[p_l]

The critical difference is that EPIC (the architecture model of Itanium) essentially exposed CPU pipelines naked to the code - so you didn't just have to reorder instructions as optimizers do today, you also had to figure out changes that experience so far suggests is doable either in hw with runtime-only data, or in very tight numerical code. This includes compiler taking the place of branch predictor as well as OOOE scheduling, as well as no on-cpu instruction reordering or out of order retirement, and IIRC a branch mispredict was quite costly.

More over, EPIC pretty much meant thar you couldn't apply similar chip-level IPC improvements as you could elsewhere, at least originally.

[cogman10]

I'm not sure that branch prediction would need to go to the compiler, but definitely agree it'd likely subsume the OOOE scheduling (at very least, it'd be less effective).

That, though, seems like it might make for a good power/performance tradeoff. Those circuits aren't free. We just didn't get to the point where compilers were doing a good job of that OOOE reordering (not until after EPIC died).

The real reason, though, that itanium died (IMO) is most businesses insisted on emulating their x86 code at a 70% performance cost. So costly that it seems like intel/hp spent most of their hardware engineering budget making that portion fast enough.

[p_l]

The x86 emulator built into Itanium 1 was very bad, yes, but it didn't matter that much outside of workstation use. HP build Itanium 2 without it, and provided software emulators for x86 and HP-PA that worked apparently "well enough".

The real deal breaker was Itanium being ridiculously expensive and quickly destroying any possibility of increased market by pricing itself out of it - and even in the markets that had the money, it was considered overpriced (nicest thing I heard about Itanium was "overpriced DSP masquerading as general purpose CPU"). I remember reading intel's published roadmaps before news about amd64 landed - We would be running 32bit x86 much longer under it, with Itanium being kept at extra premium prices.

Even customers that had Itanium as the only upgrade path available - thanks to HP - found the performance so bad - on natively compiled code! - they effectively forced HP to produce Alpha till Itanium was pretty much confirmed dead and the customers migrated out of HP vendor-locked stack (at one of the largest mobile telcos in Poland we migrated from Alpha to IBM POWER, many OpenVMS customers kept buying/hoarding Wildfire and Marvel architecture servers).