[Bug rtl-optimization/47602] New: Permit inline asm to clobber PIC register

[Bug rtl-optimization/47602] New: Permit inline asm to clobber PIC register

[ian at airs dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47602

           Summary: Permit inline asm to clobber PIC register
           Product: gcc
           Version: 4.6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: ian@airs.com


This fails for i386 32-bit when using -fpic:

void foo() { asm volatile ("movl $0,%%ebx" : : : "ebx"); }

The compiler reports

foo.c:1:14: error: PIC register clobbered by ‘ebx’ in ‘asm’

This forces people to write their asm statements differently for PIC and
non-PIC code.  That is not useful.  The compiler could save and restore the PIC
register around the asm statement.  That would be more useful, and that is what
we should do.

[Bug rtl-optimization/47602] Permit inline asm to clobber PIC register

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47602

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |DUPLICATE

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> 2011-02-03 19:54:19 UTC ---
This was added on purpose.  See PR 8340.

*** This bug has been marked as a duplicate of bug 8340 ***

[Bug rtl-optimization/47602] Permit inline asm to clobber PIC register

[ian at airs dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47602

Ian Lance Taylor <ian at airs dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |REOPENED
   Last reconfirmed|                            |2011.02.03 23:49:50
         Resolution|DUPLICATE                   |
     Ever Confirmed|0                           |1

--- Comment #2 from Ian Lance Taylor <ian at airs dot com> 2011-02-03 23:49:50 UTC ---
No, this is not a duplicate of PR 8340.  I am suggesting a different way of
handling the situation, which would still produce correct code.  I am
suggesting that when an asm clobbers the PIC register, that we explicitly save
and restore the register around the asm.

[Bug rtl-optimization/47602] Permit inline asm to clobber PIC register

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47602

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> 2011-02-03 23:52:33 UTC ---
(In reply to comment #2)
> No, this is not a duplicate of PR 8340.  I am suggesting a different way of
> handling the situation, which would still produce correct code.  I am
> suggesting that when an asm clobbers the PIC register, that we explicitly save
> and restore the register around the asm.

The reason why it is a dup is that we decided we did not want to fix it that
way.  If you want to reopen the old bug that is a different story.

[Bug middle-end/47602] Permit inline asm to clobber PIC register

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47602

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|rtl-optimization            |middle-end
           Severity|normal                      |enhancement

[Bug middle-end/47602] Permit inline asm to clobber PIC register

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47602

--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> 2011-02-04 00:04:36 UTC ---
http://gcc.gnu.org/ml/gcc-patches/2002-11/msg01372.html

[Bug middle-end/47602] Permit inline asm to clobber PIC register

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47602

--- Comment #5 from Andrew Pinski <pinskia at gcc dot gnu.org> 2011-02-04 00:06:36 UTC ---
What happens on targets like PPC where r2 (the PIC register) is a fixed
register should we save and restore it or error out.  I say we should error
out.  Really I think it is a good thing to error out and force people to write
their PIC vs no PIC inline-asm differently.  As it forces people to think about
register pressure and such.

[Bug middle-end/47602] Permit inline asm to clobber PIC register

[ian at airs dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47602

--- Comment #6 from Ian Lance Taylor <ian at airs dot com> 2011-02-04 00:08:42 UTC ---
The old bug was about bad code generation, and that was fixed.  I don't think
it needs to be reopened.

I can not find any discussion about fixing this issue in any way other than
forbidding it.

I have no problem with forbidding clobbering fixed registers.  That is a
related but distinct issue.

I do not think it is helpful to force people to consider pic vs. non-pic
issues.  They can still consider them if they choose, but I do not think it is
helpful to force them.

[Bug middle-end/47602] Permit inline asm to clobber PIC register

[hjl.tools at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47602

--- Comment #7 from H.J. Lu <hjl.tools at gmail dot com> 2011-02-04 01:42:03 UTC ---
(In reply to comment #6)
>
> I can not find any discussion about fixing this issue in any way other than
> forbidding it.
> 

EBX is fixed in PIC. If it is changed in asm, it should be saved and
restored in asm. Otherwise, non-leaf functions may not work properly
even if it is saved/restored in prologue/epilogue.

[Bug middle-end/47602] Permit inline asm to clobber PIC register

[ian at airs dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47602

--- Comment #8 from Ian Lance Taylor <ian at airs dot com> 2011-02-04 05:31:08 UTC ---
I'm suggesting saving and restoring it around the asm statement itself, not in
the prologue and epilogue.

[Bug middle-end/47602] Permit inline asm to clobber PIC register

[ebotcazou at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47602

Eric Botcazou <ebotcazou at gcc dot gnu.org> changed:

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

--- Comment #9 from Eric Botcazou <ebotcazou at gcc dot gnu.org> 2011-02-04 08:48:46 UTC ---
> I'm suggesting saving and restoring it around the asm statement itself, not in
> the prologue and epilogue.

Although the error was primarily RTH's idea, I still think it's very
reasonable.
When you write inline asm, you're expecting a certain degree of control over
the generated code; knowing that you're clobbering the PIC register can be
useful, especially on a register-starved architecture like the x86.  If you
stand by your clobbering, then writing the conditional save-and-restore code is
trivial.

[Bug middle-end/47602] Permit inline asm to clobber PIC register

[ian at airs dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47602

--- Comment #10 from Ian Lance Taylor <ian at airs dot com> 2011-02-04 14:16:32 UTC ---
What advantage do we bring to our users by requiring them to be aware of the
details of the PIC register when writing inline asm code?  Again, those who
*are* aware of the PIC register have complete control.  But many people are
able to write assembler code but have no idea what the PIC register is.  In
what way are we helping them by forcing them to know about it?  How does that
help them write inline assembler which, e.g., uses the cpuid instruction, or
makes a kernel system call which passes a parameter in %ebx?

[Bug middle-end/47602] Permit inline asm to clobber PIC register

[schwab@linux-m68k.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47602

--- Comment #11 from Andreas Schwab <schwab@linux-m68k.org> 2011-02-04 14:28:00 UTC ---
If you are writing assembler code you need to know your ABI in and out.

[Bug middle-end/47602] Permit inline asm to clobber PIC register

[hjl.tools at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47602

--- Comment #12 from H.J. Lu <hjl.tools at gmail dot com> 2011-02-04 15:01:56 UTC ---
(In reply to comment #10)
>
> what way are we helping them by forcing them to know about it?  How does that
> help them write inline assembler which, e.g., uses the cpuid instruction, or

Use <cpuid.h> provided by gcc.

> makes a kernel system call which passes a parameter in %ebx?

You can use syscall () or take a look at the C library source
to see how system call is inlined.

[Bug middle-end/47602] Permit inline asm to clobber PIC register

[ebotcazou at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47602

--- Comment #13 from Eric Botcazou <ebotcazou at gcc dot gnu.org> 2011-02-04 15:23:22 UTC ---
> In what way are we helping them by forcing them to know about it?  How does
> that help them write inline assembler which, e.g., uses the cpuid instruction,
> or makes a kernel system call which passes a parameter in %ebx?

In these cases, that won't probably help them indeed.  But, in other cases,
like someone wanting to write very efficiently a hot loop in asm for a library,
making it explicit that the %ebx register is not freely available like the
others might help him to find the most efficient approach.  Maybe we could just
warn though.

[Bug middle-end/47602] Permit inline asm to clobber PIC register

[pcpa at mandriva dot com.br]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47602

Paulo César Pereira de Andrade <pcpa at mandriva dot com.br> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |pcpa at mandriva dot com.br

--- Comment #14 from Paulo César Pereira de Andrade <pcpa at mandriva dot com.br> 2011-11-25 19:22:09 UTC ---
  In a first try on updating to valgrind 3.7.0 I run into this:

make[3]: Entering directory `/home/pcpa/rpm/BUILD/valgrind-3.7.0/coregrind'
x86_64-mandriva-linux-gnu-gcc -DHAVE_CONFIG_H -I. -I..  -I.. -I../include
-I../VEX/pub -DVGA_x86=1 -DVGO_linux=1 -DVGP_x86_linux=1
-DVGPV_x86_linux_vanilla=1 -I../coregrind -DVG_LIBDIR="\"/usr/lib64/valgrind"\"
-DVG_PLATFORM="\"x86-linux\""  -m32 -O2 -g -Wall -Wmissing-prototypes -Wshadow
-Wpointer-arith -Wstrict-prototypes -Wmissing-declarations
-Wno-format-zero-length -fno-strict-aliasing -fno-builtin -Wno-long-long -O2 -g
-frecord-gcc-switches -Wstrict-aliasing=2 -pipe -Wformat
-Werror=format-security -Wp,-D_FORTIFY_SOURCE=2 -fstack-protector
--param=ssp-buffer-size=4 -fPIC -Wno-pointer-sign -fno-stack-protector -MT
libcoregrind_x86_linux_a-syswrap-linux.o -MD -MP -MF
.deps/libcoregrind_x86_linux_a-syswrap-linux.Tpo -c -o
libcoregrind_x86_linux_a-syswrap-linux.o `test -f 'm_syswrap/syswrap-linux.c'
|| echo './'`m_syswrap/syswrap-linux.c
m_syswrap/syswrap-linux.c: In function 'run_a_thread_NORETURN':
m_syswrap/syswrap-linux.c:202:7: error: PIC register clobbered by 'ebx' in
'asm'
make[3]: *** [libcoregrind_x86_linux_a-syswrap-linux.o] Error 1

The code is like this:

static void run_a_thread_NORETURN ( Word tidW )
[...]
      /* We have to use this sequence to terminate the thread to
         prevent a subtle race.  If VG_(exit_thread)() had left the
         ThreadState as Empty, then it could have been reallocated,
         reusing the stack while we're doing these last cleanups.
         Instead, VG_(exit_thread) leaves it as Zombie to prevent
         reallocation.  We need to make sure we don't touch the stack
         between marking it Empty and exiting.  Hence the
         assembler. */
#if defined(VGP_x86_linux)
      asm volatile (
         "movl    %1, %0\n"    /* set tst->status = VgTs_Empty */
         "movl    %2, %%eax\n"    /* set %eax = __NR_exit */
         "movl    %3, %%ebx\n"    /* set %ebx = tst->os_state.exitcode */
         "int    $0x80\n"    /* exit(tst->os_state.exitcode) */
         : "=m" (tst->status)
         : "n" (VgTs_Empty), "n" (__NR_exit), "m" (tst->os_state.exitcode)
         : "eax", "ebx"
      );
#elif defined(VGP_amd64_linux)

In valgrind 3.6.1 it works because it does not tell about the clobbered
registers.

[Bug middle-end/47602] Permit inline asm to clobber PIC register

[kaffeemonster at googlemail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47602

Jan Seiffert <kaffeemonster at googlemail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |kaffeemonster at googlemail
                   |                            |dot com

--- Comment #15 from Jan Seiffert <kaffeemonster at googlemail dot com> 2011-12-24 15:54:23 UTC ---
As a heavy inline asm user myself, i can understand the pain to handle PIC
yourself, but there is no way around that.

You can "accidentally" use the PIC register by a memory operand ("m"). Then the
compiler can not save it for you.

Greetings
Jan

[Bug middle-end/47602] Permit inline asm to clobber PIC register

[kyukhin at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=47602

--- Comment #16 from Kirill Yukhin <kyukhin at gcc dot gnu.org> ---
Author: kyukhin
Date: Mon Oct 13 17:26:49 2014
New Revision: 216154

URL: https://gcc.gnu.org/viewcvs?rev=216154&root=gcc&view=rev
Log:

gcc/
    PR target/8340
    PR middle-end/47602
    PR rtl-optimization/55458
    * config/i386/i386.c (ix86_use_pseudo_pic_reg): New.
    (ix86_init_pic_reg): New.
    (ix86_select_alt_pic_regnum): Add check on pseudo register.
    (ix86_save_reg): Likewise.
    (ix86_expand_prologue): Remove PIC register initialization
    now performed in ix86_init_pic_reg.
    (ix86_output_function_epilogue): Add check on pseudo register.
    (set_pic_reg_ever_alive): New.
    (legitimize_pic_address): Replace df_set_regs_ever_live with new
    set_pic_reg_ever_alive.
    (legitimize_tls_address): Likewise.
    (ix86_pic_register_p): New check.
    (ix86_delegitimize_address): Add check on pseudo register.
    (ix86_expand_call): Insert move from pseudo PIC register to ABI
    defined REAL_PIC_OFFSET_TABLE_REGNUM.
    (TARGET_INIT_PIC_REG): New.
    (TARGET_USE_PSEUDO_PIC_REG): New.
    * config/i386/i386.h (PIC_OFFSET_TABLE_REGNUM): Return INVALID_REGNUM
    if pic_offset_table_rtx exists.
    * doc/tm.texi.in (TARGET_USE_PSEUDO_PIC_REG, TARGET_INIT_PIC_REG):
    Document.
    * doc/tm.texi: Regenerate.
    * function.c (assign_parms): Generate pseudo register for PIC.
    * init-regs.c (initialize_uninitialized_regs): Ignor pseudo PIC
    register.
    * ira-color.c (color_pass): Add check on pseudo register.
    * ira-emit.c (change_loop): Don't create copies for PIC pseudo
    register.
    * ira.c (split_live_ranges_for_shrink_wrap): Add check on pseudo
    register.
    (ira): Add target specific PIC register initialization.
    (do_reload): Keep PIC pseudo register.
    * lra-assigns.c (spill_for): Add checks on pseudo register.
    * lra-constraints.c (contains_symbol_ref_p): New.
    (lra_constraints): Enable lra risky transformations when PIC is pseudo
    register.
    * shrink-wrap.c (try_shrink_wrapping): Add check on pseudo register.
    * target.def (use_pseudo_pic_reg): New.
    (init_pic_reg): New.

gcc/testsuite/
    PR target/8340
    PR middle-end/47602
    PR rtl-optimization/55458
    * gcc.target/i386/pic-1.c: Remove dg-error as test should pass now.
    * gcc.target/i386/pr55458.c: Likewise.
    * gcc.target/i386/pr47602.c: New.
    * gcc.target/i386/pr23098.c: Move to XFAIL.

Modified:
    trunk/gcc/ChangeLog
    trunk/gcc/config/i386/i386.c
    trunk/gcc/config/i386/i386.h
    trunk/gcc/doc/tm.texi
    trunk/gcc/doc/tm.texi.in
    trunk/gcc/function.c
    trunk/gcc/init-regs.c
    trunk/gcc/ira-color.c
    trunk/gcc/ira-emit.c
    trunk/gcc/ira.c
    trunk/gcc/lra-assigns.c
    trunk/gcc/lra-constraints.c
    trunk/gcc/shrink-wrap.c
    trunk/gcc/target.def
    trunk/gcc/testsuite/ChangeLog
    trunk/gcc/testsuite/gcc.target/i386/pic-1.c
    trunk/gcc/testsuite/gcc.target/i386/pr23098.c
    trunk/gcc/testsuite/gcc.target/i386/pr55458.c

[Bug middle-end/47602] Permit inline asm to clobber PIC register

[law at redhat dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=47602

Jeffrey A. Law <law at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|REOPENED                    |RESOLVED
                 CC|                            |law at redhat dot com
         Resolution|---                         |FIXED

--- Comment #17 from Jeffrey A. Law <law at redhat dot com> ---
Now that the PIC register is a pseudo for i686, this "just works".