Re: [Qemu-devel] [PATCH v2] tcg: optimise memory layout of TCGTemp

["Alex Bennée" <alex.bennee@linaro.org>]

Emilio G. Cota <cota@braap.org> writes:

> This brings down the size of the struct from 56 to 32 bytes on 64-bit,
> and to 20 bytes on 32-bit. This leads to memory savings:
>
> 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
>   40883   29800      88   70771   11473 ./aarch64-softmmu/tcg/tcg.o
>   37473   29416      96   66985   105a9 ./x86_64-linux-user/tcg/tcg.o
>   38858   28816      96   67770   108ba ./arm-linux-user/tcg/tcg.o
>   40554   29096      88   69738   1106a ./arm-softmmu/tcg/tcg.o
>   39169   29672      88   68929   10d41 ./x86_64-softmmu/tcg/tcg.o
>
> Note that using an entire byte for some enums that need less than
> that wastes a few bits (noticeable in 32 bits, where we use
> 20 bytes instead of 16) but avoids extraction code, which overall
> is a win--I've tested several variations of the patch, and the appended
> is the best performer for OpenSSL's bntest by a very small margin:
>
> Before:
> $ taskset -c 0 perf stat -r 15 -- x86_64-linux-user/qemu-x86_64 img/bntest-x86_64 >/dev/null
> [...]
>  Performance counter stats for 'x86_64-linux-user/qemu-x86_64 img/bntest-x86_64' (15 runs):
>
>       10538.479833 task-clock (msec)         #    0.999 CPUs utilized            ( +-  0.38% )
>                772 context-switches          #    0.073 K/sec                    ( +-  2.03% )
>                  0 cpu-migrations            #    0.000 K/sec                    ( +-100.00% )
>              2,207 page-faults               #    0.209 K/sec                    ( +-  0.08% )
>       10.552871687 seconds time elapsed                                          ( +-  0.39% )
>
> After:
> $ taskset -c 0 perf stat -r 15 -- x86_64-linux-user/qemu-x86_64 img/bntest-x86_64 >/dev/null
>  Performance counter stats for 'x86_64-linux-user/qemu-x86_64 img/bntest-x86_64' (15 runs):
>
>       10459.968847 task-clock (msec)         #    0.999 CPUs utilized            ( +-  0.30% )
>                739 context-switches          #    0.071 K/sec                    ( +-  1.71% )
>                  0 cpu-migrations            #    0.000 K/sec                    ( +- 68.14% )
>              2,204 page-faults               #    0.211 K/sec                    ( +-  0.10% )
>       10.473900411 seconds time elapsed
>                ( +-  0.30% )

I'll take that as a win condition ;-)

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>

>
> 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 | 26 ++++++++++++++------------
>  1 file changed, 14 insertions(+), 12 deletions(-)
>
> diff --git a/tcg/tcg.h b/tcg/tcg.h
> index add7f75..7f95132 100644
> --- a/tcg/tcg.h
> +++ b/tcg/tcg.h
> @@ -417,20 +417,19 @@ static inline TCGCond tcg_high_cond(TCGCond c)
>      }
>  }
>  
> -#define TEMP_VAL_DEAD  0
> -#define TEMP_VAL_REG   1
> -#define TEMP_VAL_MEM   2
> -#define TEMP_VAL_CONST 3
> +typedef enum TCGTempVal {
> +    TEMP_VAL_DEAD,
> +    TEMP_VAL_REG,
> +    TEMP_VAL_MEM,
> +    TEMP_VAL_CONST,
> +} TCGTempVal;
>  
> -/* 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;
> +    TCGTempVal val_type:8;
> +    TCGType base_type:8;
> +    TCGType type:8;
>      unsigned int fixed_reg:1;
>      unsigned int mem_coherent:1;
>      unsigned int mem_allocated:1;
> @@ -438,6 +437,9 @@ 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;

-- 
Alex Bennée