phc-discussions - Re: [PHC] Compute time hardness

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Date: Sun, 3 May 2015 18:21:12 +0300
From: Solar Designer <solar@...nwall.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] Compute time hardness

On Fri, Apr 03, 2015 at 03:13:06PM +0300, Solar Designer wrote:
> Here's another relevant detail I recalled:
> 
> Pentium 4 (some or all of them? not sure) had double-pumped ALU, where
> it could perform ADDs at double the clock rate (so up to 7.6 GHz, at
> stock clocks).
> 
> http://www.anandtech.com/show/1611/7
> http://forums.anandtech.com/showthread.php?t=603812
> https://news.ycombinator.com/item?id=8255157
> http://en.wikipedia.org/wiki/NetBurst_(microarchitecture)#Rapid_Execution_Engine
> 
> Apparently, this could only execute two dependent ADDs in a cycle if
> they are 16-bit each.

I've just tested.  32-bit dependent ADDs also execute at 2 per cycle at
least on:

vendor_id       : GenuineIntel
cpu family      : 15
model           : 2
model name      : Intel(R) Pentium(R) 4 CPU 2.80GHz
stepping        : 7

I tried sequences of 1000 "addl %eax,%eax" and "addl %esp,%eax" and
"addl %edx,%eax; addl %eax,%edx" and even similar things with SUB and
bitwise ops.  All give me 2 instructions per cycle, despite of the data
dependencies between every instruction.

In fact, my CPU clock frequency detection program that correctly works
on all other x86 CPUs I tested it on (ranging from original Pentium to
Haswell), prints double the clock rate on Pentium 4 ("5569 MHz" for this
one).  I guess this can be said to be correct as well.

So it's a reality that ADDs (and indeed bitwise ops) are implementable
at least twice faster than they are implemented in current CPUs, even
including all of the overhead that occurs in CPUs as opposed to ASICs.

Repeated dependent MULs run at 15 to 18 cycles/insn on this same P4 CPU
above.  So it has latency-optimized simple ALUs, but not MUL.  Luckily,
P4 is of relatively little relevance for running yescrypt or possible
other MUL latency hardened PHC schemes.  Also, its typical DDR memory is
slow enough by modern standards that these 15 to 18 cycle latencies
probably won't be the bottleneck, if everything is tuned for MUL's 3 to
5 cycle latency along with modern RAM bandwidth.

Alexander