Re: [PATCH][i386] Correct imul (r64) latency for modern Intel CPUs

[Jan Hubicka <hubicka at ucw dot cz>]

> Since Nehalem the 64bit multiplication latency is three cycles, not
> four. So update the costs to reflect reality.

I looked into the imul latencies and was a bit confused, decided to look
into it later and forgot.

Agner Fog's table http://www.agner.org/optimize/instruction_tables.pdf
lists for sandybridge-skylake costs of 3 cycles for everything except for
16bit (which is 4 cycles).

For earlier chips this is more varying.
 Merom (core duo) and Wolfdale is 3 or everything except for 5 for 64bit.
 Nehalem is 3 for everything however 64bit has smaller throughput.

We do not have spearate table for sandybridge, but I guess we do not care
that much about older cores (Vladimir is still running a tester on one,
so we do benchmark them).

So I guess the change is OK.  I do not see this as very good reason to split
the sandybridge table out, but perhaps drop a comment in case we will want
to do it in future.

Honza
> 
> Tested on X86_64.
> OK for trunk?
> 
> Thanks.
> 
> 	* x86-tune-costs.h (skylake_cost, core_cost): Decrease r64 multiply
> 	latencies.
> 
> 	* gcc.target/i386/wmul-3.c: New test.
> 
> diff --git a/gcc/config/i386/x86-tune-costs.h b/gcc/config/i386/x86-tune-costs.h
> index 648219338308..ddb47ba44056 100644
> --- a/gcc/config/i386/x86-tune-costs.h
> +++ b/gcc/config/i386/x86-tune-costs.h
> @@ -1538,8 +1538,8 @@ struct processor_costs skylake_cost = {
>    {COSTS_N_INSNS (3),			/* cost of starting multiply for QI */
>     COSTS_N_INSNS (4),			/*				 HI */
>     COSTS_N_INSNS (3),			/*				 SI */
> -   COSTS_N_INSNS (4),			/*				 DI */
> -   COSTS_N_INSNS (4)},			/*			      other */
> +   COSTS_N_INSNS (3),			/*				 DI */
> +   COSTS_N_INSNS (3)},			/*			      other */
>    0,					/* cost of multiply per each bit set */
>    /* Expanding div/mod currently doesn't consider parallelism. So the cost
>       model is not realistic. We compensate by increasing the latencies a bit.  */
> @@ -2341,8 +2341,8 @@ struct processor_costs core_cost = {
>    {COSTS_N_INSNS (3),			/* cost of starting multiply for QI */
>     COSTS_N_INSNS (4),			/*				 HI */
>     COSTS_N_INSNS (3),			/*				 SI */
> -   COSTS_N_INSNS (4),			/*				 DI */
> -   COSTS_N_INSNS (4)},			/*			      other */
> +   COSTS_N_INSNS (3),			/*				 DI */
> +   COSTS_N_INSNS (3)},			/*			      other */
>    0,					/* cost of multiply per each bit set */
>    /* Expanding div/mod currently doesn't consider parallelism. So the cost
>       model is not realistic. We compensate by increasing the latencies a bit.  */
> diff --git a/gcc/testsuite/gcc.target/i386/wmul-3.c b/gcc/testsuite/gcc.target/i386/wmul-3.c
> new file mode 100644
> index 000000000000..66c077c2cc0d
> --- /dev/null
> +++ b/gcc/testsuite/gcc.target/i386/wmul-3.c
> @@ -0,0 +1,66 @@
> +/* { dg-do compile { target { ! ia32 } } } */
> +/* { dg-options "-O2 -march=haswell" } */
> +
> +#include <stdint.h>
> +#include <string.h>
> +
> +static const char b100_tab[200] = {
> +    '0', '0', '0', '1', '0', '2', '0', '3', '0', '4',
> +    '0', '5', '0', '6', '0', '7', '0', '8', '0', '9',
> +    '1', '0', '1', '1', '1', '2', '1', '3', '1', '4',
> +    '1', '5', '1', '6', '1', '7', '1', '8', '1', '9',
> +    '2', '0', '2', '1', '2', '2', '2', '3', '2', '4',
> +    '2', '5', '2', '6', '2', '7', '2', '8', '2', '9',
> +    '3', '0', '3', '1', '3', '2', '3', '3', '3', '4',
> +    '3', '5', '3', '6', '3', '7', '3', '8', '3', '9',
> +    '4', '0', '4', '1', '4', '2', '4', '3', '4', '4',
> +    '4', '5', '4', '6', '4', '7', '4', '8', '4', '9',
> +    '5', '0', '5', '1', '5', '2', '5', '3', '5', '4',
> +    '5', '5', '5', '6', '5', '7', '5', '8', '5', '9',
> +    '6', '0', '6', '1', '6', '2', '6', '3', '6', '4',
> +    '6', '5', '6', '6', '6', '7', '6', '8', '6', '9',
> +    '7', '0', '7', '1', '7', '2', '7', '3', '7', '4',
> +    '7', '5', '7', '6', '7', '7', '7', '8', '7', '9',
> +    '8', '0', '8', '1', '8', '2', '8', '3', '8', '4',
> +    '8', '5', '8', '6', '8', '7', '8', '8', '8', '9',
> +    '9', '0', '9', '1', '9', '2', '9', '3', '9', '4',
> +    '9', '5', '9', '6', '9', '7', '9', '8', '9', '9',
> +};
> +
> +void uint64_to_ascii_ta7_32_base100(uint64_t val, char *dst) {
> +  const int64_t POW10_10 = ((int64_t)10) * 1000 * 1000 * 1000;
> +  const uint64_t POW2_57_DIV_POW100_4 =
> +      ((int64_t)(1) << 57) / 100 / 100 / 100 / 100 + 1;
> +  const uint64_t MASK32 = ((int64_t)(1) << 32) - 1;
> +  int64_t hix = val / POW10_10;
> +  int64_t lox = val % POW10_10;
> +  int64_t lor = lox & (uint64_t)(-2);
> +  uint64_t hi = hix * POW2_57_DIV_POW100_4;
> +  uint64_t lo = lor * POW2_57_DIV_POW100_4;
> +  memcpy(dst + 0 * 10 + 0, &b100_tab[(hi >> 57) * 2], 2);
> +  memcpy(dst + 1 * 10 + 0, &b100_tab[(lo >> 57) * 2], 2);
> +  hi = (hi >> 25) + 1;
> +  lo = (lo >> 25) + 1;
> +  hi = (hi & MASK32) * 100;
> +  lo = (lo & MASK32) * 100;
> +  memcpy(dst + 0 * 10 + 2, &b100_tab[(hi >> 32) * 2], 2);
> +  hi = (hi & MASK32) * 100;
> +  memcpy(dst + 1 * 10 + 2, &b100_tab[(lo >> 32) * 2], 2);
> +  lo = (lo & MASK32) * 100;
> +  memcpy(dst + 0 * 10 + 4, &b100_tab[(hi >> 32) * 2], 2);
> +  hi = (hi & MASK32) * 100;
> +  memcpy(dst + 1 * 10 + 4, &b100_tab[(lo >> 32) * 2], 2);
> +  lo = (lo & MASK32) * 100;
> +  memcpy(dst + 0 * 10 + 6, &b100_tab[(hi >> 32) * 2], 2);
> +  hi = (hi & MASK32) * 100;
> +  memcpy(dst + 1 * 10 + 6, &b100_tab[(lo >> 32) * 2], 2);
> +  lo = (lo & MASK32) * 100;
> +  hi >>= 32;
> +  lo >>= 32;
> +  lo = (lo & (-2)) | (lox & 1);
> +  memcpy(dst + 0 * 10 + 8, &b100_tab[hi * 2], 2);
> +  memcpy(dst + 1 * 10 + 8, &b100_tab[lo * 2], 2);
> +  dst[2 * 10] = 0;
> +}
> +
> +/* { dg-final { scan-assembler-times "imulq" 11 } } */
> -- 
> Markus