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-Copyright 2000, 2001, 2002, 2003, 2004, 2005 Free Software Foundation, Inc. |
- |
-This file is part of the GNU MP Library. |
- |
-The GNU MP Library is free software; you can redistribute it and/or modify |
-it under the terms of the GNU Lesser General Public License as published by |
-the Free Software Foundation; either version 3 of the License, or (at your |
-option) any later version. |
- |
-The GNU MP Library is distributed in the hope that it will be useful, but |
-WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY |
-or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public |
-License for more details. |
- |
-You should have received a copy of the GNU Lesser General Public License |
-along with the GNU MP Library. If not, see http://www.gnu.org/licenses/. |
- |
- |
- |
- 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_1 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 tests/devel/try. 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 |
- |
- |
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