[Info-vax] Itanium support is back in GCC 15

[John Reagan]

On 2/24/2025 4:43 PM, Arne Vajhøj wrote:
> On 2/24/2025 4:22 PM, Michael S wrote:
>> On Mon, 24 Feb 2025 15:08:57 -0500
>> Arne Vajhøj <arne at vajhoej.dk> wrote:
>>> On 2/24/2025 12:42 PM, Michael S wrote:
> 
>>> C++ VMS x86-64 is clang which in the (older) clang version used
>>> should mean C++14 while C++ VMS Itanium is very very old (like
>>> C++ 98 old).
>>>
>>>> According to the benchmarks that you posted here several months (a
>>>> year?) ago, VMS x86-64 compilers are quite awful comparatively to
>>>> x86-64 compilers available on Windows/Linux/BSD.
>>>> Do you want to say that VMS Itanium compilers are worse?
>>>
>>> I believe the conclusion was that the VMS x86-64 compilers except C++
>>> was slower than C/C++ on other OS and C++ on VMS.
>>
>> Somehow I got an impression that C++ compilers were also significantly
>> slower than C++ compilers on other platforms.
>> Do I misremember?
> 
> I don't even remember that I posted non-VMS numbers here. Age! :-)
> 
> But I just checked VMS C++ latest (CXX/OPT=LEVEL:5 and clang -O3) vs a 
> random Windows GCC 14.1 (g++ -O3):
> 
> VMS is a little faster for integer
> they are about the same for floating point
> Windows is a lot faster for string
> 
> And given that this is a micro-benchmark with in reality just an inner
> loop evaluating a single expression, which means huge uncertainty, then
> I don't see this as proof of a significant difference.
> 
> Arne
> 
We are aware of the string/char performance issues.

On Alpha and Itanium, the lowlevel routines inside of LIBOTS for things 
like OTS$MOVE, string compare, memmove, etc. are all written in 
hand-crafted assembly.  For x86, we are still using a set of BLISS 
reference code that is simple.  Plus the LIBOTS we all have on our 
systems was compiled with a non-optimizing BLISS cross-compiler.

We are currently playing with native compiled LIBOTS code and doing some 
benchmarks.  Besides the brain-dead BLISS code, we have versions that 
loop with larger chunks of data which are even faster.  The fastest 
we've seen so far is a native assembly version that uses the REP 
instruction prefix on the MOVSB.  That version didn't check for 
overlapping source/dest however so any real version gets a little 
slower.  I'm not sure when we can incorporate these, but I'm trying to 
push them as soon as possible.

A fun reference to read is

https://cdrdv2-public.intel.com/814198/248966-Optimization-Reference-Manual-V1-050.pdf