[Qemu-devel] [PATCH] tcg: optimise memory layout of TCGTemp

["Emilio G. Cota" <cota@braap.org>]

This brings down the size of the struct from 56 to 32 bytes on 64-bit,
and to 16 bytes on 32-bit.

The appended adds macros to prevent us from mistakenly overflowing
the bitfields when more elements are added to the corresponding
enums/macros.

Note that reg/mem_reg need only 6 bits (for ia64) but for performance
is probably better to align them to a byte address.

Given that TCGTemp is used in large arrays this leads to a few KBs
of savings. However, unpacking the bits takes additional code, so
the net effect depends on the target (host is x86_64):

Before:
$ find . -name 'tcg.o' | xargs size
   text    data     bss     dec     hex filename
  41131   29800      88   71019   1156b ./aarch64-softmmu/tcg/tcg.o
  37969   29416      96   67481   10799 ./x86_64-linux-user/tcg/tcg.o
  39354   28816      96   68266   10aaa ./arm-linux-user/tcg/tcg.o
  40802   29096      88   69986   11162 ./arm-softmmu/tcg/tcg.o
  39417   29672      88   69177   10e39 ./x86_64-softmmu/tcg/tcg.o

After:
$ find . -name 'tcg.o' | xargs size
   text    data     bss     dec     hex filename
  41187   29800      88   71075   115a3 ./aarch64-softmmu/tcg/tcg.o
  37777   29416      96   67289   106d9 ./x86_64-linux-user/tcg/tcg.o
  39162   28816      96   68074   109ea ./arm-linux-user/tcg/tcg.o
  40858   29096      88   70042   1119a ./arm-softmmu/tcg/tcg.o
  39473   29672      88   69233   10e71 ./x86_64-softmmu/tcg/tcg.o

Suggested-by: Stefan Weil <sw@weilnetz.de>
Suggested-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Emilio G. Cota <cota@braap.org>
---
 tcg/tcg.h | 22 +++++++++++++---------
 1 file changed, 13 insertions(+), 9 deletions(-)

diff --git a/tcg/tcg.h b/tcg/tcg.h
index add7f75..71ae7b2 100644
--- a/tcg/tcg.h
+++ b/tcg/tcg.h
@@ -193,7 +193,7 @@ typedef struct TCGPool {
 typedef enum TCGType {
     TCG_TYPE_I32,
     TCG_TYPE_I64,
-    TCG_TYPE_COUNT, /* number of different types */
+    TCG_TYPE_COUNT, /* number of different types, see TCG_TYPE_NR_BITS */
 
     /* An alias for the size of the host register.  */
 #if TCG_TARGET_REG_BITS == 32
@@ -217,6 +217,9 @@ typedef enum TCGType {
 #endif
 } TCGType;
 
+/* used for bitfield packing to save space */
+#define TCG_TYPE_NR_BITS 1
+
 /* Constants for qemu_ld and qemu_st for the Memory Operation field.  */
 typedef enum TCGMemOp {
     MO_8     = 0,
@@ -421,16 +424,14 @@ static inline TCGCond tcg_high_cond(TCGCond c)
 #define TEMP_VAL_REG   1
 #define TEMP_VAL_MEM   2
 #define TEMP_VAL_CONST 3
+#define TEMP_VAL_NR_BITS 2
 
-/* XXX: optimize memory layout */
 typedef struct TCGTemp {
-    TCGType base_type;
-    TCGType type;
-    int val_type;
-    int reg;
-    tcg_target_long val;
-    int mem_reg;
-    intptr_t mem_offset;
+    unsigned int reg:8;
+    unsigned int mem_reg:8;
+    unsigned int val_type:TEMP_VAL_NR_BITS;
+    unsigned int base_type:TCG_TYPE_NR_BITS;
+    unsigned int type:TCG_TYPE_NR_BITS;
     unsigned int fixed_reg:1;
     unsigned int mem_coherent:1;
     unsigned int mem_allocated:1;
@@ -438,6 +439,9 @@ typedef struct TCGTemp {
                                   basic blocks. Otherwise, it is not
                                   preserved across basic blocks. */
     unsigned int temp_allocated:1; /* never used for code gen */
+
+    tcg_target_long val;
+    intptr_t mem_offset;
     const char *name;
 } TCGTemp;
 
-- 
1.9.1