[Bug c++/106743] New: Illegal assembly code with -march=skylake

[Bug c++/106743] New: Illegal assembly code with -march=skylake

[stayprivate at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106743

            Bug ID: 106743
           Summary: Illegal assembly code with -march=skylake
           Product: gcc
           Version: 12.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: stayprivate at gmail dot com
  Target Milestone: ---

#include <stdint.h>
#include <iostream>

#define swapw(__val)                                                           
                                                           \
        ({                                                                     
                                                                 \
                uint16_t __tmp = __val;                                        
                                                                       \
                __asm__("xchgb %b0, %h0" : "=q"(__tmp) : "0"(__tmp));          
                                                                      \
                __tmp;                                                         
                                                                       \
        })


int main()
{
    int value = rand();
    int a = swapw(value);
    std::cout << std::hex << "First value is " << value << " " << a << '\n';
}

Compile with gcc (gcc 12 on Ubuntu ), and also with trunk. As shown on
https://godbolt.org/z/oGn9WK6GE the assembler will generate the following
error:
: register type mismatch for `xchg'

This is only when using -march=skylake or above and using -O1 or above.

The code works in gcc-9/10/11.

[Bug target/106743] Illegal assembly code with -march=skylake

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106743

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
        xchgb %bpl, %bp

[Bug target/106743] Illegal assembly code with -march=skylake

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106743

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

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

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
The inline-asm is wrong.
It should be:
                __asm__("xchgb %b0, %h0 ## %0 " : "=Q"(__tmp) : "0"(__tmp));


q
Any register accessible as rl. In 32-bit mode, a, b, c, and d; in 64-bit mode,
any integer register.

Q
Any register accessible as rh: a, b, c, and d.

Because you are accessing it as rh so you need to Q rather than q.

[Bug target/106743] Illegal assembly code with -march=skylake

[marxin at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106743

Martin Liška <marxin at gcc dot gnu.org> changed:

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

--- Comment #3 from Martin Liška <marxin at gcc dot gnu.org> ---
Reduced test-case: 

cat pr106743.ii
struct basic_ostream {};
template <typename _Traits> void operator<<(_Traits, char);
basic_ostream cout;
int main___tmp;

int main() {
  int a = ({
    __asm__("xchgb %b0, %h0" : "=q"(main___tmp));
    main___tmp;
  });
  cout << a;
}