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

[Richard Henderson <rth@twiddle.net>]

On 03/25/2015 12:50 PM, Emilio G. Cota wrote:
> 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

I'd rather you moved TCG_TYPE_COUNT out of the enum and into a define.  Perhaps
even as (1 << TCG_TYPE_NR_BITS).

> @@ -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

And make this an enumeration.

>  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;

And do *not* change these from the enumeration to an unsigned int.

I know why you did this -- to keep the compiler from warning that the TCGType
enum didn't fit in the bitfield, because of TCG_TYPE_COUNT being an enumerator,
rather than an unrelated number.  Except that's exactly the warning we want to
keep, on the off-chance that someone modifies the enums without modifying the
_NR_BITS defines.


r~