8 wide vs 6 wide

By: David Kanter (dkanter.delete@this.realworldtech.com), November 7, 2016 8:43 pm
Room: Moderated Discussions
Wilco (Wilco.Dijkstra.delete@this.ntlworld.com) on November 7, 2016 2:45 am wrote:
> juanrga (noemail.delete@this.juanrga.com) on November 6, 2016 4:06 pm wrote:
> > Wilco (Wilco.Dijkstra.delete@this.ntlworld.com) on November 6, 2016 4:15 am wrote:
> > > juanrga (noemail.delete@this.juanrga.com) on November 4, 2016 4:39 pm wrote:
> > > > Wilco (Wilco.Dijkstra.delete@this.ntlworld.com) on November 4, 2016 1:12 pm wrote:
> > > > > anon (spam.delete.delete@this.this.spam.com) on November 4, 2016 6:12 am wrote:
> > > > > > Yes, but I still don't see any reason why that means the A10 is 6 wide.
> > > > > >
> > > > > > If Sandy/Ivy Bridge is 6 wide then that doesn't mean Haswell/Broadwell is 6 wide.
> > > > >
> > > > > Neither are 6-wide using the standard definition.
> > > >
> > > > If you bother to read one of the resources given (the first link
> > > > I gave), you will learn that there is no "standard" definition:
> > >
> > > Actually that same link does give a standard definition:
> > >
> > >
The number of instructions able to be issued, executed or completed per cycle is called a processor's
> > > width. Note that the issue width is less than the number of functional units – this is typical.

> >
> > Now continue reading the reference until you get to the point where he discusses if Haswell
> > would be considered 4-wide, 5-wide, or 8-wide, why it depends on what definition of "wide"
> > you use,
>
> No the definition by itself is clear. Instructions are instructions, there
> is no room for argument. No Intel core can decode/execute 8 instructions
> per cycle. Apple A7 can do 6, Haswell can do 4, it's as simple as that.

I believe that Intel cores can actually do 5 instructions/clock with the macro-op fusion.

> > and why he choses 8-wide (as myself did, as the other references given also did).
>
> The reason some people choose 8-wide is to pretend Intel's cores are wider than other
> CPUs. When you look internally both Apple A7 and Cortex-A57 are wider than Haswell.

Even width itself is only relevant as an approximation for IPC. Having an 8-wide CPU with one load/store unit is no good for general purpose code (would work for some DSP, maybe).

Also, not all instructions are equal. In particular, on vector codes, AVX could give x86 an advantage. Similarly, on spill heavy code, ARMv7 may have an advantage using LDM/STM.

> In fact Apple A7 is wider than Haswell in every regard. So anyone claiming Haswell is 8 wide
> but A7 is only 6 wide is simply lying because by the same measure A7 is actually 9 wide.

It certainly appears that the A7 has similar IPC at low frequencies.

> > > So it's the number of instructions that one can process, not micro-ops
> > > (as those vary significantly with the microarchitecture).
> > >
> >
> > The issue is that what you call "instructions" is not what is reordered, issued,
> > executed, tracked, and retired in the metal of a modern chip as Haswell, or
> > Cyclone, or A72, or Hurricane, or Zen, or Vulcan, or Kaby Lake, or...
>
> There is an almost 1:1 correspondence between instructions
> and micro-ops, so yes instructions are what matters.

The median instruction decodes to 1 uop. But when you run into nasty ones, they are worth noting. I would always carefully consider both uops and instructions.

In fact with Intel's uop cache, it's not even clear that you measure front-end width in instructions anymore...it could operate solely in the uop domain for extended periods.

David
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              Broadwell includes LLC, just for comparisionjuanrga2016/10/23 06:09 AM
                Broadwell includes LLC, just for comparisionanon2016/10/23 07:25 AM
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                        Even without the point you have invalid comparisonHeikki Kultala2016/10/28 09:03 AM
                          8 wide vs 6 widejuanrga2016/10/29 10:41 PM
                            8 wide vs 6 wideWilco2016/10/30 05:00 AM
                              8 wide vs 6 wideDoug S2016/10/30 12:20 PM
                                8 wide vs 6 wideWilco2016/10/30 01:12 PM
                                8 wide vs 6 widejuanrga2016/10/30 02:56 PM
                                  8 wide vs 6 wideTravis2016/10/30 07:13 PM
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                                                8 wide vs 6 wideWilco2016/11/04 01:12 PM
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                                                    8 wide vs 6 widejuanrga2016/11/04 05:34 PM
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                                                              8 wide vs 6 widejuanrga2016/11/20 06:56 AM
                                                                8 wide vs 6 wideWilco2016/11/21 05:54 PM
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                    Broadwell includes LLC, just for comparisionanon2016/10/26 03:10 AM
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