[Bug rtl-optimization/49169] New: ARM: optimisations strip the Thumb/ARM mode bit off function pointers

[Bug rtl-optimization/49169] New: ARM: optimisations strip the Thumb/ARM mode bit off function pointers

[michael.hope at linaro dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49169

           Summary: ARM: optimisations strip the Thumb/ARM mode bit off
                    function pointers
           Product: gcc
           Version: 4.6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: michael.hope@linaro.org


ARM devices encode the instruction set mode in the LSB of the function address.
 Functions are word aligned on ARM.  If you try to test the LSB of a function
pointer then GCC assumes that the two least significant bits are zero and
optimises away the test.

This problem is seen in Mono and was originally reported at:
 https://bugs.launchpad.net/ubuntu/+source/gcc-4.5/+bug/721531

A reduced test case is:

void main() {
        void *p = main;
        if ((int)p & 1) printf ("HIT!\n");
}

When compiled with -march=armv7-a -mthumb -O0 then the word 'HIT!' will show. 
When compiled with -O2, the branch is not taken.

The problem does not occur in 4.4.5.  It does occur in 4.5.2, 4.6.0, and trunk
r174044.

[Bug rtl-optimization/49169] ARM: optimisations strip the Thumb/ARM mode bit off function pointers

[mikpe at it dot uu.se]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49169

Mikael Pettersson <mikpe at it dot uu.se> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |mikpe at it dot uu.se

--- Comment #1 from Mikael Pettersson <mikpe at it dot uu.se> 2011-05-26 07:56:05 UTC ---
Try passing the function pointer through an opaque identity transform:

    asm("" : "=r"(p) : "0"(main));
    if ((uintptr_t)p & 1) /* do thumb case */

[Bug rtl-optimization/49169] ARM: optimisations strip the Thumb/ARM mode bit off function pointers

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49169

--- Comment #2 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-05-26 08:20:22 UTC ---
There are several places in the compiler where we assume DECL_ALIGN
constraints the lower bits of the address of the DECL.

See several similar bugs in the past (PR47239 comes to my mind).

fold-const.c:get_pointer_modulus_and_residue looks suspicious to me here,
so you might want to try

Index: gcc/fold-const.c
===================================================================
--- gcc/fold-const.c    (revision 174266)
+++ gcc/fold-const.c    (working copy)
@@ -9219,7 +9219,8 @@ get_pointer_modulus_and_residue (tree ex
   *residue = 0;

   code = TREE_CODE (expr);
-  if (code == ADDR_EXPR)
+  if (code == ADDR_EXPR
+      && TREE_CODE (TREE_OPERAND (expr, 0)) != FUNCTION_DECL)
     {
       unsigned int bitalign;
       bitalign = get_object_alignment_1 (TREE_OPERAND (expr, 0), residue);

[Bug rtl-optimization/49169] ARM: optimisations strip the Thumb/ARM mode bit off function pointers

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49169

--- Comment #3 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-05-26 08:30:21 UTC ---
Btw, we finally should introduce a target hook for this I think.

[Bug rtl-optimization/49169] ARM: optimisations strip the Thumb/ARM mode bit off function pointers

[rsandifo at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49169

rsandifo@gcc.gnu.org <rsandifo at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |rsandifo at gcc dot gnu.org

--- Comment #4 from rsandifo at gcc dot gnu.org <rsandifo at gcc dot gnu.org> 2011-06-07 07:32:37 UTC ---
(In reply to comment #3)
> Btw, we finally should introduce a target hook for this I think.

Thanks for the patch in comment #2.  How strongly do you feel
about the hook though?  In PR35705, it sounded like a lot of
targets actually need an opt-out for functions, either because
of ISA encoding (ARM, MIPS, SH) or because of function
descriptors (IA-64, PA, PPC).

I notice that ARM and mcore also have optimisation-dependent
FUNCTION_BOUNDARYs.  Arguably (very arguably) that's a bug,
and they should be using align_functions instead.  But if we
make a deliberate decision to honour DECL_ALIGN on functions,
then FUNCTION_BOUNDARY really will be an ABI property.
I'm just worried that the combination of that and the need
to identify exactly which targets should define the hook
might be more hassle than the optimisation is worth.

You said in that bug that masking function addresses isn't
likely to be a common operation, and TBH, I still agree.

Richard

[Bug rtl-optimization/49169] ARM: optimisations strip the Thumb/ARM mode bit off function pointers

[rsandifo at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49169

--- Comment #5 from rsandifo at gcc dot gnu.org <rsandifo at gcc dot gnu.org> 2011-06-27 09:33:10 UTC ---
Author: rsandifo
Date: Mon Jun 27 09:33:06 2011
New Revision: 175427

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=175427
Log:
gcc/
2011-07-24  Richard Guenther  <rguenther@suse.de>

    PR tree-optimization/49169
    * fold-const.c (get_pointer_modulus_and_residue): Don't rely on
    the alignment of function decls.

gcc/testsuite/
2011-07-24  Michael Hope  <michael.hope@linaro.org>
        Richard Sandiford  <richard.sandiford@linaro.org>

    PR tree-optimization/49169
    * gcc.dg/torture/pr49169.c: New test.

Added:
    trunk/gcc/testsuite/gcc.dg/torture/pr49169.c
Modified:
    trunk/gcc/ChangeLog
    trunk/gcc/fold-const.c
    trunk/gcc/testsuite/ChangeLog

[Bug rtl-optimization/49169] ARM: optimisations strip the Thumb/ARM mode bit off function pointers

[ramana at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49169

Ramana Radhakrishnan <ramana at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |wrong-code
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2011.07.08 15:04:22
                 CC|                            |ramana at gcc dot gnu.org
     Ever Confirmed|0                           |1

[Bug rtl-optimization/49169] ARM: optimisations strip the Thumb/ARM mode bit off function pointers

[jye2 at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49169

--- Comment #6 from jye2 at gcc dot gnu.org 2011-09-19 08:13:09 UTC ---
Author: jye2
Date: Mon Sep 19 08:13:02 2011
New Revision: 178955

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=178955
Log:
2011-09-19  Jiangning Liu  <jiangning.liu@arm.com>

    Backport r175427 from mainline
    2011-06-27  Richard Guenther  <rguenther@suse.de>

    PR tree-optimization/49169
    * fold-const.c (get_pointer_modulus_and_residue): Don't rely on
    the alignment of function decls.

2011-09-19  Jiangning Liu  <jiangning.liu@arm.com>

    Backport r175208 from mainline
    2011-06-20  Ramana Radhakrishnan  <ramana.radhakrishnan@linaro.org>

    PR target/49385
    * config/arm/thumb2.md (*thumb2_movhi_insn): Make sure atleast
    one of the operands is a register.

2011-09-19  Jiangning Liu  <jiangning.liu@arm.com>

    Backport r174803 from mainline
    2011-06-08  Julian Brown  <julian@codesourcery.com>

    * config/arm/arm.c (arm_libcall_uses_aapcs_base): Use correct ABI
    for double-precision helper functions in hard-float mode if only
    single-precision arithmetic is supported in hardware.


Modified:
    branches/ARM/embedded-4_6-branch/gcc/ChangeLog.arm
    branches/ARM/embedded-4_6-branch/gcc/config/arm/arm.c
    branches/ARM/embedded-4_6-branch/gcc/config/arm/thumb2.md
    branches/ARM/embedded-4_6-branch/gcc/fold-const.c

[Bug rtl-optimization/49169] ARM: optimisations strip the Thumb/ARM mode bit off function pointers

[ramana at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49169

Ramana Radhakrishnan <ramana at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |FIXED
   Target Milestone|---                         |4.7.0

--- Comment #7 from Ramana Radhakrishnan <ramana at gcc dot gnu.org> 2012-07-31 01:00:46 UTC ---
This should be marked FIXED as of 4.7.0 . 

Ramana