[Qemu-devel] [PATCH 3/3] tcg: don't keep dead outputs in registers

[Aurelien Jarno <aurelien@aurel32.net>]

If an op with dead outputs is not removed, because it has side effects
or has multiple output arguments and only one dead, mark the TCG 
registers as dead instead of keeping them allocated. This avoid a few 
register spills on TCG targets with low register count, especially with
div2 and mul2 ops, or when a qemu_ld* result is not used (prefetch 
emulation for example).

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
---
 tcg/tcg.c |   28 ++++++++++++++++++----------
 1 files changed, 18 insertions(+), 10 deletions(-)

diff --git a/tcg/tcg.c b/tcg/tcg.c
index 82d3e1d..fad92f9 100644
--- a/tcg/tcg.c
+++ b/tcg/tcg.c
@@ -1782,12 +1782,16 @@ static void tcg_reg_alloc_op(TCGContext *s,
             if (!ts->fixed_reg) {
                 if (ts->val_type == TEMP_VAL_REG)
                     s->reg_to_temp[ts->reg] = -1;
-                ts->val_type = TEMP_VAL_REG;
-                ts->reg = reg;
-                /* temp value is modified, so the value kept in memory is
-                   potentially not the same */
-                ts->mem_coherent = 0; 
-                s->reg_to_temp[reg] = arg;
+                if (IS_DEAD_ARG(i)) {
+                    ts->val_type = TEMP_VAL_DEAD;
+                } else {
+                    ts->val_type = TEMP_VAL_REG;
+                    ts->reg = reg;
+                    /* temp value is modified, so the value kept in memory is
+                       potentially not the same */
+                    ts->mem_coherent = 0;
+                    s->reg_to_temp[reg] = arg;
+               }
             }
         oarg_end:
             new_args[i] = reg;
@@ -1981,10 +1985,14 @@ static int tcg_reg_alloc_call(TCGContext *s, const TCGOpDef *def,
         } else {
             if (ts->val_type == TEMP_VAL_REG)
                 s->reg_to_temp[ts->reg] = -1;
-            ts->val_type = TEMP_VAL_REG;
-            ts->reg = reg;
-            ts->mem_coherent = 0; 
-            s->reg_to_temp[reg] = arg;
+            if (IS_DEAD_ARG(i)) {
+                ts->val_type = TEMP_VAL_DEAD;
+            } else {
+                ts->val_type = TEMP_VAL_REG;
+                ts->reg = reg;
+                ts->mem_coherent = 0;
+                s->reg_to_temp[reg] = arg;
+            }
         }
     }
     
-- 
1.7.2.3