1 # SPDX-License-Identifier: LGPL-2.1-or-later
2 # See Notices.txt for copyright information
5 Copyright (C) 2020 Luke Kenneth Casson Leighton <lkcl@lkcl.net>
6 Copyright (C) 2020 Michael Nolan <mtnolan2640@gmail.com>
8 dynamically partitionable shifter. Unlike part_shift_scalar, both
9 operands can be partitioned
13 * http://libre-riscv.org/3d_gpu/architecture/dynamic_simd/shift/
14 * http://bugs.libre-riscv.org/show_bug.cgi?id=173
16 from nmigen
import Signal
, Module
, Elaboratable
, Cat
, Mux
, C
17 from ieee754
.part_mul_add
.partpoints
import PartitionPoints
21 class PartitionedDynamicShift(Elaboratable
):
22 def __init__(self
, width
, partition_points
):
24 self
.partition_points
= PartitionPoints(partition_points
)
26 self
.a
= Signal(width
, reset_less
=True)
27 self
.b
= Signal(width
, reset_less
=True)
28 self
.output
= Signal(width
, reset_less
=True)
30 def elaborate(self
, platform
):
34 pwid
= self
.partition_points
.get_max_partition_count(width
)-1
35 gates
= Signal(pwid
, reset_less
=True)
36 comb
+= gates
.eq(self
.partition_points
.as_sig())
39 keys
= list(self
.partition_points
.keys()) + [self
.width
]
42 # break out both the input and output into partition-stratified blocks
48 for i
in range(len(keys
)):
50 widths
.append(width
- start
)
51 a_intervals
.append(self
.a
[start
:end
])
52 b_intervals
.append(self
.b
[start
:end
])
53 intervals
.append([start
,end
])
56 min_bits
= math
.ceil(math
.log2(intervals
[0][1] - intervals
[0][0]))
57 max_bits
= math
.ceil(math
.log2(width
))
59 # shifts are normally done as (e.g. for 32 bit) result = a & (b&0b11111)
60 # truncating the b input. however here of course the size of the
61 # partition varies dynamically.
63 for i
in range(len(b_intervals
)):
64 mask
= Signal(b_intervals
[i
].shape(), name
="shift_mask%d" % i
,
66 bits
= Signal(pwid
-i
, name
="bits%d" % i
, reset_less
=True)
68 for idx
, j
in enumerate(range(i
, pwid
)):
70 bl
.append((~gates
[j
]) & bits
[idx
-1])
73 # XXX ARGH, really annoying: simulation bug, can't use Cat(*bl).
74 for j
in range(bits
.shape()[0]):
75 comb
+= bits
[j
].eq(bl
[j
])
76 comb
+= mask
.eq(Cat((1 << min_bits
)-1, bits
)
77 & ((1 << max_bits
)-1))
78 shifter_masks
.append(mask
)
82 # Instead of generating the matrix described in the wiki, I
83 # instead calculate the shift amounts for each partition, then
84 # calculate the partial results of each partition << shift
85 # amount. On the wiki, the following table is given for output #3:
87 # 0 0 0 | a0b0[31:24] | a1b0[23:16] | a2b0[15:8] | a3b0[7:0]
88 # 0 0 1 | a0b0[31:24] | a1b1[23:16] | a2b1[15:8] | a3b1[7:0]
89 # 0 1 0 | a0b0[31:24] | a1b0[23:16] | a2b2[15:8] | a3b2[7:0]
90 # 0 1 1 | a0b0[31:24] | a1b1[23:16] | a2b2[15:8] | a3b2[7:0]
91 # 1 0 0 | a0b0[31:24] | a1b0[23:16] | a2b0[15:8] | a3b3[7:0]
92 # 1 0 1 | a0b0[31:24] | a1b1[23:16] | a2b1[15:8] | a3b3[7:0]
93 # 1 1 0 | a0b0[31:24] | a1b0[23:16] | a2b2[15:8] | a3b3[7:0]
94 # 1 1 1 | a0b0[31:24] | a1b1[23:16] | a2b2[15:8] | a3b3[7:0]
96 # Each output for o3 is given by a3bx and the partial results
97 # for o2 (namely, a2bx, a1bx, and a0b0). If I calculate the
98 # partial results [a0b0, a1bx, a2bx, a3bx], I can use just
99 # those partial results to calculate a0, a1, a2, and a3
100 element
= b_intervals
[0] & shifter_masks
[0]
102 partial
= Signal(width
, name
="partial0", reset_less
=True)
103 comb
+= partial
.eq(a_intervals
[0] << element
)
104 partial_results
.append(partial
)
105 for i
in range(1, len(keys
)):
106 reswid
= width
- intervals
[i
][0]
107 shiftbits
= math
.ceil(math
.log2(reswid
+1))+1 # hmmm...
108 print ("partial", reswid
, width
, intervals
[i
], shiftbits
)
110 masked
= Signal(b_intervals
[i
].shape(), name
="masked%d" % i
,
112 comb
+= masked
.eq(b_intervals
[i
] & shifter_masks
[i
])
113 element
= Mux(gates
[i
-1], masked
, element
)
114 elmux
= Signal(b_intervals
[i
].shape(), name
="elmux%d" % i
,
116 comb
+= elmux
.eq(element
)
119 # This calculates which partition of b to select the
120 # shifter from. According to the table above, the
121 # partition to select is given by the highest set bit in
122 # the partition mask, this calculates that with a mux
125 # This computes the partial results table. note that
126 # the shift amount is truncated because there's no point
127 # trying to shift data by 64 bits if the result width
129 shifter
= Signal(shiftbits
, name
="shifter%d" % i
,
131 with m
.If(element
> shiftbits
):
132 comb
+= shifter
.eq(shiftbits
)
134 comb
+= shifter
.eq(element
)
135 comb
+= shifter
.eq(element
)
136 partial
= Signal(reswid
, name
="partial%d" % i
, reset_less
=True)
137 comb
+= partial
.eq(a_intervals
[i
] << shifter
)
139 partial_results
.append(partial
)
143 # This calculates the outputs o0-o3 from the partial results
144 # table above. Note: only relevant bits of the partial result equal
145 # to the width of the output column are accumulated in a Mux-cascade.
147 result
= partial_results
[0]
148 out
.append(result
[s
:e
])
149 for i
in range(1, len(keys
)):
150 start
, end
= (intervals
[i
][0], width
)
151 reswid
= width
- start
152 sel
= Mux(gates
[i
-1], 0, result
[intervals
[0][1]:][:end
-start
])
153 print("select: [%d:%d]" % (start
, end
))
154 res
= Signal(end
-start
+1, name
="res%d" % i
, reset_less
=True)
155 comb
+= res
.eq(partial_results
[i
] | sel
)
160 comb
+= self
.output
.eq(Cat(*out
))