[AArch64] Emit tighter strong atomic compare-exchange loop when comparing against zero

* config/aarch64/aarch64.c (aarch64_split_compare_and_swap):
	Emit CBNZ inside loop when doing a strong exchange and comparing
	against zero.  Generate the CC flags after the loop.

	* gcc.target/aarch64/atomic_cmp_exchange_zero_strong_1.c: New test.

From-SVN: r248832

gcc/ChangeLog

@@ -1,3 +1,9 @@
+2017-06-02  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>
+
+	* config/aarch64/aarch64.c (aarch64_split_compare_and_swap):
+	Emit CBNZ inside loop when doing a strong exchange and comparing
+	against zero.  Generate the CC flags after the loop.
+
 2017-06-02  David Edelsohn  <dje.gcc@gmail.com>
 
 	* dwarf2out.c (DWARF_INITIAL_LENGTH_SIZE_STR): New.

gcc/config/aarch64/aarch64.c

@@ -12111,6 +12111,17 @@ aarch64_split_compare_and_swap (rtx operands[])
   mode = GET_MODE (mem);
   model = memmodel_from_int (INTVAL (model_rtx));
 
+  /* When OLDVAL is zero and we want the strong version we can emit a tighter
+    loop:
+    .label1:
+	LD[A]XR	rval, [mem]
+	CBNZ	rval, .label2
+	ST[L]XR	scratch, newval, [mem]
+	CBNZ	scratch, .label1
+    .label2:
+	CMP	rval, 0.  */
+  bool strong_zero_p = !is_weak && oldval == const0_rtx;
+
   label1 = NULL;
   if (!is_weak)
     {
@@ -12127,11 +12138,21 @@ aarch64_split_compare_and_swap (rtx operands[])
   else
     aarch64_emit_load_exclusive (mode, rval, mem, model_rtx);
 
+  if (strong_zero_p)
+    {
+      x = gen_rtx_NE (VOIDmode, rval, const0_rtx);
+      x = gen_rtx_IF_THEN_ELSE (VOIDmode, x,
+				gen_rtx_LABEL_REF (Pmode, label2), pc_rtx);
+      aarch64_emit_unlikely_jump (gen_rtx_SET (pc_rtx, x));
+    }
+  else
+    {
       cond = aarch64_gen_compare_reg (NE, rval, oldval);
       x = gen_rtx_NE (VOIDmode, cond, const0_rtx);
       x = gen_rtx_IF_THEN_ELSE (VOIDmode, x,
 				 gen_rtx_LABEL_REF (Pmode, label2), pc_rtx);
       aarch64_emit_unlikely_jump (gen_rtx_SET (pc_rtx, x));
+    }
 
   aarch64_emit_store_exclusive (mode, scratch, mem, newval, model_rtx);
 
@@ -12150,7 +12171,15 @@ aarch64_split_compare_and_swap (rtx operands[])
     }
 
   emit_label (label2);
-
+  /* If we used a CBNZ in the exchange loop emit an explicit compare with RVAL
+     to set the condition flags.  If this is not used it will be removed by
+     later passes.  */
+  if (strong_zero_p)
+    {
+      cond = gen_rtx_REG (CCmode, CC_REGNUM);
+      x = gen_rtx_COMPARE (CCmode, rval, const0_rtx);
+      emit_insn (gen_rtx_SET (cond, x));
+    }
   /* Emit any final barrier needed for a __sync operation.  */
   if (is_mm_sync (model))
     aarch64_emit_post_barrier (model);

gcc/testsuite/ChangeLog

@@ -1,3 +1,7 @@
+2017-06-02  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>
+
+	* gcc.target/aarch64/atomic_cmp_exchange_zero_strong_1.c: New test.
+
 2017-06-02  Will Schmidt  <will_schmidt@vnet.ibm.com>
 
 	* gcc.target/powerpc/fold-vec-abs-char.c: New.

gcc/testsuite/gcc.target/aarch64/atomic_cmp_exchange_zero_strong_1.c

@@ -0,0 +1,12 @@
+/* { dg-do compile } */
+/* { dg-options "-O2" } */
+
+int
+foo (int *a)
+{
+  int x = 0;
+  return __atomic_compare_exchange_n (a, &x, 4, 0,
+				      __ATOMIC_ACQUIRE, __ATOMIC_ACQUIRE);
+}
+
+/* { dg-final { scan-assembler-times "cbnz\\tw\[0-9\]+" 2 } } */