Re: Microarchitectural support for counting

On Wed, 25 Dec 2024 20:35:29 +0000
mitchalsup@aol.com (MitchAlsup1) wrote:

On Sat, 5 Oct 2024 15:11:29 +0000, EricP wrote:
 

MitchAlsup1 wrote: 

On Fri, 4 Oct 2024 18:11:23 +0000, EricP wrote:
 

--------------------------

>
Not necessary, you purge all of the younger instructions from the
thread at retirement, but none of the instructions associated with
the new <interrupt> thread at the front. 

>
That's difficult with a circular buffer for the instruction
queue/rob as you can't edit the order. For a branch mispredict you
might be able to mark a circular range of entries as voided, and
leave the entries to be recovered serially at retire. 

 
Sooner or later, the pipeline designer needs to recognize the of
occuring
code sequence pictured as::
 
     INST
     INST
     BC-------\
     INST     |
     INST     |
     INST     |
/----BR       |
|    INST<----/
|    INST
|    INST
\--->INST
     INST
 
So that the branch predictor predicts as usual, but DECODER recognizes
the join point of this prediction, so if the prediction is wrong, one
only nullifies the mispredicted instructions and then inserts the
alternate instructions while holding the join point instructions until
the alternate instruction complete.
 

Yes, compilers often generate such code.
When coding in asm, I typically know at least something about
probability of branches, so I tend to code it differently:

inst
inst
bc colder_section
inst
inst
inst
merge_flow:
inst
inst
...
ret

colder_section:
inst
inst
inst
br merge_flow


Intel's "efficiency" cores family starting from Tremont has weird
"clustered" front end design. It often prefers [predicted] taken
branches over [predicted] non-taken branches. On front ends like that
my optimization is likely to become pessimization.