Re: Itanium support is back in GCC 15

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

In article <vpfoc7$2o5$1@panix2.panix.com>, kludge@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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