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From: "andrey.vihrov at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug inline-asm/101438] New: Compiler hang on inline asm with local
 register and VLA operands
Date: Tue, 13 Jul 2021 14:36:05 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D101438

            Bug ID: 101438
           Summary: Compiler hang on inline asm with local register and
                    VLA operands
           Product: gcc
           Version: 11.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: inline-asm
          Assignee: unassigned at gcc dot gnu.org
          Reporter: andrey.vihrov at gmail dot com
  Target Milestone: ---
            Target: x86_64-pc-linux-gnu

Compiling this C source:

  int main(void)
  {
    register long long rax __asm__("rax") =3D 0;
    register long long rbx __asm__("rbx") =3D 0;
    register long long rcx __asm__("rcx") =3D 0;
    register long long rdx __asm__("rdx") =3D 0;
    register long long rsi __asm__("rsi") =3D 0;
    register long long rdi __asm__("rdi") =3D 0;
    register long long r8  __asm__("r8")  =3D 0;
    register long long r9  __asm__("r9")  =3D 0;
    register long long r10 __asm__("r10") =3D 0;
    register long long r11 __asm__("r11") =3D 0;
    register long long r12 __asm__("r12") =3D 0;
    register long long r13 __asm__("r13") =3D 0;
    register long long r14 __asm__("r14") =3D 0;
    register long long r15 __asm__("r15") =3D 0;

    volatile int x =3D 1;
    char arr[x];

    __asm__ (""
      :
      : "r"(rax),
        "r"(rbx),
        "r"(rcx),
        "r"(rdx),
        "r"(rsi),
        "r"(rdi),
        "r"(r8),
        "r"(r9),
        "r"(r10),
        "r"(r11),
        "r"(r12),
        "r"(r13),
        "r"(r14),
        "r"(r15),
        "m"(arr)
    );
  }

with "gcc -O1 test.c" causes the compiler to hang in an infinite loop.

Note: This code could possibly be formally incorrect, as [1] warns about
potential clobbering after initialization of local register variables.=20

[1] https://gcc.gnu.org/onlinedocs/gcc/Local-Register-Variables.html=