[Bug c/103979] New: asm goto is not considered volatile

[Bug c/103979] New: asm goto is not considered volatile

[gareth.webb+gccbugzilla at outlook dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103979

            Bug ID: 103979
           Summary: asm goto is not considered volatile
           Product: gcc
           Version: 11.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gareth.webb+gccbugzilla at outlook dot com
  Target Milestone: ---

Created attachment 52165
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=52165&action=edit
Minimal reproduction of bug

I have discovered a bug with GCC's 'asm goto'. It is documented that "note that
an asm goto statement is always implicitly considered volatile." but it is not.
It is using the same heuristic as non-goto asm (no outputs => volatile. outputs
=> not-volatile, unless explicitly specified).

This can result in the optimizer removing the asm block completely.

Confirmed the bug is present on Ubuntu 20.10's gcc package, as well as my own
build from source (tag: releases/gcc-11.2.0).

A minimal pre-processed reproduction is attached. Compile with gcc -O2 -c
asmgoto.c -o asmgoto.o, observe the disassembled output contains nothing but
endb64/ret, no jmp.

I also received an ICE at one point when trying to create a minimal
reproduction, but could not reproduce. It's included below, but as I can't
reproduce I don't know how useful it will be.

Ubuntu package:

Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/11/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none:amdgcn-amdhsa
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu 11.2.0-7ubuntu2'
--with-bugurl=file:///usr/share/doc/gcc-11/README.Bugs --enable-languages=c,
ada,c++,go,brig,d,fortran,objc,obj-c++,m2 --prefix=/usr
--with-gcc-major-version-only --program-suffix=-11
--program-prefix=x86_64-linux-gnu- --enable-share
d --enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext
--enable-threads=posix --libdir=/usr/lib --enable-nls --enable-bootstrap
--enabl
e-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes
--with-default-libstdcxx-abi=new --enable-gnu-unique-object
--disable-vtable-verify --ena
ble-plugin --enable-default-pie --with-system-zlib
--enable-libphobos-checking=release --with-target-system-zlib=auto
--enable-objc-gc=auto --enable-multiar
ch --disable-werror --enable-cet --with-arch-32=i686 --with-abi=m64
--with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic
--enable-offload
-targets=nvptx-none=/build/gcc-11-ZPT0kp/gcc-11-11.2.0/debian/tmp-nvptx/usr,amdgcn-amdhsa=/build/gcc-11-ZPT0kp/gcc-11-11.2.0/debian/tmp-gcn/usr
--without-cu
da-driver --enable-checking=release --build=x86_64-linux-gnu
--host=x86_64-linux-gnu --target=x86_64-linux-gnu
--with-build-config=bootstrap-lto-lean --enab
le-link-serialization=2
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 11.2.0 (Ubuntu 11.2.0-7ubuntu2) 

Myy own build:

Using built-in specs.
COLLECT_GCC=x86_64-elf-gcc
COLLECT_LTO_WRAPPER=/home/gwebb/opt/cross/libexec/gcc/x86_64-elf/11.2.0/lto-wrapper
Target: x86_64-elf
Configured with: ../gcc/configure --target=x86_64-elf
--prefix=/home/gwebb/opt/cross --disable-nls --enable-languages=c
--without-headers
Thread model: single
Supported LTO compression algorithms: zlib
gcc version 11.2.0 (GCC)

Possibly related ICE?

during RTL pass: fwprop1
gdt.c: In function 'flush_gdt':
gdt.c:66:1: internal compiler error: in purge_dead_edges, at cfgrtl.c:3346
  66 | }
     | ^
0x5d253d purge_dead_edges(basic_block_def*)
       ../../gcc/gcc/cfgrtl.c:3346
0x143dc1f delete_trivially_dead_insns(rtx_insn*, int)
       ../../gcc/gcc/cse.c:7178
0x145baa9 fwprop_done
       ../../gcc/gcc/fwprop.c:917
0x145baa9 fwprop
       ../../gcc/gcc/fwprop.c:1001
Please submit a full bug report,
with preprocessed source if appropriate.
Please include the complete backtrace with any bug report.
See <https://gcc.gnu.org/bugs/> for instructions.

[Bug rtl-optimization/103979] asm goto is not considered volatile with output operands

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103979

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|asm goto is not considered  |asm goto is not considered
                   |volatile                    |volatile with output
                   |                            |operands
   Last reconfirmed|                            |2022-01-11
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
           Keywords|                            |ice-on-valid-code,
                   |                            |inline-asm, wrong-code
          Component|c                           |rtl-optimization

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
I suspect the code was not updated once output operands were allowed to have it
marked as volatile. As inline-asm without output operands is always considered
volatile.

[Bug rtl-optimization/103979] asm goto is not considered volatile with output operands

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103979

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |ndesaulniers at google dot com

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
*** Bug 108548 has been marked as a duplicate of this bug. ***

[Bug rtl-optimization/103979] asm goto is not considered volatile with output operands

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103979

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

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

--- Comment #3 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
struct S { char a[64]; };

int
foo (void)
{
  struct S s, t;
  asm goto ("" : "=g" (s) : : : l);
  t = s;
  asm goto ("" : "=g" (s) : : : l);
  return s.a[0] + t.a[63];
l:
  return -1;
}

also ICEs at -O2, not during fwprop (note, I can't reproduce the #c0 testcase,
neither with trunk nor 11 branch on x86_64-linux), but during sra.

I think we shouldn't just follow blindly the docs, but see what e.g. Linux
kernel as heavy user of asm goto with outputs (and I think mostly without
volatile) wants.

[Bug rtl-optimization/103979] asm goto is not considered volatile with output operands

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103979

--- Comment #4 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Tried to reproduce the #c0 ICE all the way back to
r11-5002-ge3b3b59683c1e7d31a9d313dd97394abebf644be but couldn't, even that
version successfully DCEs the asm goto (though, in all cases during RTL
optimizations, GIMPLE keeps them).

[Bug middle-end/103979] asm goto is not considered volatile with output operands

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103979

--- Comment #5 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Jakub Jelinek from comment #3)
> I think we shouldn't just follow blindly the docs, but see what e.g. Linux
> kernel as heavy user of asm goto with outputs (and I think mostly without
> volatile) wants.

They will need volatile, at least on powerpc. The reason is they use asm goto
now for loading/storing from/to userspace. PR 110420 is another case where asm
goto is removed on accident due not to being volatile.

[Bug middle-end/103979] asm goto is not considered volatile with output operands

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103979

--- Comment #6 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Jakub Jelinek from comment #3)
> struct S { char a[64]; };
> 
> int
> foo (void)
> {
>   struct S s, t;
>   asm goto ("" : "=g" (s) : : : l);
>   t = s;
>   asm goto ("" : "=g" (s) : : : l);
>   return s.a[0] + t.a[63];
> l:
>   return -1;
> }
> 

That is going to be turned into a different issue ...

[Bug middle-end/103979] asm goto is not considered volatile with output operands

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103979

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Assignee|unassigned at gcc dot gnu.org      |pinskia at gcc dot gnu.org
             Status|NEW                         |ASSIGNED

--- Comment #7 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
MIne, I filed PR 110422  for the SRA issue.

[Bug middle-end/103979] asm goto is not considered volatile with output operands

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103979

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                URL|                            |https://gcc.gnu.org/piperma
                   |                            |il/gcc-patches/2023-June/62
                   |                            |2906.html
           Keywords|                            |patch

--- Comment #8 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Patch posted:
https://gcc.gnu.org/pipermail/gcc-patches/2023-June/622906.html

[Bug middle-end/103979] asm goto is not considered volatile with output operands

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103979

--- Comment #9 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The trunk branch has been updated by Andrew Pinski <pinskia@gcc.gnu.org>:

https://gcc.gnu.org/g:478840a2ca491fbff44371caee4983d1e7b7b7cf

commit r14-2133-g478840a2ca491fbff44371caee4983d1e7b7b7cf
Author: Andrew Pinski <apinski@marvell.com>
Date:   Mon Jun 26 17:14:06 2023 -0700

    Mark asm goto with outputs as volatile

    The manual references asm goto as being implicitly volatile already
    and that was done when asm goto could not have outputs. When outputs
    were added to `asm goto`, only asm goto without outputs were still being
    marked as volatile. Now some parts of GCC decide, removing the `asm goto`
    is ok if the output is not used, though not updating the CFG (this happens
    on both the RTL level and the gimple level). Since the biggest user of `asm
goto`
    is the Linux kernel and they expect them to be volatile (they use them to
    copy to/from userspace), we should just mark the inline-asm as volatile.

    OK? Bootstrapped and tested on x86_64-linux-gnu.

            PR middle-end/110420
            PR middle-end/103979
            PR middle-end/98619

    gcc/ChangeLog:

            * gimplify.cc (gimplify_asm_expr): Mark asm with labels as
volatile.

    gcc/testsuite/ChangeLog:

            * gcc.c-torture/compile/asmgoto-6.c: New test.

[Bug middle-end/103979] asm goto is not considered volatile with output operands

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103979

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
      Known to work|                            |14.0

--- Comment #10 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Fixed in GCC 14 and will be backporting to GCC 12.x and 13.x.

[Bug middle-end/103979] asm goto is not considered volatile with output operands

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103979

--- Comment #11 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-13 branch has been updated by Andrew Pinski
<pinskia@gcc.gnu.org>:

https://gcc.gnu.org/g:81fa6dfe1691e93664ef3b4fa66c1e057acea281

commit r13-7484-g81fa6dfe1691e93664ef3b4fa66c1e057acea281
Author: Andrew Pinski <apinski@marvell.com>
Date:   Mon Jun 26 17:14:06 2023 -0700

    Mark asm goto with outputs as volatile

    The manual references asm goto as being implicitly volatile already
    and that was done when asm goto could not have outputs. When outputs
    were added to `asm goto`, only asm goto without outputs were still being
    marked as volatile. Now some parts of GCC decide, removing the `asm goto`
    is ok if the output is not used, though not updating the CFG (this happens
    on both the RTL level and the gimple level). Since the biggest user of `asm
goto`
    is the Linux kernel and they expect them to be volatile (they use them to
    copy to/from userspace), we should just mark the inline-asm as volatile.

    OK? Bootstrapped and tested on x86_64-linux-gnu.

            PR middle-end/110420
            PR middle-end/103979
            PR middle-end/98619

    gcc/ChangeLog:

            * gimplify.cc (gimplify_asm_expr): Mark asm with labels as
volatile.

    gcc/testsuite/ChangeLog:

            * gcc.c-torture/compile/asmgoto-6.c: New test.

    (cherry picked from commit 478840a2ca491fbff44371caee4983d1e7b7b7cf)

[Bug middle-end/103979] asm goto is not considered volatile with output operands

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103979

--- Comment #12 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-12 branch has been updated by Andrew Pinski
<pinskia@gcc.gnu.org>:

https://gcc.gnu.org/g:ee3bb7cb5d2ecfc64adcfd61afb390e72cc08661

commit r12-9732-gee3bb7cb5d2ecfc64adcfd61afb390e72cc08661
Author: Andrew Pinski <apinski@marvell.com>
Date:   Mon Jun 26 17:14:06 2023 -0700

    Mark asm goto with outputs as volatile

    The manual references asm goto as being implicitly volatile already
    and that was done when asm goto could not have outputs. When outputs
    were added to `asm goto`, only asm goto without outputs were still being
    marked as volatile. Now some parts of GCC decide, removing the `asm goto`
    is ok if the output is not used, though not updating the CFG (this happens
    on both the RTL level and the gimple level). Since the biggest user of `asm
goto`
    is the Linux kernel and they expect them to be volatile (they use them to
    copy to/from userspace), we should just mark the inline-asm as volatile.

    OK? Bootstrapped and tested on x86_64-linux-gnu.

            PR middle-end/110420
            PR middle-end/103979
            PR middle-end/98619

    gcc/ChangeLog:

            * gimplify.cc (gimplify_asm_expr): Mark asm with labels as
volatile.

    gcc/testsuite/ChangeLog:

            * gcc.c-torture/compile/asmgoto-6.c: New test.

    (cherry picked from commit 478840a2ca491fbff44371caee4983d1e7b7b7cf)

[Bug middle-end/103979] asm goto is not considered volatile with output operands

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103979

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |12.4
      Known to work|                            |12.3.1, 13.1.1
             Status|ASSIGNED                    |RESOLVED
         Resolution|---                         |FIXED

--- Comment #13 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Fixed.