Re: Computer architects leaving Intel...

David Brown <david.brown@hesbynett.no> wrote:

On 17/09/2024 03:36, Waldek Hebisch wrote:

David Brown <david.brown@hesbynett.no> wrote:

On 16/09/2024 19:51, MitchAlsup1 wrote:

On Mon, 16 Sep 2024 8:34:19 +0000, David Brown wrote:
 

On 15/09/2024 21:13, MitchAlsup1 wrote:

 
As to HW sadism:: this not not <realistically> any harder than mis-
aligned DW accesses from the cache. Many ISA from the rather distant
past could do these rather efficiently {360 SRDL,...}
 

 
Anyone who designs a data structure with a bit-field that spans two
64-bit parts of a struct is probably ignorant of C bit-fields and
software in general.  It is highly unlikely to be necessary or even
beneficial from the hardware viewpoint, but really inconvenient on the
software side (whether you use bit-fields or not).

 
Sometimes you don't have a choice::
x86-64 segment registers.
PCIe MMI/O registers,
..

 
The folks designing those register setups had a choice, and made a bad
choice from the viewpoint of software (whether it be C, assembly, or any
other language).
 
It's conceivable that it was the right choice on balance, considering
many factors.  And it's certainly more believable that it was an
appropriate choice when sizes were smaller.  It is less believable that
there is an overwhelming need to cross a 64-bit boundary.

 
Several pieces of software discoverd that "bad" smaller data
structures lead to faster execution.  Simply, smaller data structures
lead to better utilization of caches and busses, and efect due to
this was larger than cost of extra instructions.  So need to cross
64-bit boundary may be rare, but there will be cases when it is best
choice.
 

 
It is possible, but I think it is rare.
 
Perhaps my perception is biased from working with microcontrollers,
where you often don't have caches and instruction speeds are not nearly
as much faster than ram access speeds as you see in modern x86 systems.

I personally got lots of 20% speedups by restructuring data on PlayStation
2 code.

The C rules for data structure layout is stupid, a programmer would add a
int in front of a vector and fail to wonder why his structure grew by 16
bytes. Never mind that he used that 4 byte int to hold a value that had a
max of 15.

Had to annotate the data structures with 16 byte comment boundaries to stop
endless stupidity.

The other thing I don't like about split bit-fields is that there is
typically no way to do atomic updates, which can mean you need extra
care to keep things correct.