[Qemu-devel] [RFC v2 PATCH] tcg: Optimize fence instructions

[Pranith Kumar <bobby.prani@gmail.com>]

This commit optimizes fence instructions. Two optimizations are
currently implemented. These are:

1. Unnecessary duplicate fence instructions

   If the same fence instruction is detected consecutively, we remove
   one instance of it.

   ex: mb; mb => mb, strl; strl => strl

2. Merging weaker fence with subsequent/previous stronger fence

   load-acquire/store-release fence can be combined with a full fence
   without relaxing the ordering constraint.

   ex: a) ld; ldaq; mb => ld; mb
       b) mb; strl; st => mb; st

Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
---
v2:
  - Properly remove current op
  - Reset only when you encounter memory operations or end of block
  - Review comments from v1

 tcg/optimize.c | 84 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 tcg/tcg.c      |  4 +++
 tcg/tcg.h      |  1 +
 3 files changed, 89 insertions(+)

diff --git a/tcg/optimize.c b/tcg/optimize.c
index cffe89b..5963a39 100644
--- a/tcg/optimize.c
+++ b/tcg/optimize.c
@@ -538,6 +538,90 @@ static bool swap_commutative2(TCGArg *p1, TCGArg *p2)
     return false;
 }
 
+/* Eliminate duplicate and unnecessary fence instructions */
+void tcg_optimize_mb(TCGContext *s)
+{
+    int oi, oi_next;
+    TCGArg prev_op_mb = -1;
+    TCGOp *prev_op = NULL;
+
+    for (oi = s->gen_op_buf[0].next; oi != 0; oi = oi_next) {
+        TCGOp *op = &s->gen_op_buf[oi];
+        TCGArg *args = &s->gen_opparam_buf[op->args];
+        TCGOpcode opc = op->opc;
+
+        switch (opc) {
+        case INDEX_op_mb:
+        {
+            TCGBar curr_mb_type = args[0] & 0xF0;
+            TCGBar prev_mb_type = prev_op_mb & 0xF0;
+
+            if (curr_mb_type == prev_mb_type ||
+                (curr_mb_type == TCG_BAR_STRL && prev_mb_type == TCG_BAR_SC)) {
+                /* Remove the current weaker barrier op. The previous
+                 * barrier is stronger and sufficient.
+                 * mb; strl => mb; st
+                 */
+                tcg_op_remove(s, op);
+                op = prev_op;
+                break;
+            } else if (curr_mb_type == TCG_BAR_SC &&
+                       prev_mb_type == TCG_BAR_LDAQ) {
+                /* Remove the previous weaker barrier op. The current
+                 * barrier is stronger and sufficient.
+                 * ldaq; mb => ld; mb
+                 */
+                tcg_op_remove(s, prev_op);
+            } else if (curr_mb_type == TCG_BAR_STRL &&
+                       prev_mb_type == TCG_BAR_LDAQ) {
+                /* Consecutive load-acquire and store-release barriers
+                 * can be merged into one stronger SC barrier
+                 * ldaq; strl => ld; mb; st
+                 */
+                args[0] = (args[0] & 0x0F) | TCG_BAR_SC;
+                tcg_op_remove(s, prev_op);
+            }
+            prev_op_mb = args[0];
+            prev_op = op;
+            break;
+        }
+        case INDEX_op_insn_start:
+            break;
+        case INDEX_op_ld8u_i32:
+        case INDEX_op_ld8u_i64:
+        case INDEX_op_ld8s_i32:
+        case INDEX_op_ld8s_i64:
+        case INDEX_op_ld16u_i32:
+        case INDEX_op_ld16u_i64:
+        case INDEX_op_ld16s_i32:
+        case INDEX_op_ld16s_i64:
+        case INDEX_op_ld_i32:
+        case INDEX_op_ld32u_i64:
+        case INDEX_op_ld32s_i64:
+        case INDEX_op_ld_i64:
+        case INDEX_op_st8_i32:
+        case INDEX_op_st8_i64:
+        case INDEX_op_st16_i32:
+        case INDEX_op_st16_i64:
+        case INDEX_op_st_i32:
+        case INDEX_op_st32_i64:
+        case INDEX_op_st_i64:
+        case INDEX_op_call:
+            prev_op_mb = -1;
+            prev_op = NULL;
+            break;
+        default:
+            if (tcg_op_defs[opc].flags & TCG_OPF_BB_END) {
+                prev_op_mb = -1;
+                prev_op = NULL;
+            }
+            break;
+        }
+
+        oi_next = op->next;
+    }
+}
+
 /* Propagate constants and copies, fold constant expressions. */
 void tcg_optimize(TCGContext *s)
 {
diff --git a/tcg/tcg.c b/tcg/tcg.c
index 42417bd..1db319e 100644
--- a/tcg/tcg.c
+++ b/tcg/tcg.c
@@ -2587,6 +2587,10 @@ int tcg_gen_code(TCGContext *s, TranslationBlock *tb)
         }
     }
 
+#ifdef USE_TCG_OPTIMIZATIONS
+    tcg_optimize_mb(s);
+#endif
+
 #ifdef CONFIG_PROFILER
     s->la_time += profile_getclock();
 #endif
diff --git a/tcg/tcg.h b/tcg/tcg.h
index 9ed78dc..79bb5bb 100644
--- a/tcg/tcg.h
+++ b/tcg/tcg.h
@@ -921,6 +921,7 @@ void tcg_op_remove(TCGContext *s, TCGOp *op);
 TCGOp *tcg_op_insert_before(TCGContext *s, TCGOp *op, TCGOpcode opc, int narg);
 TCGOp *tcg_op_insert_after(TCGContext *s, TCGOp *op, TCGOpcode opc, int narg);
 
+void tcg_optimize_mb(TCGContext *s);
 void tcg_optimize(TCGContext *s);
 
 /* only used for debugging purposes */
-- 
2.9.2