[Qemu-devel] [PATCH] tcg: Use bitmaps for free temporaries

[Richard Henderson <rth@twiddle.net>]

We previously allocated 32-bits per temp for the next_free_temp entry.
We now allocate 4 bits per temp across the 4 bitmaps.

Using a linked list meant that if a translator is tweeked, resulting in
temps being freed in a different order, that would have follow-on effects
throughout the TB.  Always allocating the lowest free temp means that
follow-on effects are minimized, which can make it easier to diff output
when debugging the translators.

Signed-off-by: Richard Henderson <rth@twiddle.net>
---
 tcg/tcg.c | 32 +++++++++++++++-----------------
 tcg/tcg.h | 11 ++++++-----
 2 files changed, 21 insertions(+), 22 deletions(-)

diff --git a/tcg/tcg.c b/tcg/tcg.c
index fd7fb6b..41ed8b0 100644
--- a/tcg/tcg.c
+++ b/tcg/tcg.c
@@ -316,11 +316,12 @@ void tcg_set_frame(TCGContext *s, int reg, intptr_t start, intptr_t size)
 
 void tcg_func_start(TCGContext *s)
 {
-    int i;
     tcg_pool_reset(s);
     s->nb_temps = s->nb_globals;
-    for(i = 0; i < (TCG_TYPE_COUNT * 2); i++)
-        s->first_free_temp[i] = -1;
+
+    /* No temps have been previously allocated for size or locality.  */
+    memset(s->free_temps, 0, sizeof(s->free_temps));
+
     s->labels = tcg_malloc(sizeof(TCGLabel) * TCG_MAX_LABELS);
     s->nb_labels = 0;
     s->current_frame_offset = s->frame_start;
@@ -468,16 +469,15 @@ static inline int tcg_temp_new_internal(TCGType type, int temp_local)
     TCGTemp *ts;
     int idx, k;
 
-    k = type;
-    if (temp_local)
-        k += TCG_TYPE_COUNT;
-    idx = s->first_free_temp[k];
-    if (idx != -1) {
-        /* There is already an available temp with the
-           right type */
+    k = type + (temp_local ? TCG_TYPE_COUNT : 0);
+    idx = find_first_bit(s->free_temps[k].l, TCG_MAX_TEMPS);
+    if (idx < TCG_MAX_TEMPS) {
+        /* There is already an available temp with the right type.  */
+        clear_bit(idx, s->free_temps[k].l);
+
         ts = &s->temps[idx];
-        s->first_free_temp[k] = ts->next_free_temp;
         ts->temp_allocated = 1;
+        assert(ts->base_type == type);
         assert(ts->temp_local == temp_local);
     } else {
         idx = s->nb_temps;
@@ -533,7 +533,7 @@ TCGv_i64 tcg_temp_new_internal_i64(int temp_local)
     return MAKE_TCGV_I64(idx);
 }
 
-static inline void tcg_temp_free_internal(int idx)
+static void tcg_temp_free_internal(int idx)
 {
     TCGContext *s = &tcg_ctx;
     TCGTemp *ts;
@@ -550,11 +550,9 @@ static inline void tcg_temp_free_internal(int idx)
     ts = &s->temps[idx];
     assert(ts->temp_allocated != 0);
     ts->temp_allocated = 0;
-    k = ts->base_type;
-    if (ts->temp_local)
-        k += TCG_TYPE_COUNT;
-    ts->next_free_temp = s->first_free_temp[k];
-    s->first_free_temp[k] = idx;
+
+    k = ts->type + (ts->temp_local ? TCG_TYPE_COUNT : 0);
+    set_bit(idx, s->free_temps[k].l);
 }
 
 void tcg_temp_free_i32(TCGv_i32 arg)
diff --git a/tcg/tcg.h b/tcg/tcg.h
index 902c751..c45e41b 100644
--- a/tcg/tcg.h
+++ b/tcg/tcg.h
@@ -26,7 +26,7 @@
 #define TCG_H
 
 #include "qemu-common.h"
-
+#include "qemu/bitops.h"
 #include "tcg-target.h"
 
 /* Default target word size to pointer size.  */
@@ -400,8 +400,6 @@ typedef struct TCGTemp {
                                   basic blocks. Otherwise, it is not
                                   preserved across basic blocks. */
     unsigned int temp_allocated:1; /* never used for code gen */
-    /* index of next free temp of same base type, -1 if end */
-    int next_free_temp;
     const char *name;
 } TCGTemp;
 
@@ -412,6 +410,10 @@ typedef struct TCGHelperInfo {
 
 typedef struct TCGContext TCGContext;
 
+typedef struct TCGTempSet {
+    unsigned long l[BITS_TO_LONGS(TCG_MAX_TEMPS)];
+} TCGTempSet;
+
 struct TCGContext {
     uint8_t *pool_cur, *pool_end;
     TCGPool *pool_first, *pool_current, *pool_first_large;
@@ -419,8 +421,6 @@ struct TCGContext {
     int nb_labels;
     int nb_globals;
     int nb_temps;
-    /* index of free temps, -1 if none */
-    int first_free_temp[TCG_TYPE_COUNT * 2]; 
 
     /* goto_tb support */
     uint8_t *code_buf;
@@ -446,6 +446,7 @@ struct TCGContext {
 
     uint8_t *code_ptr;
     TCGTemp temps[TCG_MAX_TEMPS]; /* globals first, temps after */
+    TCGTempSet free_temps[TCG_TYPE_COUNT * 2];
 
     TCGHelperInfo *helpers;
     int nb_helpers;
-- 
1.8.1.4