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Measure the finer chain tail: 84/120 becomes 18/120, and the derivation was right by cancellation

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blit.s gains v7 -- v6's 24-pixel movem chain plus a second chain whose unit is
one `move.l (a0)+,(a2)+`. Measured over 13 span lengths by span.sh, every config
pixel-exact:

    cycles = 66.0 per span + 9.143 per COARSE pixel + 9.978 per FINE pixel

fitting all 13 to within 0.2%. v5 and v6 re-measure to FINDINGS 30 exactly, so
the harness has not drifted underneath the new variant.

Rescored against the same scsi window and the same additive model, v7 takes
84/120 frames over budget to 18/120 -- exactly what FINDINGS 39.4 derived, and
that agreement is two cancelling errors: the derivation's 2-register movem tail
is 29% too dear per pixel, and its "nothing per span" for the second chain entry
is 22.3 clocks too cheap. The plain post-incrementing move.l is the right tail
instruction, and it makes the padding quantum 2 pixels, which a run of 4x4
blocks pads to exactly zero.

The DMAC stays dropped on a measurement now rather than an argument: v7 takes
back 37 of the 43 frames the array chain would, with no reserved channel and no
timing neither emulator here can verify. Break-even against all-V1 moves from
L=4 blocks to L=2.

The fine displacement is carried mid-stream rather than in the span record, so
the decoder holds nothing across the copy and keeps all 12 payload registers --
which is the whole reason the coarse unit is 24 pixels.

span.sh is now -seconds_to_run 200 (30 s wall, 36 configs) and takes its
expected snapshot count from the generated metadata instead of a literal 23.

Claude-Session: https://claude.ai/code/session_01194oWYW8DQXK1SZ2DnChW6
This commit is contained in:
prosolis
2026-08-23 19:02:23 -07:00
parent c5ca56330e
commit c520a89e14
9 changed files with 522 additions and 52 deletions
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tools/bench/blit.s
+90
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@@ -69,6 +69,27 @@
; And with row and remainder handling gone, 12 registers are free again
; (d0-d6/a1/a3-a6), which is why the unit is 24 pixels and not V5's 16.
;
; V7 v6 with a SECOND, finer chain for the tail (FINDINGS 39.4). v6 pays for
; its 24-pixel quantum in padding: an average span wastes ~11 pixels, and
; FINDINGS 39.3 attributes 86% of the DMAC array-chain's advantage over v6
; to exactly that. V7 keeps the 24-pixel coarse chain and appends a chain
; of 2-pixel units, so a span is 24*c + 2*f pixels and the padding is at
; most one pixel -- ZERO for the real case, where a span is a run of 4x4
; blocks and its length is a multiple of 4.
;
; The fine unit is `move.l (a0)+,(a2)+` (20 cycles, 2 pixels), NOT a
; 2-register movem: movem.l (a0)+,d0-d1 plus movem.l d0-d1,(a2) plus the
; lea is 52+8 cycles for 4 pixels, so the obvious "smaller movem" tail is
; 50% dearer per pixel than the plainest instruction on the machine.
;
; The second entry point costs a second dispatch, and the trick that pays
; for it is that the fine displacement is NOT in the span record: it sits
; in the STREAM, after the coarse pixels and before the fine ones. The
; coarse chain falls out into `move.w (a0)+,d0 / jmp`, by which point d0
; is dead payload and a0 is pointing exactly at it. So v7 holds nothing
; extra across the copy and keeps all 12 payload registers -- a record is
; still {u32 address, u16 displacement}, with one more u16 mid-span.
;
; 12 registers per movem burst (d0-d7/a2-a5 = 48 bytes) is the maximum
; available: a0=src, a1=dst, a6=end sentinel. The row counter lives in the
; a1-vs-a6 compare rather than a d-register for exactly this reason.
@@ -88,6 +109,10 @@ DSTE = $C38000 ; GVRAM + 224*1024 (one past last)
ROWS = 192 ; picture rows a V5 stream describes
V6UNIT = 12 ; bytes of code per V6 chain unit
V6MAX = 11 ; chain units = 11*24 = 264 pixels >= one row
V7CU = 12 ; bytes of code per V7 COARSE unit (24 px)
V7CN = 11 ; coarse units: 11*24 = 264 px >= one row
V7FU = 2 ; bytes of code per V7 FINE unit (2 px)
V7FN = 11 ; fine units: 11*2 = 22 px > one coarse unit
org $10000
start:
@@ -103,6 +128,8 @@ start:
beq v5
cmp.l #6,d0
beq v6
cmp.l #7,d0
beq v7
bra v3
; ---------------------------------------------------------------- V1
@@ -282,5 +309,68 @@ v6ch:
bne v6
bra done
; ---------------------------------------------------------------- V7
; a0 stream, a2 destination, d7 spans remaining; everything else is payload.
; Stream per span: u32 dest, u16 coarse disp, c*48 B pixels,
; u16 fine disp, f*4 B pixels.
v7: move.l SPTR.l,a0
move.w (a0)+,d7 ; total spans in the frame
subq.w #1,d7
v7span: move.l (a0)+,a2 ; absolute GVRAM destination
move.w (a0)+,d0 ; (V7CN - coarse) * V7CU
jmp v7ch(pc,d0.w)
v7ch:
movem.l (a0)+,d0-d6/a1/a3-a6
movem.l d0-d6/a1/a3-a6,(a2)
lea 48(a2),a2
movem.l (a0)+,d0-d6/a1/a3-a6
movem.l d0-d6/a1/a3-a6,(a2)
lea 48(a2),a2
movem.l (a0)+,d0-d6/a1/a3-a6
movem.l d0-d6/a1/a3-a6,(a2)
lea 48(a2),a2
movem.l (a0)+,d0-d6/a1/a3-a6
movem.l d0-d6/a1/a3-a6,(a2)
lea 48(a2),a2
movem.l (a0)+,d0-d6/a1/a3-a6
movem.l d0-d6/a1/a3-a6,(a2)
lea 48(a2),a2
movem.l (a0)+,d0-d6/a1/a3-a6
movem.l d0-d6/a1/a3-a6,(a2)
lea 48(a2),a2
movem.l (a0)+,d0-d6/a1/a3-a6
movem.l d0-d6/a1/a3-a6,(a2)
lea 48(a2),a2
movem.l (a0)+,d0-d6/a1/a3-a6
movem.l d0-d6/a1/a3-a6,(a2)
lea 48(a2),a2
movem.l (a0)+,d0-d6/a1/a3-a6
movem.l d0-d6/a1/a3-a6,(a2)
lea 48(a2),a2
movem.l (a0)+,d0-d6/a1/a3-a6
movem.l d0-d6/a1/a3-a6,(a2)
lea 48(a2),a2
movem.l (a0)+,d0-d6/a1/a3-a6
movem.l d0-d6/a1/a3-a6,(a2)
lea 48(a2),a2
v7cx: move.w (a0)+,d0 ; (V7FN - fine) * V7FU, from mid-stream
jmp v7fh(pc,d0.w)
v7fh:
move.l (a0)+,(a2)+
move.l (a0)+,(a2)+
move.l (a0)+,(a2)+
move.l (a0)+,(a2)+
move.l (a0)+,(a2)+
move.l (a0)+,(a2)+
move.l (a0)+,(a2)+
move.l (a0)+,(a2)+
move.l (a0)+,(a2)+
move.l (a0)+,(a2)+
move.l (a0)+,(a2)+
dbra d7,v7span
subq.l #1,ITER.l
bne v7
bra done
done: move.l #$FF,FLAG.l ; timer stops here
halt: bra.s halt