Commit: 34f939218ce78163171addd63750e1e0300376ab - qemu (git)

Commit MetaInfo

• Revisión: 34f939218ce78163171addd63750e1e0300376ab (tree)
• Tiempo: 2016-09-17 00:12:12
• Autor: Pranith Kumar <bobby.prani@gmai...>
• Commiter: Richard Henderson

Log Message

tcg: Optimize fence instructions

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 <[e-mail]>
Message-Id: <[e-mail]>
Signed-off-by: Richard Henderson <[e-mail]>

Cambiar Resumen

• modified: tcg/optimize.c

Diferencia incremental

--- 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 +1296,43 @@ void tcg_optimize(TCGContext *s)
             }
             break;
         }
+
+        /* Eliminate duplicate and redundant fence instructions.  */
+        if (prev_mb_args) {
+            switch (opc) {
+            case INDEX_op_mb:
+                /* Merge two barriers of the same type into one,
+                 * or a weaker barrier into a stronger one,
+                 * or two weaker barriers into a stronger one.
+                 *   mb X; mb Y => mb X|Y
+                 *   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.
+                 */
+                prev_mb_args[0] |= args[0];
+                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;
+        }
     }
 }