/45704] [4.5 Regression] load byte instruction is used for volatile int

[rguenth at gcc dot gnu dot org]

------- Comment #2 from rguenth at gcc dot gnu dot org  2010-09-20 15:54 -------
Subject: Bug 45704

Author: rguenth
Date: Mon Sep 20 15:54:03 2010
New Revision: 164439

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=164439
Log:
2010-09-20  Richard Guenther  <rguenther@suse.de>

        PR middle-end/45704
        * gimplify.c (gimplify_modify_expr_rhs): Preserve volatileness.

        * gcc.dg/torture/pr45704.c: New testcase.

Added:
    trunk/gcc/testsuite/gcc.dg/torture/pr45704.c
Modified:
    trunk/gcc/ChangeLog
    trunk/gcc/gimplify.c
    trunk/gcc/testsuite/ChangeLog


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45704

[Bug tree-optimization/45704] [4.5 Regression] load byte instruction is used for volatile int

[rguenth at gcc dot gnu dot org]

------- Comment #4 from rguenth at gcc dot gnu dot org  2010-09-21 10:47 -------
Subject: Bug 45704

Author: rguenth
Date: Tue Sep 21 10:47:28 2010
New Revision: 164475

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=164475
Log:
2010-09-21  Richard Guenther  <rguenther@suse.de>

        PR middle-end/45704
        * gimplify.c (gimplify_modify_expr_rhs): Preserve volatileness.

        * gcc.dg/torture/pr45704.c: New testcase.

Added:
    branches/gcc-4_5-branch/gcc/testsuite/gcc.dg/torture/pr45704.c
Modified:
    branches/gcc-4_5-branch/gcc/ChangeLog
    branches/gcc-4_5-branch/gcc/gimplify.c
    branches/gcc-4_5-branch/gcc/testsuite/ChangeLog


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45704

[Bug tree-optimization/45704] [4.5 Regression] load byte instruction is used for volatile int

[rguenth at gcc dot gnu dot org]

------- Comment #3 from rguenth at gcc dot gnu dot org  2010-09-21 10:47 -------
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=45704

[Bug tree-optimization/45704] [4.5 Regression] load byte instruction is used for volatile int

[tschwinge at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45704

Thomas Schwinge <tschwinge at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |domen at cba dot si

--- Comment #11 from Thomas Schwinge <tschwinge at gcc dot gnu.org> 2011-02-03 18:27:10 UTC ---
*** Bug 46762 has been marked as a duplicate of this bug. ***

[Bug tree-optimization/45704] [4.5 Regression] load byte instruction is used for volatile int

[anemo at mba dot ocn.ne.jp]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45704

--- Comment #10 from Atsushi Nemoto <anemo at mba dot ocn.ne.jp> 2010-10-01 12:48:47 UTC ---
(In reply to comment #9)
> > Is that "two more changes" are not bugfix but improvement?
> 
> Yes.

I see, thank you for clarification.

[Bug tree-optimization/45704] [4.5 Regression] load byte instruction is used for volatile int

[rguenther at suse dot de]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45704

--- Comment #9 from rguenther at suse dot de <rguenther at suse dot de> 2010-09-30 14:14:13 UTC ---
On Thu, 30 Sep 2010, anemo at mba dot ocn.ne.jp wrote:

> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45704
> 
> --- Comment #8 from Atsushi Nemoto <anemo at mba dot ocn.ne.jp> 2010-09-30 13:59:34 UTC ---
> (In reply to comment #0)
> > The PR 42956 bugzilla shows same fix was applied to both 4.5.0 and 4.4.4,
> > but they behave differently on this test case.
> > 
> > Comparing patches for 4.4 branch and 4.5, I see the latter contains two more
> > changes for gimplify.c:
> > A STRIP_NOP line and lines starting with "/* *(foo *)&complexfoo => __real__
> > complexfoo */" comment.
> 
> In the first place, why PR 42956 fixes are different for 4.4 and 4.5?
> The commit logs are same, but actual changes for gimplify.c were differ.
> 
> Is that "two more changes" are not bugfix but improvement?

Yes.

[Bug tree-optimization/45704] [4.5 Regression] load byte instruction is used for volatile int

[anemo at mba dot ocn.ne.jp]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45704

--- Comment #8 from Atsushi Nemoto <anemo at mba dot ocn.ne.jp> 2010-09-30 13:59:34 UTC ---
(In reply to comment #0)
> The PR 42956 bugzilla shows same fix was applied to both 4.5.0 and 4.4.4,
> but they behave differently on this test case.
> 
> Comparing patches for 4.4 branch and 4.5, I see the latter contains two more
> changes for gimplify.c:
> A STRIP_NOP line and lines starting with "/* *(foo *)&complexfoo => __real__
> complexfoo */" comment.

In the first place, why PR 42956 fixes are different for 4.4 and 4.5?
The commit logs are same, but actual changes for gimplify.c were differ.

Is that "two more changes" are not bugfix but improvement?

[Bug tree-optimization/45704] [4.5 Regression] load byte instruction is used for volatile int

[anemo at mba dot ocn.ne.jp]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45704

--- Comment #7 from Atsushi Nemoto <anemo at mba dot ocn.ne.jp> 2010-09-28 15:34:59 UTC ---
(In reply to comment #6)
> It should be as this is likely a problem with unaligned access
> support.  I think you can't generally expect unaligned volatile
> accesses to work (on ia64 for example they certainly wouldn't).

OK, filed as PR 45819.
ARM linux kernel depends on this unaligned volatile access.

[Bug tree-optimization/45704] [4.5 Regression] load byte instruction is used for volatile int

[rguenther at suse dot de]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45704

--- Comment #6 from rguenther at suse dot de <rguenther at suse dot de> 2010-09-27 15:54:06 UTC ---
On Mon, 27 Sep 2010, anemo at mba dot ocn.ne.jp wrote:

> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45704
> 
> --- Comment #5 from Atsushi Nemoto <anemo at mba dot ocn.ne.jp> 2010-09-27 15:48:32 UTC ---
> Here is a similar test case with "packed" attribute, which still produces bad
> result.
> Four load-byte instructions are generated instead of one load-word.
> 
> struct st {
>     int ptr;
> } __attribute__ ((packed));
> 
> int foo(struct st *st)
> {
>     int v = *(volatile int *)&st->ptr;
>     return v & 0xff;
> }
> 
> gcc 4.4.4 works fine (same result with the first test case)
> 
> gcc 4.6.0 20100927 (or 4.5.2 20100927) generates:
>     lbu    $2,0($4)
>     lbu    $3,1($4)
>     andi    $2,$2,0x00ff
>     lbu    $3,2($4)
>     lbu    $3,3($4)
>     j    $31
>     nop
> 
> I do not know this should be another PR or not.

It should be as this is likely a problem with unaligned access
support.  I think you can't generally expect unaligned volatile
accesses to work (on ia64 for example they certainly wouldn't).

[Bug tree-optimization/45704] [4.5 Regression] load byte instruction is used for volatile int

[anemo at mba dot ocn.ne.jp]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45704

--- Comment #5 from Atsushi Nemoto <anemo at mba dot ocn.ne.jp> 2010-09-27 15:48:32 UTC ---
Here is a similar test case with "packed" attribute, which still produces bad
result.
Four load-byte instructions are generated instead of one load-word.

struct st {
    int ptr;
} __attribute__ ((packed));

int foo(struct st *st)
{
    int v = *(volatile int *)&st->ptr;
    return v & 0xff;
}

gcc 4.4.4 works fine (same result with the first test case)

gcc 4.6.0 20100927 (or 4.5.2 20100927) generates:
    lbu    $2,0($4)
    lbu    $3,1($4)
    andi    $2,$2,0x00ff
    lbu    $3,2($4)
    lbu    $3,3($4)
    j    $31
    nop

I do not know this should be another PR or not.