[Qemu-devel] [PULL 18/18] tcg: Optimize fence instructions

[Richard Henderson <rth@twiddle.net>]

From: Pranith Kumar <bobby.prani@gmail.com>

This commit optimizes fence instructions.  Two optimizations are
currently implemented: (1) unnecessary duplicate fence instructions,
and (2) merging weaker fences into a stronger fence.

[rth: Merge tcg_optimize_mb back into tcg_optimize, so that we only
loop over the opcode stream once.  Merge "unrelated" weaker barriers
into one stronger barrier.]

Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
Message-Id: <20160823134825.32578-1-bobby.prani@gmail.com>
Signed-off-by: Richard Henderson <rth@twiddle.net>
---
 tcg/optimize.c | 54 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 54 insertions(+)

diff --git a/tcg/optimize.c b/tcg/optimize.c
index cffe89b..0455285 100644
--- a/tcg/optimize.c
+++ b/tcg/optimize.c
@@ -542,6 +542,7 @@ static bool swap_commutative2(TCGArg *p1, TCGArg *p2)
 void tcg_optimize(TCGContext *s)
 {
     int oi, oi_next, nb_temps, nb_globals;
+    TCGArg *prev_mb_args = NULL;
 
     /* Array VALS has an element for each temp.
        If this temp holds a constant then its value is kept in VALS' element.
@@ -1295,5 +1296,58 @@ void tcg_optimize(TCGContext *s)
             }
             break;
         }
+
+        /* Eliminate duplicate and redundant fence instructions.  */
+        if (prev_mb_args) {
+            TCGArg pop, cop;
+            TCGBar pty, cty;
+
+            switch (opc) {
+            case INDEX_op_mb:
+                pop = prev_mb_args[0];
+                cop = args[0];
+                pty = pop & 0xF0;
+                cty = cop & 0xF0;
+
+                if (cty == pty) {
+                    /* Two barriers of the same type.  Merge the set of
+                     * memories to which this applies.  */
+                    pop |= cop & 0x0F;
+                } else {
+                    /* Merge a weaker barrier into a stronger one,
+                     * or two weaker barriers into a stronger one.
+                     *   mb; strl => mb; st
+                     *   ldaq; mb => ld; mb
+                     *   ldaq; strl => ld; mb; st
+                     * Other combinations are also merged into a strong
+                     * barrier.  This is stricter than specified but for
+                     * the purposes of TCG is better than not optimizing.
+                     */
+                    pop = TCG_BAR_SC | ((cop | pop) & 0x0F);
+                }
+                /* Change the previous barrier to the merged state.
+                 * Then we can remove the current barrier.  */
+                prev_mb_args[0] = pop;
+                tcg_op_remove(s, op);
+                break;
+
+            default:
+                /* Opcodes that end the block stop the optimization.  */
+                if ((def->flags & TCG_OPF_BB_END) == 0) {
+                    break;
+                }
+                /* fallthru */
+            case INDEX_op_qemu_ld_i32:
+            case INDEX_op_qemu_ld_i64:
+            case INDEX_op_qemu_st_i32:
+            case INDEX_op_qemu_st_i64:
+            case INDEX_op_call:
+                /* Opcodes that touch guest memory stop the optimization.  */
+                prev_mb_args = NULL;
+                break;
+            }
+        } else if (opc == INDEX_op_mb) {
+            prev_mb_args = args;
+        }
     }
 }
-- 
2.7.4