https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80080
Jakub Jelinek <jakub at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |jakub at gcc dot gnu.org
--- Comment #4 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
(In reply to Dominik Vogt from comment #3)
> Most of this can be fixed in the backend, but all attempts to get rid of the
> additional load ("lr %r3,%31") in "bar" within the backend have failed.
> This is caused by overly complicated code being generated in
> expand_compare_and_swap_loop(). I'm currently testing the patch below which
> eliminates the superfluous variable cmp_reg (and the register allocated for
> it). Is there any chance to get something like this into Gcc7? The patch
> could possibly also help other targets. (Will post on gcc-patches when the
> tests are complete.)
>
>
> --- a/gcc/optabs.c
> +++ b/gcc/optabs.c
> @@ -5798,17 +5798,15 @@ find_cc_set (rtx x, const_rtx pat, void *data)
> static bool
> expand_compare_and_swap_loop (rtx mem, rtx old_reg, rtx new_reg, rtx seq)
> {
> - machine_mode mode = GET_MODE (mem);
> rtx_code_label *label;
> - rtx cmp_reg, success, oldval;
> + rtx success, oldval;
>
> /* The loop we want to generate looks like
>
> - cmp_reg = mem;
> + old_reg = mem;
> label:
> - old_reg = cmp_reg;
> seq;
> - (success, cmp_reg) = compare-and-swap(mem, old_reg, new_reg)
> + (success, old_reg) = compare-and-swap(mem, old_reg, new_reg)
> if (success)
> goto label;
>
> @@ -5816,23 +5814,21 @@ expand_compare_and_swap_loop (rtx mem, rtx old_reg,
> rtx new_reg, rtx seq)
> iterations use the value loaded by the compare-and-swap pattern. */
>
> label = gen_label_rtx ();
> - cmp_reg = gen_reg_rtx (mode);
>
> - emit_move_insn (cmp_reg, mem);
> + emit_move_insn (old_reg, mem);
> emit_label (label);
> - emit_move_insn (old_reg, cmp_reg);
> if (seq)
> emit_insn (seq);
>
> success = NULL_RTX;
> - oldval = cmp_reg;
> + oldval = old_reg;
> if (!expand_atomic_compare_and_swap (&success, &oldval, mem, old_reg,
> new_reg, false, MEMMODEL_SYNC_SEQ_CST,
> MEMMODEL_RELAXED))
> return false;
>
> - if (oldval != cmp_reg)
> - emit_move_insn (cmp_reg, oldval);
> + if (oldval != old_reg)
> + emit_move_insn (old_reg, oldval);
>
> /* Mark this jump predicted not taken. */
> emit_cmp_and_jump_insns (success, const0_rtx, EQ, const0_rtx,
That doesn't look correct. expand_compare_and_swap_loop is called from two
places and at least in the first case it wants old_reg to contain on success
something that is the target that will be returned.
While with your patch it might be something different.
