[Bug c/20972] New: Compiler-generated code produces an assembler warning

[Bug c/20972] New: Compiler-generated code produces an assembler warning

[jconner at apple dot com]

Test gcc.c-torture/compile/930210-1.c, when compiled with:

    -O2 -c

produces the warning:

    /var/tmp//ccSVOi5X.s: Assembler messages:
    /var/tmp//ccSVOi5X.s:23: Warning: source register same as write-back base

Because of the following instruction generated in the assembly:

    strb    r2, [r2], #1

The assembler warning is produced because the instruction is unpredictable when the source register 
and the base destination register are the same and post-increment addressing is used.

Note that while this test doesn't fail for the arm-none-elf target, a similar test case does demonstrate 
the same issue:

void f(void)
{
  char  c1, c2;
  char *p1, *p2;

  c1 = c2 = *p1++;
  while (c1--)
    *p2++ = *p1++;
}

This behavior is present in gcc-4.1.0-20050405, and gcc-4.0.0-20050410 (RC1).
Configuration options: --target=arm-unknown-elf --program-prefix=arm-elf- --disable-nls --
enable-languages=c,c++ --with-newlib --enable-multilib --disable-shared

-- 
           Summary: Compiler-generated code produces an assembler warning
           Product: gcc
           Version: 4.1.0
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: jconner at apple dot com
                CC: gcc-bugs at gcc dot gnu dot org
  GCC host triplet: powerpc-apple-darwin7.8.0
GCC target triplet: arm-unknown-elf


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20972

[Bug target/20972] Compiler-generated code produces an assembler warning

[pinskia at gcc dot gnu dot org]

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|c                           |target
           Keywords|                            |wrong-code


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20972

[Bug target/20972] Compiler-generated code produces an assembler warning

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-04-12 17:08 -------
Patch here: <http://gcc.gnu.org/ml/gcc-patches/2005-04/msg01291.html>.

Do you know if this is a regression from say 3.4.x?

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |pinskia at gcc dot gnu dot
                   |                            |org
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|                            |1
           Keywords|                            |patch
   Last reconfirmed|0000-00-00 00:00:00         |2005-04-12 17:08:16
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20972

[Bug target/20972] Compiler-generated code produces an assembler warning

[jconner at apple dot com]

------- Additional Comments From jconner at apple dot com  2005-04-12 17:24 -------
Subject: Re:  Compiler-generated code produces an assembler warning

I don't see this same behavior on 3.4.3.

- Josh

On Apr 12, 2005, at 10:08 AM, pinskia at gcc dot gnu dot org wrote:

>
> ------- Additional Comments From pinskia at gcc dot gnu dot org   
> 2005-04-12 17:08 -------
> Patch here: <http://gcc.gnu.org/ml/gcc-patches/2005-04/msg01291.html>.
>
> Do you know if this is a regression from say 3.4.x?
>
> --  
>            What    |Removed                     |Added
> ----------------------------------------------------------------------- 
> -----
>                  CC|                            |pinskia at gcc dot  
> gnu dot
>                    |                            |org
>              Status|UNCONFIRMED                 |NEW
>      Ever Confirmed|                            |1
>            Keywords|                            |patch
>    Last reconfirmed|0000-00-00 00:00:00         |2005-04-12 17:08:16
>                date|                            |
>
>
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20972
>
> ------- You are receiving this mail because: -------
> You reported the bug, or are watching the reporter.
>



-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20972

[Bug target/20972] [4.0/4.1 Regression] Compiler-generated code produces an assembler warning

[pinskia at gcc dot gnu dot org]

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|Compiler-generated code     |[4.0/4.1 Regression]
                   |produces an assembler       |Compiler-generated code
                   |warning                     |produces an assembler
                   |                            |warning
   Target Milestone|---                         |4.0.1


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20972

[Bug rtl-optimization/20972] [4.0/4.1 Regression] Can't describe an early-clobber by an auto-inc

[rearnsha at gcc dot gnu dot org]

------- Additional Comments From rearnsha at gcc dot gnu dot org  2005-05-12 11:36 -------
The ARM ARM says that

  STR Rd, [Rn], #4

and 

  STR Rd, [Rn, #4]!

are unpredictable if Rd == Rn.  That is, the auto-inc side effect is permitted
to take place before Rd is read.
 
There's currently no way for a back-end to describe that an auto-inc operation
might be an early-clobber.  The obvious '&<>' in the constraint doesn't work,
causing an abort in find_reloads().

This bug isn't really a regression (the limitation has always been there), but
the vagaries of register allocation means that it is only recently that it's
started doing this (either that, or the assembler has only recently started
whining about it :-).

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |rearnsha at gcc dot gnu dot
                   |                            |org
          Component|target                      |rtl-optimization
   Last reconfirmed|2005-04-12 17:08:16         |2005-05-12 11:36:06
               date|                            |
            Summary|[4.0/4.1 Regression]        |[4.0/4.1 Regression] Can't
                   |Compiler-generated code     |describe an early-clobber by
                   |produces an assembler       |an auto-inc
                   |warning                     |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20972

[Bug rtl-optimization/20972] [4.0/4.1 Regression] Can't describe an early-clobber by an auto-inc

[mmitchel at gcc dot gnu dot org]

------- Additional Comments From mmitchel at gcc dot gnu dot org  2005-07-06 16:53 -------
Postponed until 4.0.2.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|4.0.1                       |4.0.2


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20972

[Bug rtl-optimization/20972] [4.0/4.1 Regression] Can't describe an early-clobber by an auto-inc

[mmitchel at gcc dot gnu dot org]

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|4.0.2                       |4.0.3


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20972

[Bug rtl-optimization/20972] [4.0/4.1 Regression] Can't describe an early-clobber by an auto-inc

[bonzini at gcc dot gnu dot org]

------- Comment #5 from bonzini at gcc dot gnu dot org  2005-10-13 16:01 -------
Maybe you could add a reload_completed split to not use autoincrements if Rd ==
Rn?


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20972