84 lines
2.1 KiB
ArmAsm
84 lines
2.1 KiB
ArmAsm
.global _gint_image_p4_loop
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/* gint's image renderer: 4-bit indexed entry point
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P4 compacts pixel data further than P8 by restricting values to a 16-color
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palette and packing 2 pixels in each byte. This severely restricts our
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ability to use sub-images because odd positions land within bytes.
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Fortunately, we can solve this by using more edge pixels. The simplest way
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to write a P4 loop is to process 2 pixels from a 2-aligned source image
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position in a single iteration. Other structures don't even come close in
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terms of CPU performance (which, as a reminder, is the main bottleneck in
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Azur but not in gint): selecting nibbles individually is too long, while not
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unrolling is still clearly inefficient. So it becomes very important to
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forcibly align the sub-image on byte-aligned input boundaries and stick to
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that grid.
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Obviously, this approach causes up to one extra pixel to be overwritten on
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each side of every line. We solve this problem by adding *another* edge
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pixel on the left side. In the renderer this is called the left edge or
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edge_1, while the standard one is called right edge or edge_2.
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r0: - (initially: cmd.effect)
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r1: Number of lines remaining to draw
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r2: Number of columns per line
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r3: Input pointer
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r4: Input stride
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r5: Output pointer
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r6: Output stride
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r7: Right edge pointer
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r8: - (initially: cmd)
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r9: - (initially: cmd.loop)
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r10: Left edge pointer */
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_gint_image_p4_loop:
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/* r4: int output_width (pixels)
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r5: struct gint_image_cmd *cmd */
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mov.b @(1,r5), r0 /* cmd.effect */
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add #2, r5
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mov.w @r5+, r2 /* cmd.columns */
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mov r4, r6
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mov.l r8, @-r15
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mov r5, r8
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/* For here on the command is r8 */
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mov.l r9, @-r15
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sub r2, r6
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mov.w @r8+, r4 /* cmd.input_stride */
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add r6, r6
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mov.b @r8+, r1 /* cmd.lines */
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nop
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mov.l r10, @-r15
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extu.b r1, r1
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mov.b @r8+, r10 /* cmd.edge_1 */
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nop
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mov.l @r8+, r9
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shlr r0 /* T bit is now VFLIP */
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mov.l @r8+, r5 /* cmd.output */
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nop
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bf.s _NO_VFLIP
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mov.l @r8+, r3 /* cmd.input */
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_VFLIP:
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neg r4, r4
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nop
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_NO_VFLIP:
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mov r2, r7
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shlr r7
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jmp @r9
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subc r7, r4
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