prosolis
29eb78a599
FINDINGS 29 priced a literal-span mode at 4*(50 + 4L*9.08) cycles and labelled
the whole section DERIVED. Session 8 step 0 was to measure it before optimising
over the mode set it implies. Two variants in blit.s, one stream per span length
from prep_spans.py, timed by span.lua, driven by span.sh in ~25 s:
v5, handed (x, npix) and left to work the copy out: 97.9/span + 10.459/px
v6, handed an address and a jump displacement: 43.7/span + 9.152/px
29 assumed 50.0/span + 9.080/px
So 29's arithmetic was right about a format nobody had written. The difference
is not tuning: v5 spends ~122 cycles a span computing a destination, dividing
npix into bursts and handling a 0..15 remainder, all of which the encoder knows
at build time. v6's record is {u32 absolute GVRAM address, u16 jump
displacement} into an unrolled chain of 24-pixel copy units -- no loop, no
remainder, no arithmetic -- and it fits 11 span lengths to 0.3%.
Three things that measurement showed and derivation could not:
- The per-pixel cost is a function of REGISTER PRESSURE. FINDINGS 24's 9.08
was a fixed blit with 12 registers free; v5 can spare 8 and pays 10.46; v6
gets 12 back only because the encoder holds the state.
- Short spans die in the remainder path -- a 12-pixel span costs MORE than a
16-pixel one -- and the fix is padding, not avoidance.
- Odd-x alignment is free (259.0 vs 261.8 cycles/span), as a 16-bit bus
implies but nobody had checked.
Re-priced against the unchanged mode maps, sasi: median 74.4% -> 52.0% (29 said
43.0), misses 37 -> 10/120 (29 said 8), 448.0 KB/s. Break-even moved from runs
of 2 blocks to runs of 4. 29.4 survives: a scene cut needs x >= 0.196 of the
frame as spans and the bus allows x <= 0.373, so it fits at 12fps.
All 23 timing configs are also checked pixel-exact, so none of this was timed
against a decoder that quietly skipped work.
FINDINGS 30. Next: lever B, the cost-aware mode decision.
Claude-Session: https://claude.ai/code/session_01194oWYW8DQXK1SZ2DnChW6
287 lines
12 KiB
ArmAsm
287 lines
12 KiB
ArmAsm
; Full-frame GVRAM blit cost on a stock 68000 @ 10MHz.
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;
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; Answers: what fraction of a 12fps frame budget (833,333 cycles) does simply
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; PUTTING a decoded 256x192 frame on screen cost, before any decoding?
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;
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; Geometry (tools/bench/crtc_mode.lua): 256-colour page, one pixel per WORD of
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; CPU address space, 1024-byte line stride, picture in rows 32..223 of a
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; 256-row page. So a row is 512 contiguous bytes of writes, then a 512-byte
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; skip. 192 rows = 98,304 bytes of GVRAM write traffic per frame.
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;
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; Confirmed from MAME 0.277 x68k_crtc.cpp:501 (gvram_w, case 0x0100): a CPU
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; write in 256-colour mode is masked to 0x00ff, so the HIGH byte of every word
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; written is discarded by the hardware. V1 exploits this -- it never has to
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; clear the odd bytes of its source.
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;
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; Three variants, selected by VAR, each looped ITER times:
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; V1 movem.l blit from a word-expanded RAM frame (96KB). The realistic
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; "decode to RAM, then blit" design. Reads 96KB, writes 96KB.
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; V2 naive byte-source expansion (move.b / move.w per pixel). The obvious
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; implementation, kept as the baseline V1 has to beat.
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; V3 write-only floor: registers preloaded once, no source read at all.
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; Nothing that puts this many pixels on screen can beat V3. The gap
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; V1-V3 is the price of reading a source frame at all.
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; V4 the SAME 96KB of writes, but issued in 4x4 BLOCK order instead of
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; row-linear order. This is the access pattern a decoder that writes
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; codewords straight into GVRAM actually has, and it is the number that
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; picks the decoder architecture: compose-in-RAM-then-blit (V1) versus
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; decode-direct-to-GVRAM (V4 scaled by the fraction of non-SKIP blocks).
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; Each block is 4 rows of 8 bytes at a 1024-byte stride, so the
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; destination displacements 0/1024/2048/3072 all fit a 16-bit offset and
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; the block needs only one base pointer. V4 deliberately scrambles the
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; picture (it reads a row-linear source in block order); it is a timing
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; probe, which is why the correctness snapshot is taken after V1.
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; V5 ROW-LINEAR LITERAL SPANS, the mode priced in FINDINGS 29 and never
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; measured. Walks a stream of per-row span records
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; row: u16 nspans, then nspans * { u16 x, u16 npix, npix*u16 pixels }
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; for 192 rows, copying each span's word-expanded pixels straight from
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; the stream buffer into GVRAM. Unlike V1-V4 the work per call is set by
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; the STREAM, not by the code, so one variant measures every span length:
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; tools/bench/prep_spans.py generates a stream per span length and
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; tools/bench/span.lua times them and fits cycles = A*spans + B*pixels.
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; The point of the measurement is A -- the per-span overhead FINDINGS 29
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; guessed at 50 cycles -- and how much B degrades from V1's 9.08 when a
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; span is too short to burst. Every config covers the whole frame, so
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; V5 draws the SAME picture V1 does and can be verified, not just timed.
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;
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; Bursts are 8 registers (d0-d3/a3-a6 = 32 bytes = 16 pixels), not V1's
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; 12: a0/a1/a2 and d4-d7 are all live across a span (stream, row base,
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; destination, and three counters). The remainder is copied move.l at a
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; time with a leading move.w when it is odd, so a 4-pixel span never
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; reaches a movem at all -- which is exactly the case FINDINGS 29's
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; full-row-width extrapolation flatters.
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;
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; V6 the SAME spans with the arithmetic moved into the encoder. V5 measures
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; a decoder that is handed (x, npix) and has to work out how to copy it;
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; most of its per-span cost is that working-out, and an encoder can do it
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; once at build time instead of 12 times a second. V6's record is
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; { u32 absolute GVRAM address, u16 jump displacement } -- no row
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; structure, no counters, no remainder logic -- and the displacement
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; jumps into an unrolled chain of 24-pixel copy units, so a span of any
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; supported length is straight-line code with no loop at all.
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; GVRAM sits at a fixed $C00000 on every X68000, so absolute destinations
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; are a legitimate thing for an encoder to bake in.
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;
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; Two consequences of the format. Span lengths are multiples of 24
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; pixels, and a span may overrun the 256 visible pixels of its row by up
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; to 23 -- harmless, because the line stride is 1024 bytes and only the
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; first 512 are displayed, so the overrun lands in the invisible half.
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; And with row and remainder handling gone, 12 registers are free again
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; (d0-d6/a1/a3-a6), which is why the unit is 24 pixels and not V5's 16.
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;
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; 12 registers per movem burst (d0-d7/a2-a5 = 48 bytes) is the maximum
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; available: a0=src, a1=dst, a6=end sentinel. The row counter lives in the
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; a1-vs-a6 compare rather than a d-register for exactly this reason.
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; 512 = 10*48 + 32, hence ten 12-register bursts and one 8-register tail.
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; Destination uses (d16,a1) displacement rather than post-increment because
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; movem cannot post-increment a destination; the displacement costs 4 cycles
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; per burst but saves an 8-cycle lea, so it is the cheaper of the two.
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FLAG = $18000 ; 0 idle / 1 running / $FF done
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VAR = $18004 ; variant selector, written by Lua
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ITER = $18008 ; iteration count, written by Lua
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SPTR = $1800C ; V5 span stream pointer, written by Lua
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SRCW = $60000 ; word-expanded frame 192*512 = 96KB
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SRCB = $80000 ; byte-per-pixel frame 192*256 = 48KB
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DST0 = $C08000 ; GVRAM + 32*1024 (first picture row)
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DSTE = $C38000 ; GVRAM + 224*1024 (one past last)
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ROWS = 192 ; picture rows a V5 stream describes
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V6UNIT = 12 ; bytes of code per V6 chain unit
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V6MAX = 11 ; chain units = 11*24 = 264 pixels >= one row
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org $10000
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start:
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move.l VAR.l,d0
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move.l #1,FLAG.l ; timer starts here
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cmp.l #1,d0
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beq v1
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cmp.l #2,d0
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beq v2
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cmp.l #4,d0
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beq v4
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cmp.l #5,d0
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beq v5
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cmp.l #6,d0
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beq v6
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bra v3
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; ---------------------------------------------------------------- V1
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v1: lea SRCW,a0
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lea DST0,a1
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lea DSTE,a6
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v1row: movem.l (a0)+,d0-d7/a2-a5
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movem.l d0-d7/a2-a5,(a1)
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movem.l (a0)+,d0-d7/a2-a5
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movem.l d0-d7/a2-a5,48(a1)
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movem.l (a0)+,d0-d7/a2-a5
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movem.l d0-d7/a2-a5,96(a1)
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movem.l (a0)+,d0-d7/a2-a5
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movem.l d0-d7/a2-a5,144(a1)
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movem.l (a0)+,d0-d7/a2-a5
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movem.l d0-d7/a2-a5,192(a1)
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movem.l (a0)+,d0-d7/a2-a5
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movem.l d0-d7/a2-a5,240(a1)
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movem.l (a0)+,d0-d7/a2-a5
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movem.l d0-d7/a2-a5,288(a1)
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movem.l (a0)+,d0-d7/a2-a5
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movem.l d0-d7/a2-a5,336(a1)
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movem.l (a0)+,d0-d7/a2-a5
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movem.l d0-d7/a2-a5,384(a1)
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movem.l (a0)+,d0-d7/a2-a5
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movem.l d0-d7/a2-a5,432(a1)
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movem.l (a0)+,d0-d7
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movem.l d0-d7,480(a1)
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lea 1024(a1),a1
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cmpa.l a6,a1
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bne v1row
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subq.l #1,ITER.l
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bne v1
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bra done
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; ---------------------------------------------------------------- V2
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v2: lea SRCB,a0
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lea DST0,a1
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lea DSTE,a6
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v2row: move.w #255,d1
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v2px: move.b (a0)+,d0
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move.w d0,(a1)+ ; high byte is discarded by gvram_w
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dbra d1,v2px
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lea 512(a1),a1 ; skip the unused half of the line
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cmpa.l a6,a1
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bne v2row
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subq.l #1,ITER.l
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bne v2
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bra done
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; ---------------------------------------------------------------- V3
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v3: lea SRCW,a0
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movem.l (a0),d0-d7/a2-a5 ; load the burst once, outside the loop
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lea DST0,a1
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lea DSTE,a6
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v3row: movem.l d0-d7/a2-a5,(a1)
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movem.l d0-d7/a2-a5,48(a1)
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movem.l d0-d7/a2-a5,96(a1)
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movem.l d0-d7/a2-a5,144(a1)
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movem.l d0-d7/a2-a5,192(a1)
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movem.l d0-d7/a2-a5,240(a1)
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movem.l d0-d7/a2-a5,288(a1)
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movem.l d0-d7/a2-a5,336(a1)
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movem.l d0-d7/a2-a5,384(a1)
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movem.l d0-d7/a2-a5,432(a1)
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movem.l d0-d7,480(a1)
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lea 1024(a1),a1
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cmpa.l a6,a1
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bne v3row
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subq.l #1,ITER.l
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bne v3
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bra done
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; ---------------------------------------------------------------- V4
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v4: lea SRCW,a0
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lea DST0,a3 ; base of the current block row
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lea DSTE,a4 ; one past the last block row
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v4brow: move.l a3,a1
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lea 512(a3),a5 ; 64 blocks * 8 bytes
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v4blk: movem.l (a0)+,d0-d7 ; 32 bytes = one 4x4 block, expanded
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movem.l d0-d1,(a1)
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movem.l d2-d3,1024(a1)
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movem.l d4-d5,2048(a1)
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movem.l d6-d7,3072(a1)
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addq.l #8,a1
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cmpa.l a5,a1
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bne.s v4blk
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lea 4096(a3),a3 ; next block row is 4 picture lines
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cmpa.l a4,a3
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bne v4brow
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subq.l #1,ITER.l
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bne v4
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bra done
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; ---------------------------------------------------------------- V5
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; a0 stream, a1 row base, a2 span destination, d7 rows, d6 spans, d5 pixels,
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; d4 burst/tail counter. Everything else (d0-d3/a3-a6) is burst payload.
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v5: move.l SPTR.l,a0
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lea DST0,a1
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move.w #ROWS-1,d7
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v5row: move.w (a0)+,d6 ; spans in this row
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subq.w #1,d6
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bmi.s v5eor ; a row may legitimately have none
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v5span: move.w (a0)+,d0 ; x, in pixels
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add.w d0,d0 ; one pixel = one word
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lea 0(a1,d0.w),a2
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move.w (a0)+,d5 ; pixels in this span
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move.w d5,d4
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lsr.w #4,d4 ; 16-pixel bursts
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beq.s v5tail
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subq.w #1,d4
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v5burst: movem.l (a0)+,d0-d3/a3-a6 ; 32 bytes straight out of the stream
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movem.l d0-d3/a3-a6,(a2)
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lea 32(a2),a2
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dbra d4,v5burst
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v5tail: moveq #15,d4
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and.w d5,d4 ; 0..15 pixels left
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beq.s v5eos
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lsr.w #1,d4 ; C = odd pixel count
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bcc.s v5t2
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move.w (a0)+,(a2)+
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v5t2: subq.w #1,d4
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bmi.s v5eos
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v5tl: move.l (a0)+,(a2)+
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dbra d4,v5tl
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v5eos: dbra d6,v5span
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v5eor: lea 1024(a1),a1
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dbra d7,v5row
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subq.l #1,ITER.l
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bne v5
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bra done
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; ---------------------------------------------------------------- V6
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; a0 stream, a2 destination, d7 spans remaining; everything else is payload.
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v6: move.l SPTR.l,a0
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move.w (a0)+,d7 ; total spans in the frame
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subq.w #1,d7
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v6span: move.l (a0)+,a2 ; absolute GVRAM destination
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move.w (a0)+,d0 ; (V6MAX - units) * V6UNIT, from the encoder
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jmp v6ch(pc,d0.w)
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v6ch:
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movem.l (a0)+,d0-d6/a1/a3-a6
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movem.l d0-d6/a1/a3-a6,(a2)
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lea 48(a2),a2
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movem.l (a0)+,d0-d6/a1/a3-a6
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movem.l d0-d6/a1/a3-a6,(a2)
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lea 48(a2),a2
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movem.l (a0)+,d0-d6/a1/a3-a6
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movem.l d0-d6/a1/a3-a6,(a2)
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lea 48(a2),a2
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movem.l (a0)+,d0-d6/a1/a3-a6
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movem.l d0-d6/a1/a3-a6,(a2)
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lea 48(a2),a2
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movem.l (a0)+,d0-d6/a1/a3-a6
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movem.l d0-d6/a1/a3-a6,(a2)
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lea 48(a2),a2
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movem.l (a0)+,d0-d6/a1/a3-a6
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movem.l d0-d6/a1/a3-a6,(a2)
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lea 48(a2),a2
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movem.l (a0)+,d0-d6/a1/a3-a6
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movem.l d0-d6/a1/a3-a6,(a2)
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lea 48(a2),a2
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movem.l (a0)+,d0-d6/a1/a3-a6
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movem.l d0-d6/a1/a3-a6,(a2)
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lea 48(a2),a2
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movem.l (a0)+,d0-d6/a1/a3-a6
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movem.l d0-d6/a1/a3-a6,(a2)
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lea 48(a2),a2
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movem.l (a0)+,d0-d6/a1/a3-a6
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movem.l d0-d6/a1/a3-a6,(a2)
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lea 48(a2),a2
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movem.l (a0)+,d0-d6/a1/a3-a6
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movem.l d0-d6/a1/a3-a6,(a2)
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lea 48(a2),a2
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dbra d7,v6span
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subq.l #1,ITER.l
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bne v6
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bra done
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done: move.l #$FF,FLAG.l ; timer stops here
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halt: bra.s halt
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