[Bug c/96365] New: GCC report impossible constraint on possible inline assembly output constraints

[Bug c/96365] New: GCC report impossible constraint on possible inline assembly output constraints

[141242068 at smail dot nju.edu.cn]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96365

            Bug ID: 96365
           Summary: GCC report impossible constraint on possible inline
                    assembly output constraints
           Product: gcc
           Version: 9.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: 141242068 at smail dot nju.edu.cn
  Target Milestone: ---

$ cat a.c
int main() {
  int x = 0, y = 0, z = 0;
  asm("" : "=ab"(x), "=bc"(y), "=c"(z));
  return 0;
}

Inline assembly at line 3 is a possible constraint, compiler can allocate eax
for x, ebx for y, ecx for z, but GCC report that this asm has impossible
constraint as below:

-> tmp $ gcc a.c
asm.c: In function ‘main’:
asm.c:3:3: error: ‘asm’ operand has impossible constraints
    3 |   asm("" : "=ab"(x), "=bc"(y), "=c"(z));
      |   ^~~

My gcc version:

-> tmp $ gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/9/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none:hsa
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu 9.3.0-10ubuntu2'
--with-bugurl=file:///usr/share/doc/gcc-9/README.Bugs
--enable-languages=c,ada,c++,go,brig,d,fortran,objc,obj-c++,gm2 --prefix=/usr
--with-gcc-major-version-only --program-suffix=-9
--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 --enable-clocale=gnu --enable-libstdcxx-debug
--enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new
--enable-gnu-unique-object --disable-vtable-verify --enable-plugin
--enable-default-pie --with-system-zlib --with-target-system-zlib=auto
--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,hsa
--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 9.3.0 (Ubuntu 9.3.0-10ubuntu2)

[Bug c/96365] GCC report impossible constraint on possible inline assembly output constraints

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96365

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

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

--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Any reason why you can't just write "=a"(x), "=b"(y), "=c"(z) if you mean that
as the only possibility?

[Bug c/96365] GCC report impossible constraint on possible inline assembly output constraints

[141242068 at smail dot nju.edu.cn]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96365

--- Comment #2 from wierton <141242068 at smail dot nju.edu.cn> ---
(In reply to Jakub Jelinek from comment #1)
> Any reason why you can't just write "=a"(x), "=b"(y), "=c"(z) if you mean
> that as the only possibility?

Emm, I was studying the inline assembly by reading the manual on
https://gcc.gnu.org/onlinedocs/gcc/Using-Assembly-Language-with-C.html#Using-Assembly-Language-with-C.
This example was accidently found because I wondered that if GCC really
considers constraints allocation as constraint solving problem. It seems that
GCC takes simpler strategy such as greedy algorithm and this may violate the
specification. I was not very sure if it is really inconsistent with
specification (I failed in finding accurate description), but I thought posting
this example may help improve the quality of GCC (or specification).

[Bug middle-end/96365] GCC report impossible constraint on possible inline assembly output constraints

[141242068 at smail dot nju.edu.cn]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96365

wierton <141242068 at smail dot nju.edu.cn> changed:

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

--- Comment #3 from wierton <141242068 at smail dot nju.edu.cn> ---
Compiler has no need to take it as a CSP problem.