[Qemu-devel] [PATCH] x86_64: optimise muldiv64 for x86_64 architecture

[Qemu-devel] [PATCH] x86_64: optimise muldiv64 for x86_64 architecture

[Frediano Ziglio]

As this platform can do multiply/divide using 128 bit precision use
these instruction to implement it.

Signed-off-by: Frediano Ziglio <frediano.ziglio@huawei.com>
---
 include/qemu-common.h | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/include/qemu-common.h b/include/qemu-common.h
index f862214..5366220 100644
--- a/include/qemu-common.h
+++ b/include/qemu-common.h
@@ -370,6 +370,7 @@ static inline uint8_t from_bcd(uint8_t val)
 }
 
 /* compute with 96 bit intermediate result: (a*b)/c */
+#ifndef __x86_64__
 static inline uint64_t muldiv64(uint64_t a, uint32_t b, uint32_t c)
 {
     union {
@@ -392,6 +393,18 @@ static inline uint64_t muldiv64(uint64_t a, uint32_t b, uint32_t c)
     res.l.low = (((rh % c) << 32) + (rl & 0xffffffff)) / c;
     return res.ll;
 }
+#else
+static inline uint64_t muldiv64(uint64_t a, uint32_t b, uint32_t c)
+{
+    uint64_t res;
+
+    asm ("mulq %2\n\tdivq %3"
+         : "=a"(res)
+         : "a"(a), "qm"((uint64_t) b), "qm"((uint64_t)c)
+         : "rdx", "cc");
+    return res;
+}
+#endif
 
 /* Round number down to multiple */
 #define QEMU_ALIGN_DOWN(n, m) ((n) / (m) * (m))
-- 
1.9.1

Re: [Qemu-devel] [PATCH] x86_64: optimise muldiv64 for x86_64 architecture

[Richard Henderson]

On 01/09/2015 01:53 AM, Frediano Ziglio wrote:
> As this platform can do multiply/divide using 128 bit precision use
> these instruction to implement it.
> 
> Signed-off-by: Frediano Ziglio <frediano.ziglio@huawei.com>
> ---
>  include/qemu-common.h | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/include/qemu-common.h b/include/qemu-common.h
> index f862214..5366220 100644
> --- a/include/qemu-common.h
> +++ b/include/qemu-common.h
> @@ -370,6 +370,7 @@ static inline uint8_t from_bcd(uint8_t val)
>  }
>  
>  /* compute with 96 bit intermediate result: (a*b)/c */
> +#ifndef __x86_64__
>  static inline uint64_t muldiv64(uint64_t a, uint32_t b, uint32_t c)
>  {
>      union {
> @@ -392,6 +393,18 @@ static inline uint64_t muldiv64(uint64_t a, uint32_t b, uint32_t c)
>      res.l.low = (((rh % c) << 32) + (rl & 0xffffffff)) / c;
>      return res.ll;
>  }
> +#else
> +static inline uint64_t muldiv64(uint64_t a, uint32_t b, uint32_t c)
> +{
> +    uint64_t res;
> +
> +    asm ("mulq %2\n\tdivq %3"
> +         : "=a"(res)
> +         : "a"(a), "qm"((uint64_t) b), "qm"((uint64_t)c)
> +         : "rdx", "cc");
> +    return res;
> +}
> +#endif

Honestly, this ought to move into qemu/host-utils.h, and it should
use __int128 for targets that support it.  Which includes x86_64,
but also other 64-bit hosts.


r~

[Qemu-devel] [PATCH] x86_64: optimise muldiv64 for x86_64 architecture

[Frediano Ziglio]

As this platform can do multiply/divide using 128 bit precision use
these instruction to implement it.

Signed-off-by: Frediano Ziglio <frediano.ziglio@huawei.com>
---
 include/qemu-common.h | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/include/qemu-common.h b/include/qemu-common.h
index f862214..5366220 100644
--- a/include/qemu-common.h
+++ b/include/qemu-common.h
@@ -370,6 +370,7 @@ static inline uint8_t from_bcd(uint8_t val)
 }
 
 /* compute with 96 bit intermediate result: (a*b)/c */
+#ifndef __x86_64__
 static inline uint64_t muldiv64(uint64_t a, uint32_t b, uint32_t c)
 {
     union {
@@ -392,6 +393,18 @@ static inline uint64_t muldiv64(uint64_t a, uint32_t b, uint32_t c)
     res.l.low = (((rh % c) << 32) + (rl & 0xffffffff)) / c;
     return res.ll;
 }
+#else
+static inline uint64_t muldiv64(uint64_t a, uint32_t b, uint32_t c)
+{
+    uint64_t res;
+
+    asm ("mulq %2\n\tdivq %3"
+         : "=a"(res)
+         : "a"(a), "qm"((uint64_t) b), "qm"((uint64_t)c)
+         : "rdx", "cc");
+    return res;
+}
+#endif
 
 /* Round number down to multiple */
 #define QEMU_ALIGN_DOWN(n, m) ((n) / (m) * (m))
-- 
1.9.1

Re: [Qemu-devel] [PATCH] x86_64: optimise muldiv64 for x86_64 architecture

[Paolo Bonzini]

On 09/01/2015 11:27, Frediano Ziglio wrote:
> 
> Signed-off-by: Frediano Ziglio <frediano.ziglio@huawei.com>
> ---
>  include/qemu-common.h | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/include/qemu-common.h b/include/qemu-common.h
> index f862214..5366220 100644
> --- a/include/qemu-common.h
> +++ b/include/qemu-common.h
> @@ -370,6 +370,7 @@ static inline uint8_t from_bcd(uint8_t val)
>  }
>  
>  /* compute with 96 bit intermediate result: (a*b)/c */
> +#ifndef __x86_64__
>  static inline uint64_t muldiv64(uint64_t a, uint32_t b, uint32_t c)
>  {
>      union {
> @@ -392,6 +393,18 @@ static inline uint64_t muldiv64(uint64_t a, uint32_t b, uint32_t c)
>      res.l.low = (((rh % c) << 32) + (rl & 0xffffffff)) / c;
>      return res.ll;
>  }
> +#else
> +static inline uint64_t muldiv64(uint64_t a, uint32_t b, uint32_t c)
> +{
> +    uint64_t res;
> +
> +    asm ("mulq %2\n\tdivq %3"
> +         : "=a"(res)
> +         : "a"(a), "qm"((uint64_t) b), "qm"((uint64_t)c)
> +         : "rdx", "cc");
> +    return res;
> +}
> +#endif
>  

Good idea.  However, if you have __int128, you can just do

   return (__int128)a * b / c

and the compiler should generate the right code.  Conveniently, there is
already CONFIG_INT128 that you can use.

Paolo

Re: [Qemu-devel] [PATCH] x86_64: optimise muldiv64 for x86_64 architecture

[Frediano Ziglio]

2015-01-09 10:35 GMT+00:00 Paolo Bonzini <pbonzini@redhat.com>:
>
>
> On 09/01/2015 11:27, Frediano Ziglio wrote:
>>
>> Signed-off-by: Frediano Ziglio <frediano.ziglio@huawei.com>
>> ---
>>  include/qemu-common.h | 13 +++++++++++++
>>  1 file changed, 13 insertions(+)
>>
>> diff --git a/include/qemu-common.h b/include/qemu-common.h
>> index f862214..5366220 100644
>> --- a/include/qemu-common.h
>> +++ b/include/qemu-common.h
>> @@ -370,6 +370,7 @@ static inline uint8_t from_bcd(uint8_t val)
>>  }
>>
>>  /* compute with 96 bit intermediate result: (a*b)/c */
>> +#ifndef __x86_64__
>>  static inline uint64_t muldiv64(uint64_t a, uint32_t b, uint32_t c)
>>  {
>>      union {
>> @@ -392,6 +393,18 @@ static inline uint64_t muldiv64(uint64_t a, uint32_t b, uint32_t c)
>>      res.l.low = (((rh % c) << 32) + (rl & 0xffffffff)) / c;
>>      return res.ll;
>>  }
>> +#else
>> +static inline uint64_t muldiv64(uint64_t a, uint32_t b, uint32_t c)
>> +{
>> +    uint64_t res;
>> +
>> +    asm ("mulq %2\n\tdivq %3"
>> +         : "=a"(res)
>> +         : "a"(a), "qm"((uint64_t) b), "qm"((uint64_t)c)
>> +         : "rdx", "cc");
>> +    return res;
>> +}
>> +#endif
>>
>
> Good idea.  However, if you have __int128, you can just do
>
>    return (__int128)a * b / c
>
> and the compiler should generate the right code.  Conveniently, there is
> already CONFIG_INT128 that you can use.
>
> Paolo

Well, it works but in our case b <= c, that is a * b / c is always <
2^64. This lead to no integer overflow in the last division. However
the compiler does not know this so it does the entire (a*b) / c
division which is mainly consists in two integer division instead of
one (not taking into account that is implemented using a helper
function).

I think that I'll write two patches. One implementing using the int128
as you suggested (which is much easier to read that current one and
assembly ones) that another for x86_64 optimization.

Frediano

Re: [Qemu-devel] [PATCH] x86_64: optimise muldiv64 for x86_64 architecture

[Paolo Bonzini]

On 09/01/2015 12:04, Frediano Ziglio wrote:
> 2015-01-09 10:35 GMT+00:00 Paolo Bonzini <pbonzini@redhat.com>:
>>
>>
>> On 09/01/2015 11:27, Frediano Ziglio wrote:
>>>
>>> Signed-off-by: Frediano Ziglio <frediano.ziglio@huawei.com>
>>> ---
>>>  include/qemu-common.h | 13 +++++++++++++
>>>  1 file changed, 13 insertions(+)
>>>
>>> diff --git a/include/qemu-common.h b/include/qemu-common.h
>>> index f862214..5366220 100644
>>> --- a/include/qemu-common.h
>>> +++ b/include/qemu-common.h
>>> @@ -370,6 +370,7 @@ static inline uint8_t from_bcd(uint8_t val)
>>>  }
>>>
>>>  /* compute with 96 bit intermediate result: (a*b)/c */
>>> +#ifndef __x86_64__
>>>  static inline uint64_t muldiv64(uint64_t a, uint32_t b, uint32_t c)
>>>  {
>>>      union {
>>> @@ -392,6 +393,18 @@ static inline uint64_t muldiv64(uint64_t a, uint32_t b, uint32_t c)
>>>      res.l.low = (((rh % c) << 32) + (rl & 0xffffffff)) / c;
>>>      return res.ll;
>>>  }
>>> +#else
>>> +static inline uint64_t muldiv64(uint64_t a, uint32_t b, uint32_t c)
>>> +{
>>> +    uint64_t res;
>>> +
>>> +    asm ("mulq %2\n\tdivq %3"
>>> +         : "=a"(res)
>>> +         : "a"(a), "qm"((uint64_t) b), "qm"((uint64_t)c)
>>> +         : "rdx", "cc");
>>> +    return res;
>>> +}
>>> +#endif
>>>
>>
>> Good idea.  However, if you have __int128, you can just do
>>
>>    return (__int128)a * b / c
>>
>> and the compiler should generate the right code.  Conveniently, there is
>> already CONFIG_INT128 that you can use.
> 
> Well, it works but in our case b <= c, that is a * b / c is always <
> 2^64.

This is not necessarily the case.  Quick grep:

hw/timer/hpet.c:    return (muldiv64(value, HPET_CLK_PERIOD, FS_PER_NS));
hw/timer/hpet.c:    return (muldiv64(value, FS_PER_NS, HPET_CLK_PERIOD));

One of the two must disprove your assertion. :)

But it's true that we expect no overflow.

> This lead to no integer overflow in the last division. However
> the compiler does not know this so it does the entire (a*b) / c
> division which is mainly consists in two integer division instead of
> one (not taking into account that is implemented using a helper
> function).
> 
> I think that I'll write two patches. One implementing using the int128
> as you suggested (which is much easier to read that current one and
> assembly ones) that another for x86_64 optimization.

Right, that's even better.

Out of curiosity, have you seen it in some profiles?

Paolo

Re: [Qemu-devel] [PATCH] x86_64: optimise muldiv64 for x86_64 architecture

[Peter Maydell]

On 9 January 2015 at 11:24, Paolo Bonzini <pbonzini@redhat.com> wrote:
> On 09/01/2015 12:04, Frediano Ziglio wrote:
>> I think that I'll write two patches. One implementing using the int128
>> as you suggested (which is much easier to read that current one and
>> assembly ones) that another for x86_64 optimization.
>
> Right, that's even better.

Personally I would prefer we didn't write inline assembly
functions if we can avoid them. So I'd rather see an int128
version, and if the compiler doesn't do a good enough job
then go talk to the compiler folks to improve things.

> Out of curiosity, have you seen it in some profiles?

I would absolutely want to see significant perf uplift on
a real workload before we start putting inline asm into
qemu-common.h...

-- PMM

Re: [Qemu-devel] [PATCH] x86_64: optimise muldiv64 for x86_64 architecture

[Frediano Ziglio]

2015-01-09 11:24 GMT+00:00 Paolo Bonzini <pbonzini@redhat.com>:
>
>
> On 09/01/2015 12:04, Frediano Ziglio wrote:
>> 2015-01-09 10:35 GMT+00:00 Paolo Bonzini <pbonzini@redhat.com>:
>>>
>>>
>>> On 09/01/2015 11:27, Frediano Ziglio wrote:
>>>>
>>>> Signed-off-by: Frediano Ziglio <frediano.ziglio@huawei.com>
>>>> ---
>>>>  include/qemu-common.h | 13 +++++++++++++
>>>>  1 file changed, 13 insertions(+)
>>>>
>>>> diff --git a/include/qemu-common.h b/include/qemu-common.h
>>>> index f862214..5366220 100644
>>>> --- a/include/qemu-common.h
>>>> +++ b/include/qemu-common.h
>>>> @@ -370,6 +370,7 @@ static inline uint8_t from_bcd(uint8_t val)
>>>>  }
>>>>
>>>>  /* compute with 96 bit intermediate result: (a*b)/c */
>>>> +#ifndef __x86_64__
>>>>  static inline uint64_t muldiv64(uint64_t a, uint32_t b, uint32_t c)
>>>>  {
>>>>      union {
>>>> @@ -392,6 +393,18 @@ static inline uint64_t muldiv64(uint64_t a, uint32_t b, uint32_t c)
>>>>      res.l.low = (((rh % c) << 32) + (rl & 0xffffffff)) / c;
>>>>      return res.ll;
>>>>  }
>>>> +#else
>>>> +static inline uint64_t muldiv64(uint64_t a, uint32_t b, uint32_t c)
>>>> +{
>>>> +    uint64_t res;
>>>> +
>>>> +    asm ("mulq %2\n\tdivq %3"
>>>> +         : "=a"(res)
>>>> +         : "a"(a), "qm"((uint64_t) b), "qm"((uint64_t)c)
>>>> +         : "rdx", "cc");
>>>> +    return res;
>>>> +}
>>>> +#endif
>>>>
>>>
>>> Good idea.  However, if you have __int128, you can just do
>>>
>>>    return (__int128)a * b / c
>>>
>>> and the compiler should generate the right code.  Conveniently, there is
>>> already CONFIG_INT128 that you can use.
>>
>> Well, it works but in our case b <= c, that is a * b / c is always <
>> 2^64.
>
> This is not necessarily the case.  Quick grep:
>
> hw/timer/hpet.c:    return (muldiv64(value, HPET_CLK_PERIOD, FS_PER_NS));
> hw/timer/hpet.c:    return (muldiv64(value, FS_PER_NS, HPET_CLK_PERIOD));
>
> One of the two must disprove your assertion. :)
>

Unless FS_PER_NS ==  HPET_CLK_PERIOD :)

> But it's true that we expect no overflow.
>

This is enough!

>> This lead to no integer overflow in the last division. However
>> the compiler does not know this so it does the entire (a*b) / c
>> division which is mainly consists in two integer division instead of
>> one (not taking into account that is implemented using a helper
>> function).
>>
>> I think that I'll write two patches. One implementing using the int128
>> as you suggested (which is much easier to read that current one and
>> assembly ones) that another for x86_64 optimization.
>
> Right, that's even better.
>
> Out of curiosity, have you seen it in some profiles?
>
> Paolo

No, just looked at the complicate code and generated code and though
"why using dozen of instructions if two are enough?" :)

Frediano