Copyright 1997, 1999-2002 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 either: * 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. or * the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version. or both in parallel, as here. 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 General Public License for more details. You should have received copies of the GNU General Public License and the GNU Lesser General Public License along with the GNU MP Library. If not, see https://www.gnu.org/licenses/. This directory contains mpn functions for 64-bit V9 SPARC RELEVANT OPTIMIZATION ISSUES Notation: IANY = shift/add/sub/logical/sethi IADDLOG = add/sub/logical/sethi MEM = ld*/st* FA = fadd*/fsub*/f*to*/fmov* FM = fmul* UltraSPARC can issue four instructions per cycle, with these restrictions: * Two IANY instructions, but only one of these may be a shift. If there is a shift and an IANY instruction, the shift must precede the IANY instruction. * One FA. * One FM. * One branch. * One MEM. * IANY/IADDLOG/MEM must be insn 1, 2, or 3 in an issue bundle. Taken branches should not be in slot 4, since that makes the delay insn come from separate bundle. * If two IANY/IADDLOG instructions are to be executed in the same cycle and one of these is setting the condition codes, that instruction must be the second one. To summarize, ignoring branches, these are the bundles that can reach the peak execution speed: insn1 iany iany mem iany iany mem iany iany mem insn2 iaddlog mem iany mem iaddlog iany mem iaddlog iany insn3 mem iaddlog iaddlog fa fa fa fm fm fm insn4 fa/fm fa/fm fa/fm fm fm fm fa fa fa The 64-bit integer multiply instruction mulx takes from 5 cycles to 35 cycles, depending on the position of the most significant bit of the first source operand. When used for 32x32->64 multiplication, it needs 20 cycles. Furthermore, it stalls the processor while executing. We stay away from that instruction, and instead use floating-point operations. Floating-point add and multiply units are fully pipelined. The latency for UltraSPARC-1/2 is 3 cycles and for UltraSPARC-3 it is 4 cycles. Integer conditional move instructions cannot dual-issue with other integer instructions. No conditional move can issue 1-5 cycles after a load. (This might have been fixed for UltraSPARC-3.) The UltraSPARC-3 pipeline is very simular to the one of UltraSPARC-1/2 , but is somewhat slower. Branches execute slower, and there may be other new stalls. But integer multiply doesn't stall the entire CPU and also has a much lower latency. But it's still not pipelined, and thus useless for our needs. STATUS * mpn_lshift, mpn_rshift: The current code runs at 2.0 cycles/limb on UltraSPARC-1/2 and 2.65 on UltraSPARC-3. For UltraSPARC-1/2, the IEU0 functional unit is saturated with shifts. * mpn_add_n, mpn_sub_n: The current code runs at 4 cycles/limb on UltraSPARC-1/2 and 4.5 cycles/limb on UltraSPARC-3. The 4 instruction recurrency is the speed limiter. * mpn_addmul_1: The current code runs at 14 cycles/limb asymptotically on UltraSPARC-1/2 and 17.5 cycles/limb on UltraSPARC-3. On UltraSPARC-1/2, the code sustains 4 instructions/cycle. It might be possible to invent a better way of summing the intermediate 49-bit operands, but it is unlikely that it will save enough instructions to save an entire cycle. The load-use of the u operand is not enough scheduled for good L2 cache performance. The UltraSPARC-1/2 L1 cache is direct mapped, and since we use temporary stack slots that will conflict with the u and r operands, we miss to L2 very often. The load-use of the std/ldx pairs via the stack are perhaps over-scheduled. It would be possible to save two instructions: (1) The mov could be avoided if the std/ldx were less scheduled. (2) The ldx of the r operand could be split into two ld instructions, saving the shifts/masks. It should be possible to reach 14 cycles/limb for UltraSPARC-3 if the fp operations where rescheduled for this processor's 4-cycle latency. * mpn_mul_1: The current code is a straightforward edit of the mpn_addmul_1 code. It would be possible to shave one or two cycles from it, with some labour. * mpn_submul_1: Simpleminded code just calling mpn_mul_1 + mpn_sub_n. This means that it runs at 18 cycles/limb on UltraSPARC-1/2 and 23 cycles/limb on UltraSPARC-3. It would be possible to either match the mpn_addmul_1 performance, or in the worst case use one more instruction group. * US1/US2 cache conflict resolving. The direct mapped L1 date cache of US1/US2 is a problem for mul_1, addmul_1 (and a prospective submul_1). We should allocate a larger cache area, and put the stack temp area in a place that doesn't cause cache conflicts.