+# daxpy
+
+This is a standard textbook algorithm demonstration for Vector and SIMD ISAs.
+
+<https://bugs.libre-soc.org/show_bug.cgi?id=1117>
+
# c code
```
- void daxpy(size_t n, double a, const double x[], double y[])
- {
- for (size_t i = 0; i < n; i++) {
- y[i] = a*x[i] + y[i];
- }
+ void daxpy(size_t n, double a, const double x[], double y[]) {
+ for (size_t i = 0; i < n; i++)
+ y[i] = a*x[i] + y[i];
}
```
+Summary
+
+| ISA | total | loop | words | notes |
+|-----|-------|------|-------|-------|
+| SVP64 | 8 | 6 | 13 | 5 64-bit, 4 32-bit |
+| RVV | 13 | 11 | 9.5 | 7 32-bit, 5 16-bit |
+| SVE | 12 | 7 | 12 | all 32-bit |
+
# SVP64 Power ISA version
-```
-
- # r5: n count
- # r6: x ptr
- # r7: y ptr
- # fp1: a mul-scalar
- mtctr 5 # move n to CTR
- addi r10,r6,0 # copy y-ptr into r10
-.L2
- setvl MAXVL=32,VL=CTR # could do more
- sv.lfdup/els *32,8(6) # load from x into fp32-63
- sv.lfdup/els *64,8(7) # load from y into fp64-95
- sv.fmadd *64,*64,1,*32 # (*y) = (*y) * (*x) + fp1
- stfdup/els *64,8(10) # store y-copy
- sv.bc/ctr .L2 # decrement VL by CTR
- blr # return
+The first instruction is simple: the plan is to use CTR for looping.
+Therefore, copy n (r5) into CTR. Next however, at the start of
+the loop (L2) is not so obvious: MAXVL is being set to 32
+elements, but at the same time VL is being set to MIN(MAXVL,CTR).
+
+This algorithm
+relies on post-increment, relies on no overlap between x and y
+in memory, and critically relies on y overwrite. x is post-incremented
+when read, but y is post-incremented on write. Load/Store Post-Increment
+is a new Draft set of instructions for the Scalar Subsets, which save
+having to pre-subtract an offset before running the loop.
+
+For `sv.lfdup`, RA is Scalar so continuously accumulates
+additions of the immediate (8):
+the last write to RA is the address for
+the next block (the next time round the CTR loop).
+To understand this it is necessary to appreciate that
+SVP64 is as if a sequence of loop-unrolled scalar instructions were
+issued. With that sequence all writing the new version of RA
+before the next element-instruction, the end result is identical
+in effect to Element-Strided, except that RA points to the start
+of the next batch.
+
+Use of Element-Strided on `sv.lfd/els`
+ensures the Immediate (8) results in a contiguous LD
+*without* modifying RA.
+The first element is loaded from RA, the second from RA+8, the third
+from RA+16 and so on. However unlike the `sv.lfdup`, RA remains
+pointing at the current block being processed of the y array.
+
+With both a part of x and y loaded into a batch of GPR
+registers starting at 32 and 64 respectively, a multiply-and-accumulate
+can be carried out. The scalar constant `a` is in fp1.
+
+Where the curret pointer to y had not been updated by the `sv.lfd/els`
+instruction, this means that y (r7) is already pointing to the
+right place to store the results. However given that we want r7
+to point to the start of the next batch, *now* we can use
+`sv.stfdup` which will post-increment RA repeatedly by 8
+
+Now that the batch of length `VL` has been done, the next step
+is to decrement CTR by VL, which we know due to the setvl instruction
+that VL and CTR will be equal or that if CTR is greater than MAXVL,
+that VL will be *equal* to MAXVL. Therefore, when `sv bc/ctr`
+performs a decrement of CTR by VL, we an be confident that CTR
+will only reach zero if there is no more of the array to process.
+
+Thus in an elegant way each RISC instruction is actually quite sophisticated,
+but not a huge CISC-like difference from the original Power ISA.
+Scalar Power ISA already has LD/ST-Update (pre-increment on RA):
+we propose adding Post-Increment (Motorola 68000 and 8086 have had
+both for decades). Power ISA branch-conditional has had Decrement-CTR
+since its inception: we propose in SVP64 to add "Decrement CTR by VL".
+The end result is an exceptionally compact daxpy that is easy to read
+and understand.
+
+```
+ # r5: n count; r6: x ptr; r7: y ptr; fp1: a
+ 1 mtctr 5 # move n to CTR
+ 2 .L2
+ 3 setvl MAXVL=32,VL=CTR # actually VL=MIN(MAXVL,CTR)
+ 4 sv.lfdup *32,8(6) # load x into fp32-63, incr x
+ 5 sv.lfd/els *64,8(7) # load y into fp64-95, NO INC
+ 6 sv.fmadd *64,*64,1,*32 # (*y) = (*y) * (*x) + a
+ 7 sv.stfdup *64,8(7) # store at y, post-incr y
+ 8 sv.bc/ctr .L2 # decr CTR by VL, jump !zero
+ 9 blr # return
```
# RVV version
+
```
# a0 is n, a1 is pointer to x[0], a2 is pointer to y[0], fa0 is a
li t0, 2<<25
15 b.first .loop // more to do?
16 ret
```
+
+[[!tag svp64_cookbook ]]
projects / libreriscv.git / blobdiff