1 # SVP64 REMAP Worked Example: Matrix Multiply
5 * [Online matrix calculator](https://matrix.reshish.com/multCalculation.php)
7 * <https://bugs.libre-soc.org/show_bug.cgi?id=701>
8 * video <https://m.youtube.com/watch?v=BbhOA8ULKt4> slides
9 <https://ftp.libre-soc.org/openpower_2021.pdf>
10 * setvl instruction: [[sv/setvl]]
11 * REMAP python function based on yield (shows how indices are generated):
14 One of the most powerful and versatile modes of the REMAP engine (a part of
15 the SVP64 feature set) is the ability to perform matrix
16 multiplication with all elements within a scalar register file.
18 This is done by converting the index used to iterate over the operand and
19 result matrices to the actual index inside the scalar register file.
21 ## Worked example - manual (conventional method)
23 The matrix multiply looks like this:
29 When multiplying non-square matrices (rows != columns), to determine the
30 dimension of the result when matrix X has `a` rows and `b` columns and matrix
31 Y has `b` rows and `c` columns:
37 The result matrix will have number of rows of the first matrix, and number of
38 columns of the second matrix.
41 For this example, the following values will be used for the operand matrices X
42 and Y, result Z shown for completeness.
44 X =| 1 2 3 | Y = | 6 7 | Z = | 52 58 |
45 | 3 4 5 | | 8 9 | | 100 112 |
48 Matrix X has 2 rows, 3 columns (2x3), and matrix Y has 3 rows, 2 columns.
50 To determine the final dimensions of the resultant matrix Z, take the number
51 of rows from matrix X (2) and number of columns from matrix Y (2).
53 The method usually taught in linear algebra course to students is the
54 following (outer product):
56 1. Start with the first row of the first matrix, and first column of the
58 2. Multiply each element in the row by each element in the column, and sum
59 with the current value of the result matrix element (multiply-add-accumulate).
60 Store result in the row matching first operand row and column matching second
62 3. Move to the next column of the second matrix, and next column of the result
63 matrix. If there are no more columns in the second matrix, go back to first
64 column (second matrix), and move to next row (first and result matrices).
65 If there are no more rows left, result matrix has been fully computed.
68 This for-loop uses the indices as shown above
71 for i in range(0, mat_X_num_rows):
72 for k in range(0, mat_Y_num_cols):
73 for j in range(0, mat_X_num_cols): # or mat_Y_num_rows
74 mat_Z[i][k] += mat_X[i][j] * mat_Y[j][k]
80 | 1 2 3 | | 6 7 | = | (1*6 + 2*8 + 3*10) (1*7 + 2*9 3*11) |
81 | 3 4 5 | * | 8 9 | | (3*6 + 4*8 + 5*10) (3*7 + 4*9 5*11) |
84 | 1 2 3 | | 6 7 | = | ( 6 + 16 + 30) ( 7 + 18 + 33) |
85 | 3 4 5 | * | 8 9 | | (18 + 32 + 50) (21 + 36 + 55) |
88 | 1 2 3 | | 6 7 | = | 52 58 |
89 | 3 4 5 | * | 8 9 | | 100 112 |
93 For the algorithm, assign indeces to matrices as follows:
100 Mat Y | 6 7 8 9 10 11 |
103 Mat Z | 52 58 100 112 |
106 (Start with the first row, then assign index left-to-right, top-to-bottom.)
111 | Mat X | Mat Y | Mat Z |
126 Worked example broken down into individual multiply-add accumulates:
128 [[!img outer_product_worked_example.jpg size="500x" ]]
130 The issue with this algorithm is that the result matrix element is the same
131 for three consecutive operations, and where each element is stored in CPU
132 registers, the same register will be written to three times and thus causing
137 A slight modification to the order of the loops in the algorithm massively
138 reduces the chance of read-after-write hazards, as the result matrix
139 element (and thus register) changes with every multiply-add operation.
144 for i in range(mat_X_num_rows):
145 for j in range(0, mat_X_num_cols): # or mat_Y_num_rows
146 for k in range(0, mat_Y_num_cols):
147 mat_Z[i][k] += mat_X[i][j] * mat_Y[j][k]
153 | Mat X | Mat Y | Mat Z |
168 Worked example for inner product:
170 [[!img inner_product_worked_example.jpg size="500x" ]]
172 The index for the result matrix changes with every operation, and thus the
173 consecutive multiply-add instruction doesn't depend on the previous write
176 Outer and inner product indices side-by-side:
179 | Outer Product | Inner Product |
180 | Mat X | Mat Y | Mat Z | Mat X | Mat Y | Mat Z |
181 | 0 | 0 | 0 | 0 | 0 | 0 |
182 | 1 | 2 | 0 | 0 | 1 | 1 |
183 | 2 | 4 | 0 | 3 | 0 | 2 |
184 | 0 | 1 | 1 | 3 | 1 | 3 |
185 | 1 | 3 | 1 | 1 | 2 | 0 |
186 | 2 | 5 | 1 | 1 | 3 | 1 |
187 | 3 | 0 | 2 | 4 | 2 | 2 |
188 | 4 | 2 | 2 | 4 | 3 | 3 |
189 | 5 | 4 | 2 | 2 | 4 | 0 |
190 | 3 | 1 | 3 | 2 | 5 | 1 |
191 | 4 | 3 | 3 | 5 | 4 | 2 |
192 | 5 | 5 | 3 | 5 | 5 | 3 |
195 # SVP64 instructions implementing matrix multiply
197 * SVP64 assembler example:
198 [unit test](https://git.libre-soc.org/?p=openpower-isa.git;a=blob;f=src/openpower/decoder/isa/test_caller_svp64_matrix.py;hb=30f2d8a8e92ad2939775f19e6a0f387499e9842b#l56)
199 * svremap and svshape instructions defined in:
201 * Multiple-Add Low Doubleword instruction pseudo-code (Power ISA 3.0C
202 Book I, section 3.3.9): [[openpower/isa/fixedarith]]
204 *(Need to check if first arg of svremap correct, then one shown works with
208 svshape 2, 2, 3, 0, 0
209 svremap 31, 1, 2, 3, 0, 0, 0
210 sv.maddld *0, *16, *32, *0
215 The `svshape` instruction is a convenient way to access the `SVSHAPE` Special
216 Purpose Registers (SPRs), which were added alongside the SVP64 looping
217 system for complex element indexing. Without having "Re-shaping" SPRs, only the most
218 basic, consecuting indexing of register elements (0,1,2,3...) would
221 ### SVSHAPE Remapping SPRs
223 * See [[sv/remap]] for the full break down of SPRs `SVSHAPE0-3`.
224 * Pseudo-code for the
225 [svshape instruction](https://git.libre-soc.org/?p=openpower-isa.git;a=blob;f=openpower/isa/simplev.mdwn;h=33a02e#l120)
227 Matrix Multiply utilises SVSHAPE0-2 SPRs.
229 (NOTE: This section is duplicated from the remap spec, and thus will be re-worked.)
233 |0:5 |6:11 | 12:17 | 18:20 | 21:23 |24:27 |28:29 |30:31|
234 |----- |----- | ------- | ------- | ------ |------|------ |---- |
235 |xdimsz|ydimsz| zdimsz | permute | invxyz |offset|skip |mode |
240 - 0b00 indicates no dimensions to be skipped
241 - 0b01 - skip '1st dim'
242 - 0b10 - skip '2nd dim'
243 - 0b11 - skip '3rd dim'
245 invxyz (3-bit; 1 for x, 1 for y, 1 for z):
247 - If corresponding dim bit is zero, start index from zero and increment
248 - If bit set, start from xdimsz-1 (x dimension size, or whichever dimension
249 bit is being looked at) and decrement down to zero.
251 offset is used to offset the result by `offset` elements (important for when
252 using element width overrides are used).
254 xdimsz, ydimsz, zdimsz are offset by 1, such that 0-0b111111 correspond to
255 1-64. A value of xdimsz=2 would indicate that in the first dimension there are
256 3 elements in the array.
258 With the example Matrix X (2 rows, 3 columns, or 2x3 matrix), xdimsz=1,
263 | permute | order | array format |
264 | ------- | ----- | ------------------------ |
265 | 000 | 0,1,2 | (xdim+1)(ydim+1)(zdim+1) |
266 | 001 | 0,2,1 | (xdim+1)(zdim+1)(ydim+1) |
267 | 010 | 1,0,2 | (ydim+1)(xdim+1)(zdim+1) |
268 | 011 | 1,2,0 | (ydim+1)(zdim+1)(xdim+1) |
269 | 100 | 2,0,1 | (zdim+1)(xdim+1)(ydim+1) |
270 | 101 | 2,1,0 | (zdim+1)(ydim+1)(xdim+1) |
271 | 110 | 0,1 | Indexed (xdim+1)(ydim+1) |
272 | 111 | 1,0 | Indexed (ydim+1)(xdim+1) |
274 Permute re-arranges the order of the nested for-loops used to iterate over the
275 three dimensions. This allows for in-place transpose, in-place rotate, matrix
276 multiply, convolutions, without the limitation of Power-of-Two matrices.
278 For normal matrix multiply, the permute setting is 0b010 (order 1,0,2,
279 or swap x and y loops).
281 (*NOTE:* This is done automatically by the Matrix-Multiply REMAP mode, `SVRM=0`.)
283 ### Limitations of Matrix REMAP
285 - Vector Length (VL) limited to 127, and up to 127 Multiply-Add Accumulates
286 (MAC), or other operations may be performed in total.
287 For matrix multiply, it means both operand matrices and result matrix can
288 have no more than 127 elements in total.
289 (Larger matrices can be split into tiles to circumvent this issue, out
290 of scope of this document).
291 - `svshape` instruction only provides part of the Matrix REMAP capability.
292 For rotation and mirroring, `SVSHAPE` SPRs must be programmed directly (thus
293 requiring more assembler instructions). Future revisions of SVP64 will
294 provide more comprehensive capacity, mitigating the need to write to `SVSHAPE`
297 Going back to the assembler instruction used to setup the shape for matrix
301 svshape 2, 2, 3, 0, 0
306 - SVxd=2, SVyd=2, SVzd=3
307 - SVRM=0 (Matrix mode, uses `SVSHAPE0` SPR)
308 - vf=0 (not using Vertical-First mode)
310 To determine the `SVxd`/`SVyd`/`SVzd` settings:
312 - `SVxd` is equal to the number of columns in the second operand matrix.
313 Matrix Y has 2 columns, so `SVxd=2`.
314 - `SVyd` is equal to the number of rows in the first operand matrix.
315 Matrix X has 2 rows, so `SVyd=2`.
316 - `SVzd` is equal to the number of columns in the first operand matrix,
317 or the number of rows in the second operand matrix.
318 Matrix X has 3 columns, matrix Y has 3 rows, so `SVzd=3`.
323 SVxd | mat_Y_num_cols
324 SVyd | mat_X_num_rows
325 SVzd | mat_X_num_cols OR mat_Y_num_rows
332 |0 |6 |11 |13 |15 |17 |19 |21 | 22:25 |26:31 |
333 | -- | -- | -- | -- | -- | -- | -- | -- | ---- | ----- |
334 | PO | SVme |mi0 | mi1 | mi2 | mo0 | mo1 | pst | rsvd | XO |
336 * svremap SVme,mi0,mi1,mi2,mo0,mo1,pst
339 svremap 31, 1, 2, 3, 0, 0, 0
344 - `SVme=31`, 5-bit bitmask determines which registers have REMAP applied.
345 Bitfields are: `RA|RB|RC|RT|EA/FRS` In this example, all input registers
346 (RA, RB, RC) of *any* scalar instruction to follow and output register (RT)
347 will have REMAP applied.
348 - `mix/mox` fields determine which shape is applied to the activated register
349 - `mi0=1`, instruction operand RA has SVSHAPE1 applied to it.
350 - `mi1=2`, instruction operand RB has SVSHAPE2 applied to it.
351 - `mi2=3`, instruction operand RA has SVSHAPE3 applied to it.
352 - `mo0=0`, instruction result RT has SVSHAPE0 applied to it.
353 - `mo1=0`, instruction result EA/FRS has SVSHAPE0 applied to it. *(not applicable
355 - `pst=0`, if set, REMAP remains enabled until explicitly disabled, or another
356 REMAP, or setvl is setup.
358 Assigns the configured SVSHAPEs to the relevant operand/result registers
359 of the consecutive instruction/s (depending on if REMAP is set to persistent).
361 The index table shown for the inner method above shows indices for a 'flattened'
362 matrix (how it would be arranged in sequential GPR registers), whereas
363 SVSHAPE0, 1, 2 registers setup the indices in relation to rows and columns
366 This is how the indices compare:
370 Flattened Indices | Mat X | Mat Y | Mat Z |
371 | Mat X | Mat Y | Mat Z | | r c | r c | r c |
372 | 0 | 0 | 0 | | 0 0 | 0 0 | 0 0 |
373 | 0 | 1 | 1 | | 0 0 | 0 1 | 0 1 |
374 | 3 | 0 | 2 | | 1 0 | 0 0 | 1 0 |
375 | 3 | 1 | 3 | | 1 0 | 0 1 | 1 1 |
376 | 1 | 2 | 0 | | 0 1 | 1 0 | 0 0 |
377 | 1 | 3 | 1 | | 0 1 | 1 1 | 0 1 |
378 | 4 | 2 | 2 | | 1 1 | 1 0 | 1 0 |
379 | 4 | 3 | 3 | | 1 1 | 1 1 | 1 1 |
380 | 2 | 4 | 0 | | 0 2 | 2 0 | 0 0 |
381 | 2 | 5 | 1 | | 0 2 | 2 1 | 0 1 |
382 | 5 | 4 | 2 | | 1 2 | 2 0 | 1 0 |
383 | 5 | 5 | 3 | | 1 2 | 2 1 | 1 1 |
386 See [[openpower/sv/remap]] Section 3.3 Matrix Mode for more information on
387 the index sequences which can be produced with SVSHAPE SPRs.
389 ## maddld - Multiply-Add Low Doubleword VA-form
392 sv.maddld *0, *16, *32, *0
395 A standard instruction available since version 3.0 of PowerISA.
397 *Temporary note:* maddld (Multiply-Add Low Doubleword) in the 3.1b version
398 of the PowerISA spec is in the Linux Compliancy Subset, not SFS or SFFS.
399 See page 1477 of the document, or page 1503 of the pdf.
401 This instruction can be used as a multiply-add accumulate by setting the
402 third operand to be the same as the result register, which functions as
407 ### Running the simulator test case
409 - Set up the LibreSOC development environment (need to have openpower-isa repo
410 setup and installed).
412 $: cd /PATH/TO/src/openpower-isa/src/openpower/decoder/isa/
413 $: python3 test_caller_svp64_matrix.py >& /tmp/f
415 (All test cases are enabled by default; extra test cases can be disabled by changing
416 `test_` to `tst_` or any other prefix.)
418 The simulator dump will be stored as in the temporary directory.
420 Particular text strings to to search for:
422 - `shape remap` - shows the indices corresponding to the operand and result matrices
423 - `flattened X,Y,expected` - shows the expected matrix result which will be used to
424 validate the result coming from the simulator.
425 - `maddld-matrix` - shows the result matrix element values coming from the simulator.
426 These values must match the expect result calculated before the simulator call.
428 If the matrix multiply worked successfully, the simulator dump will have an `OK`
429 string after the `Ran 1 test in...` summary.
433 [[!img mat_mul_sim_results.png size="600x" ]]
435 ### Remapyield showing how the matrix indices are generated
437 *(more work required not critically important right now)*
439 This table was drawn up using the `remapyield.py` code found
440 [here](https://git.libre-soc.org/?p=openpower-isa.git;a=blob;f=src/openpower/decoder/isa/remapyield.py;h=d7b4bdeff2ae5d5f319ae036e4dff70de194dc67;hb=HEAD).
442 The xdim, ydim, and zdim were set to match the values specified in the svshape
443 instruction for the above matrix multiply example.
445 `x`, 'y`, and `z` are iterators for the three loops, with a range between 0 and
446 `SVxd-1`, `SVyd-1`, `SVzd-1` respectively.
448 For the above `svshape` instruction example:
450 - `x` takes values between 0 and 1 (`SVxd=2`)
451 - `y` takes values between 0 and 1 (`SVyd=2`)
452 - `z` takes values between 0 and 2 (`SVzd=3`)
453 - `x_end`, `y_end`, and `z_end` correspond to the largest permitted values for
454 `x`, `y`, and `z` (1, 1, and 2 respectively).
457 | (x == | (y == | (z ==
458 index | x | y | z | x_end) | y_end) | z_end)
459 0 | 0 | 0 | 0 | F | F | F
460 1 | 1 | 0 | 0 | T | F | F
461 2 | 0 | 1 | 0 | F | T | F
462 3 | 1 | 1 | 0 | T | T | F
463 4 | 0 | 0 | 1 | F | F | F
464 5 | 1 | 0 | 1 | T | F | F
465 6 | 0 | 1 | 1 | F | T | F
466 7 | 1 | 1 | 1 | T | T | F
467 8 | 0 | 0 | 2 | F | F | T
468 9 | 1 | 0 | 2 | T | F | T
469 10 | 0 | 1 | 2 | F | T | T
470 11 | 1 | 1 | 2 | T | T | T
473 If the `x`, `y`, and `z` sequences are compared with the inner product index
474 table, there are some resemblances. Swapping the `x` and `y` (permute=0b10)
475 shows that matrix X indices correspond to `y`, matrix Y indices correspond to
476 `x` (if the following equation used: `x+z*z_end`), and matrix Z indices
479 [[!tag svp64_cookbook ]]