(grosged) slight optimisations (#116)

* slight optimisations

* Revert a bad optimization

_PixelPtr checks the full 24-bit y value in DE, but the y value is only passed as an 8-bit value to gfx_SetPixel. The upper 16 bits of DE will be undefined and possibly nonzero here, in which case I believe _PixelPtr would erroneously reject it as offscreen even if the 8-bit value is actually on screen.

* Optimize gfx_SetPixel

-8 cycles, -2 bytes

* Make gfx_GetPixel safer

+4 cycles, +2 bytes

I had these changes sitting around locally for a while. Since this
routine was already getting optimized in a very similar fashion, I
figured I should just slip my little extra safety changes (and better
comments) on top.

Author: adriweb

src/graphx/graphx.asm

@@ -202,14 +202,16 @@ gfx_AllocSprite:
 ;  arg2 : pointer to malloc routine
 ; Returns:
 ;  Pointer to allocated sprite, first byte width, second height
-	ld	bc,3
-	push	bc
-	pop	hl
+	ld	hl,3
 	add	hl,sp
 	ld	e,(hl)			; e = width
-	add	hl,bc
+	inc	hl
+	inc	hl
+	inc	hl
 	ld	d,(hl)			; d = height
-	add	hl,bc
+	inc	hl
+	inc	hl
+	inc	hl
 	ld	hl,(hl)			; hl = malloc
 	push	de
 	mlt	de			; de = width * height
@@ -518,18 +520,31 @@ gfx_GetPixel:
 ; Returns:
 ;  Color index of X,Y coordinate
 	ld	hl,3
-	add	hl,sp
-	ld	bc,(hl)			; bc = x coordinate
+	add	hl,sp			; hl = &x
+	inc.s	bc
+	ld	c,(hl)
 	inc	hl
+	ld	b,(hl)			; bc = (uint16_t)x
 	inc	hl
-	inc	hl			; move to next argument
-	ld	de,0
-	ld	e,(hl)			; e = y coordinate
-	call	_PixelPtr
-	ret	c			; return if out of bounds
-	ld	a,(hl)			; get the actual pixel
+	inc	hl			; hl = &y
+	ld	e,(hl)			; e = y
+	ld	d,LcdWidth/2
+	mlt	de			; de = y * (lcdWidth / 2)
+	ld	hl,(CurrentBuffer)	; hl = buffer
+	add	hl,bc
+	add	hl,de
+	add	hl,de			; hl = buffer + y * (lcdWidth / 2)*2 + (uint16_t)x
+					;    = buffer + y * lcdWidth + (uint16_t)x
+					;    = &buffer[y][x]
+; No clipping is necessary, because if the pixel is offscreen, the result is
+; undefined. All that is necessary is to ensure that there are no side effects
+; of reading outside of the buffer. In this case, the largest possible offset
+; into the buffer is 255 * lcdWidth + 65535 = 147135 bytes. Even in the case
+; that the current buffer is the second half of VRAM, the largest that this
+; pointer can be is $D52C00 + 147135 = $D76ABF. This goes beyond the end of
+; mapped RAM, but only into unmapped memory with no read side effects.
+	ld	a,(hl)			; a = buffer[y][x]
 	ret
-
 ;-------------------------------------------------------------------------------
 gfx_SetPixel:
 ; Sets the color pixel to the global color index
@@ -541,10 +556,8 @@ gfx_SetPixel:
 	ld	hl,3
 	add	hl,sp
 	ld	bc,(hl)			; bc = x coordinate
-	inc	hl
-	inc	hl
-	inc	hl			; move to next argument
-	ld	de,0
+	ld	de,3
+	add	hl,de			; move to next argument
 	ld	e,(hl)			; e = y coordinate
 _SetPixel:
 	call	_PixelPtr
@@ -803,11 +816,11 @@ _HorizLine_NoClip:
 Color_2 := $-1
 _MemorySet:
 	ld	(hl),a
-	push	hl
 	cpi
-	ex	de,hl
-	pop	hl
 	ret	po
+	ex	de,hl
+	ld	hl,-1
+	add	hl,de
 	ldir
 	ret