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

[Stephen Hoffman]

On 2025-02-23 21:29:00 +0000, John Dallman said:

> In article <vpfoc7$2o5$1 at panix2.panix.com>, kludge at panix.com (Scott 
> Dorsey) wrote:
> 
>> The whole idea of the VLIW system is that the compiler will be able to 
>> optimize the code to gain paralellism of units inside the single  
>> processor. This is a very very ingenious idea but nobody has yet been 
>> able to make a compiler that could do it well enough for it to  be a 
>> real win.
> 
> Sadly, the job is *impossible*.
> 
> The fundamental problem in optimisation for modern computers is the 
> slowness of main RAM, which isn't currently solvable at a reasonable 
> cost. We use caches to mitigate it.
> 
> Out-of-order execution addresses this problem by tracking the data 
> dependencies on memory and registers in real time and executing 
> instructions when their data is available....

The Itanium compiler optimizer just doesn't (and can't) know enough 
about the system memory state, yes. Among other (no pun intended) 
issues.

The attempt to address that included providing run-time feedback into 
the executables; providing post-link, post-execution tuning. (Caliper / 
Atom / OM / etc.)

https://www.cs.tufts.edu/comp/150PAT/tools/caliper/wiess-rev-4.pdf

This Alpha versus IA-64 Itanium paper from 1999 describes the issues 
with Itanium quite well too, for those interested:

https://web.archive.org/web/20010611202933/http://www.compaq.com/hpc/ref/ref_alpha_ia64.doc 



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