xref: /src/external/lgpl3/gmp/dist/mpn/ia64

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                      IA-64 MPN SUBROUTINES


This directory contains mpn functions for the IA-64 architecture.


CODE ORGANIZATION

	mpn/ia64          itanium-2, and generic ia64

The code here has been optimized primarily for Itanium 2.  Very few Itanium 1
chips were ever sold, and Itanium 2 is more powerful, so the latter is what
we concentrate on.



CHIP NOTES

The IA-64 ISA keeps instructions three and three in 128 bit bundles.
Programmers/compilers need to put explicit breaks `;;' when there are WAW or
RAW dependencies, with some notable exceptions.  Such "breaks" are typically
at the end of a bundle, but can be put between operations within some bundle
types too.

The Itanium 1 and Itanium 2 implementations can under ideal conditions
execute two bundles per cycle.  The Itanium 1 allows 4 of these instructions
to do integer operations, while the Itanium 2 allows all 6 to be integer
operations.

Taken cloop branches seem to insert a bubble into the pipeline most of the
time on Itanium 1.

Loads to the fp registers bypass the L1 cache and thus get extremely long
latencies, 9 cycles on the Itanium 1 and 6 cycles on the Itanium 2.

The software pipeline stuff using br.ctop instruction causes delays, since
many issue slots are taken up by instructions with zero predicates, and
since many extra instructions are needed to set things up.  These features
are clearly designed for code density, not speed.

Misc pipeline limitations (Itanium 1):
* The getf.sig instruction can only execute in M0.
* At most four integer instructions/cycle.
* Nops take up resources like any plain instructions.

Misc pipeline limitations (Itanium 2):
* The getf.sig instruction can only execute in M0.
* Nops take up resources like any plain instructions.


ASSEMBLY SYNTAX

.align pads with nops in a text segment, but gas 2.14 and earlier
incorrectly byte-swaps its nop bundle in big endian mode (eg. hpux), making
it come out as break instructions.  We use the ALIGN() macro in
mpn/ia64/ia64-defs.m4 when it might be executed across.  That macro
suppresses any .align if the problem is detected by configure.  Lack of
alignment might hurt performance but will at least be correct.

foo:: to create a global symbol is not accepted by gas.  Use separate
".global foo" and "foo:" instead.

.global is the standard global directive.  gas accepts .globl, but hpux "as"
doesn't.

.proc / .endp generates the appropriate .type and .size information for ELF,
so the latter directives don't need to be given explicitly.

.pred.rel "mutex"... is standard for annotating predicate register
relationships.  gas also accepts .pred.rel.mutex, but hpux "as" doesn't.

.pred directives can't be put on a line with a label, like
".Lfoo: .pred ...", the HP assembler on HP-UX 11.23 rejects that.
gas is happy with it, and past versions of HP had seemed ok.

// is the standard comment sequence, but we prefer "C" since it inhibits m4
macro expansion.  See comments in ia64-defs.m4.


REGISTER USAGE

Special:
   r0: constant 0
   r1: global pointer (gp)
   r8: return value
   r12: stack pointer (sp)
   r13: thread pointer (tp)
Caller-saves: r8-r11 r14-r31 f6-f15 f32-f127
Caller-saves but rotating: r32-


================================================================
mpn_add_n, mpn_sub_n:

The current code runs at 1.25 c/l on Itanium 2.

================================================================
mpn_mul_1:

The current code runs at 2 c/l on Itanium 2.

Using a blocked approach, working off of 4 separate places in the operands,
one could make use of the xma accumulation, and approach 1 c/l.

	ldf8 [up]
	xma.l
	xma.hu
	stf8  [wrp]

================================================================
mpn_addmul_1:

The current code runs at 2 c/l on Itanium 2.

It seems possible to use a blocked approach, as with mpn_mul_1.  We should
read rp[] to integer registers, allowing for just one getf.sig per cycle.

	ld8  [rp]
	ldf8 [up]
	xma.l
	xma.hu
	getf.sig
	add+add+cmp+cmp
	st8  [wrp]

These 10 instructions can be scheduled to approach 1.667 cycles, and with
the 4 cycle latency of xma, this means we need at least 3 blocks.  Using
ldfp8 we could approach 1.583 c/l.

================================================================
mpn_submul_1:

The current code runs at 2.25 c/l on Itanium 2.  Getting to 2 c/l requires
ldfp8 with all alignment headache that implies.

================================================================
mpn_addmul_N

For best speed, we need to give up using mpn_addmul_2 as the main multiply
building block, and instead take multiple v limbs per loop.  For the Itanium
1, we need to take about 8 limbs at a time for full speed.  For the Itanium
2, something like mpn_addmul_4 should be enough.

The add+cmp+cmp+add we use on the other codes is optimal for shortening
recurrencies (1 cycle) but the sequence takes up 4 execution slots.  When
recurrency depth is not critical, a more standard 3-cycle add+cmp+add is
better.

/* First load the 8 values from v */
	ldfp8		v0, v1 = [r35], 16;;
	ldfp8		v2, v3 = [r35], 16;;
	ldfp8		v4, v5 = [r35], 16;;
	ldfp8		v6, v7 = [r35], 16;;

/* In the inner loop, get a new U limb and store a result limb. */
	mov		lc = un
Loop:	ldf8		u0 = [r33], 8
	ld8		r0 = [r32]
	xma.l		lp0 = v0, u0, hp0
	xma.hu		hp0 = v0, u0, hp0
	xma.l		lp1 = v1, u0, hp1
	xma.hu		hp1 = v1, u0, hp1
	xma.l		lp2 = v2, u0, hp2
	xma.hu		hp2 = v2, u0, hp2
	xma.l		lp3 = v3, u0, hp3
	xma.hu		hp3 = v3, u0, hp3
	xma.l		lp4 = v4, u0, hp4
	xma.hu		hp4 = v4, u0, hp4
	xma.l		lp5 = v5, u0, hp5
	xma.hu		hp5 = v5, u0, hp5
	xma.l		lp6 = v6, u0, hp6
	xma.hu		hp6 = v6, u0, hp6
	xma.l		lp7 = v7, u0, hp7
	xma.hu		hp7 = v7, u0, hp7
	getf.sig	l0 = lp0
	getf.sig	l1 = lp1
	getf.sig	l2 = lp2
	getf.sig	l3 = lp3
	getf.sig	l4 = lp4
	getf.sig	l5 = lp5
	getf.sig	l6 = lp6
	add+cmp+add	xx, l0, r0
	add+cmp+add	acc0, acc1, l1
	add+cmp+add	acc1, acc2, l2
	add+cmp+add	acc2, acc3, l3
	add+cmp+add	acc3, acc4, l4
	add+cmp+add	acc4, acc5, l5
	add+cmp+add	acc5, acc6, l6
	getf.sig	acc6 = lp7
	st8		[r32] = xx, 8
	br.cloop Loop

	49 insn at max 6 insn/cycle:		8.167 cycles/limb8
	11 memops at max 2 memops/cycle:	5.5 cycles/limb8
	16 fpops at max 2 fpops/cycle:		8 cycles/limb8
	21 intops at max 4 intops/cycle:	5.25 cycles/limb8
	11+21 memops+intops at max 4/cycle	8 cycles/limb8

================================================================
mpn_lshift, mpn_rshift

The current code runs at 1 cycle/limb on Itanium 2.

Using 63 separate loops, we could use the double-word shrp instruction.
That instruction has a plain single-cycle latency.  We need 63 loops since
this instruction only accept immediate count.  That would lead to a somewhat
silly code size, but the speed would be 0.75 c/l on Itanium 2 (by using shrp
each cycle plus shl/shr going down I1 for a further limb every second
cycle).

================================================================
mpn_copyi, mpn_copyd

The current code runs at 0.5 c/l on Itanium 2.  But that is just for L1
cache hit.  The 4-way unrolled loop takes just 2 cycles, and thus load-use
scheduling isn't great.  It might be best to actually use modulo scheduled
loops, since that will allow us to do better load-use scheduling without too
much unrolling.

Depending on size or operand alignment, we get 1 c/l or 0.5 c/l on Itanium
2, according to tune/speed.  Cache bank conflicts?



REFERENCES

Intel Itanium Architecture Software Developer's Manual, volumes 1 to 3,
Intel document 245317-004, 245318-004, 245319-004 October 2002.  Volume 1
includes an Itanium optimization guide.

Intel Itanium Processor-specific Application Binary Interface (ABI), Intel
document 245370-003, May 2001.  Describes C type sizes, dynamic linking,
etc.

Intel Itanium Architecture Assembly Language Reference Guide, Intel document
248801-004, 2000-2002.  Describes assembly instruction syntax and other
directives.

Itanium Software Conventions and Runtime Architecture Guide, Intel document
245358-003, May 2001.  Describes calling conventions, including stack
unwinding requirements.

Intel Itanium Processor Reference Manual for Software Optimization, Intel
document 245473-003, November 2001.

Intel Itanium-2 Processor Reference Manual for Software Development and
Optimization, Intel document 251110-003, May 2004.

All the above documents can be found online at

    http://developer.intel.com/design/itanium/manuals.htm