https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80080
Dominik Vogt <vogt at linux dot vnet.ibm.com> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |vogt at linux dot vnet.ibm.com
--- Comment #3 from Dominik Vogt <vogt at linux dot vnet.ibm.com> ---
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,
