[Bug target/109903] New: Register misallocation in hand-crafted asm insn, no diagnostics produced

["dimitri.gorokhovik at free dot fr" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109903

            Bug ID: 109903
           Summary: Register misallocation in hand-crafted asm insn, no
                    diagnostics produced
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: dimitri.gorokhovik at free dot fr
  Target Milestone: ---

For the source code:

#if NO_BUG
unsigned long f (unsigned long a)
{
  unsigned long v;

  asm ("mrs %[_1], pmccntr_eli0\n"
       "\tldr x1, [%[_2]]\n"
       : [_1] "=r" (v)
       : [_2] "r" (a)
       );

  return v;
}


#else // BUG

extern void g(unsigned long);
void f (unsigned long a)
{
  unsigned long v;

  asm ("mrs %[_1], pmccntr_eli0\n"
       "\tldr x1, [%[_2]]\n"
       : [_1] "=r" (v)
       : [_2] "r" (a)
       );

  g(v);
}

#endif

The asm output produced is:

## BUG
#APP
# 23 "bug.c" 1
        mrs %rdi, pmccntr_eli0
        ldr x1, [%rdi]

# 0 "" 2
#NO_APP


## NO_BUG
#APP
# 6 "bug.c" 1
        mrs %rax, pmccntr_eli0
        ldr x1, [%rdi]

# 0 "" 2
#NO_APP

As one can see, in the "NO BUG" case, the registers allocated to reading "the
counter" and to reading "memory", are different, whereas they are one and the
same in the "BUG" case.

This was obtained with today's trunk built for native compilation: gcc (GCC)
14.0.0 20230518 (experimental). 

The example was compiled as:
gcc -S -O3 -Wall -Wextra -o bug.S bug.c 

no diagnostic messages are produced.

Same was also observed with gcc-aarch64-none-linux toolchain 13.2 (from Arm
download page), so "target-related issue" might not be an entirely correct
classification for this.