Re: [PATCH 00/12] x86/crypto: Fix RBP usage in several crypto .S files

[Ingo Molnar]

* Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:

> I'm still looking at the other one (sha512-avx2), but so far I haven't
> found a way to speed it back up.

Here's a couple of very quick observations with possible optimizations:

AFAICS the main effect of the RBP fixes is the introduction of a memory load into 
the critical path, into the body unrolled loop:

+       mov frame_TBL(%rsp), TBL
        vpaddq  (TBL), Y_0, XFER
        vmovdqa XFER, frame_XFER(%rsp)
        FOUR_ROUNDS_AND_SCHED

Both 'TLB' and 'T1' are mapped to R12, which is why TBL has to be spilled to be 
reloaded from the stack.

1)

Note how R12 is used immediately, right in the next instruction:

        vpaddq  (TBL), Y_0, XFER

I.e. the RBP fixes lengthen the program order data dependencies - that's a new 
constraint and a few extra cycles per loop iteration if the workload is 
address-generator bandwidth limited on that.

A simple way to ease that constraint would be to move the 'TLB' load up into the 
loop, body, to the point where 'T1' is used for the last time - which is:


        mov     a, T1           # T1 = a                                # MAJB
        and     c, T1           # T1 = a&c                              # MAJB

        add     y0, y2          # y2 = S1 + CH                          # --
        or      T1, y3          # y3 = MAJ = (a|c)&b)|(a&c)             # MAJ

+       mov frame_TBL(%rsp), TBL

        add     y1, h           # h = k + w + h + S0                    # --

        add     y2, d           # d = k + w + h + d + S1 + CH = d + t1  # --

        add     y2, h           # h = k + w + h + S0 + S1 + CH = t1 + S0# --
        add     y3, h           # h = t1 + S0 + MAJ                     # --

Note how this moves up the 'TLB' reload by 4 instructions.

2)

If this does not get back performance, then maybe another reason is that it's 
cache access latency limited, in which case a more involved optimization would be 
to look at the register patterns and usages:


			first-use	last-use		use-length
	a:		#10		#29			20
	b:		#24		#24			 1
	c:		#14		#30			17
	d:		#23		#34			12
	e:		#11		#20			10
	f:		#15		#15			 1
	g:		#18		#27			10
	h:		#13		#36			24

	y0:		#11		#31			21
	y1:		#12		#33			22
	y2:		#15		#35			21
	y3:		#10		#36			27

	T1:		#16		#32			17

The 'first-use' colums shows the number of the instruction within the loop body 
that the register gets used - with '#1' denoting the first instruction ad #36 the 
last instruction, the 'last-use' column is showing the last instruction, and the 
'use-length' colum shows the 'window' in which a register is used.

What we want are the registers that are used the most tightly, i.e. these two:

	b:		#24		#24			 1
	f:		#15		#15			 1

Of these two 'f' is the best one, because it has an earlier use and longer 
cooldown.

If alias 'TBL' with 'f' then we could reload 'TLB' for the next iteration very 
early on:

        mov     f, y2           # y2 = f                                # CH
+       mov frame_TBL(%rsp), TBL
        rorx    $34, a, T1      # T1 = a >> 34                          # S0B

And there will be 21 instructions that don't depend on TLB after this, plenty of 
time for the load to be generated and propagated.

NOTE: my pseudo-patch is naive, due to the complication caused by the RotateState 
macro name rotation. It's still fundamentally possible I believe, it's just that 
'TBL' has to be rotated too, together with the other varibles.

3)

If even this does not help, because the workload is ucode-cache limited, and the 
extra reloads pushed the critical path just beyond some cache limit, then another 
experiment to try would be to roll _back_ the loop some more: instead of 4x 
FOUR_ROUNDS_AND_SCHED unrolled loops, try just having 2.

The CPU should still be smart enough with 2x interleaving of the loop body, and 
the extra branches should be relatively small and we could get back some 
performance.

In theory ...

4)

If the workload is fundamentally cache-port bandwidth limited, then the extra 
loads from memory to reload 'TLB' take away valuable bandwidth. There's no easy 
fix for that, but to find an unused register.

Here's the (initial, pre-rotation) integer register mappings:

	a:	RAX
	b:	RBX
	c:	RCX
	d:	R8
	e:	RDX
	f:	R9
	g:	R10
	h:	R11

	y0:	R13
	y1:	R14
	y2:	R15
	y3:	RSI

	T1:	R12

	TLB:	R12 # aliased to T1

Look what's missing: I don't see RDI being used in the loop.

RDI is allocated to 'CTX', but that's only used in higher level glue code, it does 
not appear to be used in the inner loops (explicitly at least).

So if this observation of mine is true we could go back to the old code for the 
hotpath, but use RDI for TBL and not reload it in the hotpath.

Thanks,

	Ingo