Re: [PATCH 2/3][AArch64][PR target/65697] Strengthen barriers for sync-compare-swap builtins.

[Matthew Wahab <matthew.wahab@arm.com>]

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[Added PR number and updated patches]

This patch changes the code generated for __sync_type_compare_and_swap to

   ldxr reg; cmp; bne label; stlxr; cbnz; label: dmb ish; mov .., reg

This removes the acquire-barrier from the load and ends the operation with a
fence to prevent memory references appearing after the __sync operation from
being moved ahead of the store-release.

This also strengthens the acquire barrier generated for __sync_lock_test_and_set
(which, like compare-and-swap, is implemented as a form of atomic exchange):

   ldaxr; stxr; cbnz
becomes
   ldxr; stxr; cbnz; dmb ish


Tested with check-gcc for aarch64-none-linux-gnu.

Ok for trunk?
Matthew

2015-05-22  Matthew Wahab  <matthew.wahab@arm.com>

	PR target/65697
	* config/aarch64/aarch64.c (aarch64_split_compare_and_swap): Check
	for __sync memory models, emit appropriate initial and final
	barriers.


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From 1e5cda95944e7176b8934296b1bb1ec4c9fb1362 Mon Sep 17 00:00:00 2001
From: Matthew Wahab <matthew.wahab@arm.com>
Date: Fri, 15 May 2015 09:31:06 +0100
Subject: [PATCH 2/3] [AArch64] Strengthen barriers for sync-compare-swap
 builtins.

Change-Id: I335771f2f42ea951d227f20f6cb9daa07330614d
---
 gcc/config/aarch64/aarch64.c | 19 +++++++++++++++++--
 1 file changed, 17 insertions(+), 2 deletions(-)

diff --git a/gcc/config/aarch64/aarch64.c b/gcc/config/aarch64/aarch64.c
index 182dbad..5b9feee 100644
--- a/gcc/config/aarch64/aarch64.c
+++ b/gcc/config/aarch64/aarch64.c
@@ -9433,14 +9433,19 @@ aarch64_split_compare_and_swap (rtx operands[])
   bool is_weak;
   rtx_code_label *label1, *label2;
   rtx x, cond;
+  enum memmodel model;
+  rtx model_rtx;
+  rtx load_model_rtx;
 
   rval = operands[0];
   mem = operands[1];
   oldval = operands[2];
   newval = operands[3];
   is_weak = (operands[4] != const0_rtx);
+  model_rtx = operands[5];
   scratch = operands[7];
   mode = GET_MODE (mem);
+  model = memmodel_from_int (INTVAL (model_rtx));
 
   label1 = NULL;
   if (!is_weak)
@@ -9450,7 +9455,13 @@ aarch64_split_compare_and_swap (rtx operands[])
     }
   label2 = gen_label_rtx ();
 
-  aarch64_emit_load_exclusive (mode, rval, mem, operands[5]);
+  /* A __sync operation will end with a fence so the load can be relaxed.  */
+  if (is_mm_sync (model))
+    load_model_rtx = GEN_INT (MEMMODEL_RELAXED);
+  else
+    load_model_rtx = model_rtx;
+
+  aarch64_emit_load_exclusive (mode, rval, mem, load_model_rtx);
 
   cond = aarch64_gen_compare_reg (NE, rval, oldval);
   x = gen_rtx_NE (VOIDmode, cond, const0_rtx);
@@ -9458,7 +9469,7 @@ aarch64_split_compare_and_swap (rtx operands[])
 			    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, operands[5]);
+  aarch64_emit_store_exclusive (mode, scratch, mem, newval, model_rtx);
 
   if (!is_weak)
     {
@@ -9475,6 +9486,10 @@ aarch64_split_compare_and_swap (rtx operands[])
     }
 
   emit_label (label2);
+
+  /* A __sync operation may need a final fence.  */
+  if (is_mm_sync (model))
+    aarch64_emit_post_barrier (model);
 }
 
 /* Split an atomic operation.  */
-- 
1.9.1