Branch Prediction?

[David Holmes]

On 19/11/2014 4:42 AM, Christian Thalinger wrote:
> …or I could just read the next email.  Doh.

Not so obvious when the subject was changed to "Compiler branch hints". 
I had to go to the archives to find it.

My 2c. The performance focus has been on generated code, not the runtime 
code. Personally I dislike these magic macros as they clutter the code.

David

>> On Nov 18, 2014, at 10:41 AM, Christian Thalinger <christian.thalinger at oracle.com> wrote:
>>
>> I’m not sure if the silence means nobody knows or nobody cares.  Speaking for myself, I don’t know of any history on this.
>>
>>> On Nov 8, 2014, at 1:43 AM, Erik Österlund <erik.osterlund at lnu.se> wrote:
>>>
>>> Hi,
>>>
>>> Just out of curiosity, is there some good reason why we don't have a branch prediction macro?
>>> For every tight load a; cmpxchg(expect: a, addr: &x, new_val: b); loop, I feel a bit uneasy not telling the compiler that this is pretty likely to succeed, and relying on its guessing.
>>>
>>> Has it been excluded because it's considered not nice or perhaps it was simply never introduced because nobody found it useful?
>>>
>>> Could have some define like this for GCC, which for other compilers reduces to nothing:
>>>
>>> #define VM_EXPECT_TRUE(A) __builtin_expect((A), true)
>>> #define VM_EXPECT_FALSE(A) __builtin_expect((A), false)
>>>
>>> It might not lead to drastic performance improvements, but it feels weird not to tell the compiler what we know and keep secrets from it. And I think it's also nice for documentation purposes that people reading it also understand that this expression is gonna be true most of the time, and deal with it accordingly.
>>>
>>> /Erik
>>
>