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From: "dimitri.gorokhovik at free dot fr" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/109903] New: Register misallocation in hand-crafted asm
 insn, no diagnostics produced
Date: Thu, 18 May 2023 14:59:48 +0000
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X-Bugzilla-Version: 14.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109903

            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] "=3Dr" (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] "=3Dr" (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 "t=
he
counter" and to reading "memory", are different, whereas they are one and t=
he
same in the "BUG" case.

This was obtained with today's trunk built for native compilation: gcc (GCC)
14.0.0 20230518 (experimental).=20

The example was compiled as:
gcc -S -O3 -Wall -Wextra -o bug.S bug.c=20

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.=