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Date: Thu, 8 Oct 2015 10:00:53 +0200
From: Massimo Del Zotto <massimodz8@...il.com>
To: Solar Designer <solar@...nwall.com>
Cc: discussions@...sword-hashing.net
Subject: Re: [PHC] yescrypt on GPU

I have been shuffling a bit through the CL forum and I'm quite sure I
missed something.
Anyway, I have reached that conclusion (about instruction latencies being
basically 0) reading this/posting:
https://community.amd.com/thread/170261

I also read a couple of times the link you provided.

Quoting, for those who don't feel like following the link:

RealHet, 2015 Apr 18
Hi,

This example is absolutely correct, it can harvest the peak performance of
the gpu, yet it uses the same register for everything:
v_mad_f32 v0, v0, v0, v0
v_mad_f32 v0, v0, v0, v0
v_mad_f32 v0, v0, v0, v0
v_mad_f32 v0, v0, v0, v0


>From the discussion you linked:

gautam.himanshu, 2012 May 11
Actually only a quad-wavefront(16 work-items) can run parallely on a
Compute Unit. All these extra work-items are introduced, so hide the
latencies of memory i/o for data as well as **code**.


It seems GCN also switches to a different wavefront at every instruction
anyway.

I honestly haven't fully understood the latency involved by using LDS but
given the little control the CL compiler gives, I don't think I have the
tools to investigate it. I would have loved doing some ASM but I'm positive
I cannot afford it.
It's sure LDS latency isn't as instruction latency and is deterministic
only when no bank collisions occur(ed). I'm inclined to believe we're
talking about an handful of ticks (which are effectively clocks, from the
point of view of IP increment).

I agree with your conclusion of requiring occupancy at least 4/10.

Plus: I've seen S_NOP being emitted very sparingly. It burns 1 clock.
_expand32W has a few, one of them looks like

 Address;Opcode;Operands;Cycles;Instruction Type;Hex
 0x000190;V_ADD_I32;v2 vcc s6 v2;4;Vector Arithmetics;4A040406
 0x000194;V_LSHLREV_B32;v3 2 v1;4;Vector Arithmetics;34060282
 0x000198;S_LOAD_DWORDX4;s[12:15] s[2:3] 0x70;Varies;Scalar Memory
Read;C0860370
 0x00019C;S_MOV_B64;s[10:11] exec;4;Scalar Arithmetics;BE8A047E
 0x0001A0;V_MOV_B32;v8 0;4;Vector Arithmetics;7E100280
 0x0001A4;S_NOP;0x0000;1;Flow Control;BF800000
 label_006A:
 0x0001A8;V_ADD_I32;v9 vcc v2 v8;4;Vector Arithmetics;4A121102

My CubeHash kernel has two. They look like:

0x00004C;V_LSHLREV_B32;v3 2 v3;4;Vector Arithmetics;34060682
0x000050;V_LSHLREV_B32;v4 2 v4;4;Vector Arithmetics;34080882
0x000054;V_LSHLREV_B32;v5 2 v5;4;Vector Arithmetics;340A0A82
0x000058;V_LSHLREV_B32;v6 2 v6;4;Vector Arithmetics;340C0C82
0x00005C;V_ADD_I32;v3 vcc s12 v3;4;Vector Arithmetics;4A06060C
0x000060;V_ADD_I32;v4 vcc s12 v4;4;Vector Arithmetics;4A08080C
0x000064;V_ADD_I32;v5 vcc s12 v5;4;Vector Arithmetics;4A0A0A0C
0x000068;V_ADD_I32;v6 vcc s12 v6;4;Vector Arithmetics;4A0C0C0C
0x00006C;S_LOAD_DWORDX4;s[16:19] s[2:3] 0x50;Varies;Scalar Memory
Read;C0880350
0x000070;V_LSHLREV_B32;v7 2 v0;4;Vector Arithmetics;340E0082
0x000074;S_WAITCNT;lgkmcnt(0);Varies;Flow Control;BF8C007F
0x000078;V_ADD_I32;v7 vcc s0 v7;4;Vector Arithmetics;4A0E0E00
0x00007C;S_NOP;0x0000;1;Flow Control;BF800000
0x000080;TBUFFER_LOAD_FORMAT_X;v3 v3 s[4:7];0 offen
format:[BUF_DATA_FORMAT_32BUF_NUM_FORMAT_FLOAT];Varies;Vector Memory
Read;EBA01000 80010303

...

0x000478;V_ADD_I32;v20 vcc v20 v4;4;Vector Arithmetics;4A280914
0x00047C;DS_WRITE2_B32;v28 v19 v20 offset0:224
offset1:192;Varies;LDS;D838C0E0 0014131C
0x000484;S_NOP;0x0000;1;Flow Control;BF800000
0x000488;DS_READ_B32;v29 v18;Varies;LDS;D8D80000 1D000012


_expand32W has occupancy 4/10, cubehash 6/10 (oddly).

So I'm inclined to believe instruction latency is "not worry" in many
(most?) cases. It is clear it can effectively be considered 0 under some
circumstances.
My 10'000 ft decision about the rest is to investigate further at some
point in the future. I don't think I will be constrained in further
revisions.


Massimo

2015-10-07 10:42 GMT+02:00 Solar Designer <solar@...nwall.com>:

> On Wed, Oct 07, 2015 at 09:29:09AM +0200, Massimo Del Zotto wrote:
> > I have been told on the AMD CL forum (by 'realhet', who appears very
> > proficient and up to date with GCN ASM) that GCN has no instruction
> > latencies (i.e. it can consume a result in the instruction immediately
> > following), probably a nice implication of the instructions being
> processed
> > in 4-clock 'ticks'.
>
> realhet wrote an assembler/Pascal/IDE targeting GCN, so got to be very
> proficient with GCN.  Can you post an URL for that specific forum posting?
>
> In the following old thread, it was said that while 4 wavefronts could
> be sufficient for ALU bound problems, more are needed for memory bound
> problems (not surprisingly), and I think this might include local memory
> (but I don't really know):
>
> https://community.amd.com/thread/159171
>
> (In that old thread, I think comments by jeff_golds are authoritative,
> while realhet was just learning then-new GCN at the time.)
>
> Anyway, I think this means that if we put the S-boxes into local memory,
> we do incur at least those 4 cycle minimum latencies.
>
> Alexander
>

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