[PATCH] i386: Fix wrong codegen for -mrelax-cmpxchg-loop

[Uros Bizjak]

On Thu, Nov 18, 2021 at 8:37 AM Hongyu Wang <hongyu.wang@intel.com> wrote:
>
> Hi Uros,
>
> For -mrelax-cmpxchg-loop introduced by PR 103069/r12-5265, it would
> produce infinite loop. The correct code should be
>
> .L84:
>         movl    (%rdi), %ecx
>         movl    %eax, %edx
>         orl     %esi, %edx
>         cmpl    %eax, %ecx
>         jne     .L82
>         lock cmpxchgl   %edx, (%rdi)
>         jne     .L84
>         movl    %r8d, %eax  <<< retval is missing in previous impl
>         ret
> .L82:
>         rep nop
>         jmp     .L84
>
> Adjust corresponding expander to fix such issue, and fix runtime test
> so the problem would be exposed.
>
> Bootstrapped/regtested on x86_64-pc-linux-gnu{-m32,}.
> Ok for master?
>
> gcc/ChangeLog:
>
>         * config/i386/i386-expand.c (ix86_expand_atomic_fetch_op_loop):
>         Adjust generated cfg to avoid infinite loop.
>
> gcc/testsuite/ChangeLog:
>
>         * gcc.target/i386/pr103069-2.c: Adjust.

LGTM.

Thanks,
Uros.

> ---
>  gcc/config/i386/i386-expand.c              |  7 ++++++-
>  gcc/testsuite/gcc.target/i386/pr103069-2.c | 11 ++++++-----
>  2 files changed, 12 insertions(+), 6 deletions(-)
>
> diff --git a/gcc/config/i386/i386-expand.c b/gcc/config/i386/i386-expand.c
> index 3e4de64ec24..0d5d1a0e205 100644
> --- a/gcc/config/i386/i386-expand.c
> +++ b/gcc/config/i386/i386-expand.c
> @@ -23143,13 +23143,14 @@ void ix86_expand_atomic_fetch_op_loop (rtx target, rtx mem, rtx val,
>                                        bool doubleword)
>  {
>    rtx old_reg, new_reg, old_mem, success, oldval, new_mem;
> -  rtx_code_label *loop_label, *pause_label;
> +  rtx_code_label *loop_label, *pause_label, *done_label;
>    machine_mode mode = GET_MODE (target);
>
>    old_reg = gen_reg_rtx (mode);
>    new_reg = old_reg;
>    loop_label = gen_label_rtx ();
>    pause_label = gen_label_rtx ();
> +  done_label = gen_label_rtx ();
>    old_mem = copy_to_reg (mem);
>    emit_label (loop_label);
>    emit_move_insn (old_reg, old_mem);
> @@ -23207,11 +23208,15 @@ void ix86_expand_atomic_fetch_op_loop (rtx target, rtx mem, rtx val,
>                            GET_MODE (success), 1, loop_label,
>                            profile_probability::guessed_never ());
>
> +  emit_jump_insn (gen_jump (done_label));
> +  emit_barrier ();
> +
>    /* If mem is not expected, pause and loop back.  */
>    emit_label (pause_label);
>    emit_insn (gen_pause ());
>    emit_jump_insn (gen_jump (loop_label));
>    emit_barrier ();
> +  emit_label (done_label);
>  }
>
>  #include "gt-i386-expand.h"
> diff --git a/gcc/testsuite/gcc.target/i386/pr103069-2.c b/gcc/testsuite/gcc.target/i386/pr103069-2.c
> index 8ac824cc8e8..b3f2235fd55 100644
> --- a/gcc/testsuite/gcc.target/i386/pr103069-2.c
> +++ b/gcc/testsuite/gcc.target/i386/pr103069-2.c
> @@ -1,5 +1,5 @@
> -/* PR target/103068 */
> -/* { dg-do compile } */
> +/* PR target/103069 */
> +/* { dg-do run } */
>  /* { dg-additional-options "-O2 -march=x86-64 -mtune=generic" } */
>
>  #include <stdlib.h>
> @@ -37,13 +37,14 @@ FUNC_ATOMIC_RELAX (char, xor)
>  #define TEST_ATOMIC_FETCH_LOGIC(TYPE, OP) \
>  { \
>    TYPE a = 11, b = 101, res, exp; \
> +  TYPE c = 11, d = 101;        \
>    res = relax_##TYPE##_##OP##_fetch (&a, b); \
> -  exp = f_##TYPE##_##OP##_fetch (&a, b);  \
> +  exp = f_##TYPE##_##OP##_fetch (&c, d);  \
>    if (res != exp) \
>      abort (); \
> -  a = 21, b = 92; \
> +  a = c = 21, b = d = 92; \
>    res = relax_##TYPE##_fetch_##OP (&a, b); \
> -  exp = f_##TYPE##_fetch_##OP (&a, b);  \
> +  exp = f_##TYPE##_fetch_##OP (&c, d);  \
>    if (res != exp) \
>      abort (); \
>  }
> --
> 2.18.1
>