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

["gareth.webb+gccbugzilla at outlook dot com" <gcc-bugzilla@gcc.gnu.org>]

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.