[Bug c/86938] New: Invalid register name for r8w

[quasicoherent at protonmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86938

            Bug ID: 86938
           Summary: Invalid register name for r8w
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: quasicoherent at protonmail dot com
  Target Milestone: ---

Created attachment 44531
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=44531&action=edit
-save-temps output

When compiling the following code:

#include <stdio.h>

int main() {
        register unsigned short var __asm__("r8w") = 0;

        __asm__ volatile (
                "mov $123, %[var]\t\n"
        :       [var] "+rm" (var)
        );

        printf("%d\n", var);

        return 0;
}

with

gcc -Wall -Wextra bug.c

I get the error:

bug.c: In function ‘main’:
bug.c:4:26: error: invalid register name for ‘var’
  register unsigned short var __asm__("r8w") = 0;
                          ^~~

If "r8w" is replaced by, for example, "ax" the compilation works and the
resulting program outputs "123\n", as desired. 



gcc -v output:

Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/7/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu 7.3.0-16ubuntu3'
--with-bugurl=file:///usr/share/doc/gcc-7/README.Bugs
--enable-languages=c,ada,c++,go,brig,d,fortran,objc,obj-c++ --prefix=/usr
--with-gcc-major-version-only --with-as=/usr/bin/x86_64-linux-gnu-as
--with-ld=/usr/bin/x86_64-linux-gnu-ld --program-suffix=-7
--program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id
--libexecdir=/usr/lib --without-included-gettext --enable-threads=posix
--libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu
--enable-libstdcxx-debug --enable-libstdcxx-time=yes
--with-default-libstdcxx-abi=new --enable-gnu-unique-object
--disable-vtable-verify --enable-libmpx --enable-plugin --enable-default-pie
--with-system-zlib --with-target-system-zlib --enable-objc-gc=auto
--enable-multiarch --disable-werror --with-arch-32=i686 --with-abi=m64
--with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic
--enable-offload-targets=nvptx-none --without-cuda-driver
--enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu
--target=x86_64-linux-gnu
Thread model: posix
gcc version 7.3.0 (Ubuntu 7.3.0-16ubuntu3)