Optimise fz_paint_span_with_color and fz_paint_solid_color

The 2 biggest hotspots in benchmarking on the Raspberry pi at
1200dpi.

1 files changed, 544 insertions, 51 deletions

diff --git a/source/fitz/draw-paint.c b/source/fitz/draw-paint.c
index e7cfbdb4..30b9364f 100644
--- a/source/fitz/draw-paint.c
+++ b/source/fitz/draw-paint.c
@@ -86,19 +86,192 @@ fz_paint_solid_alpha(byte * restrict dp, int w, int alpha)
 	}
 }
 
-void
-fz_paint_solid_color(byte * restrict dp, int n, int w, byte *color)
+static inline void
+fz_paint_solid_color_2(byte * restrict dp, int w, byte *color)
+{
+	int sa = FZ_EXPAND(color[1]);
+	if (sa == 0)
+		return;
+	if (sa == 256)
+	{
+		while (w--)
+		{
+			dp[0] = color[0];
+			dp[1] = 255;
+			dp += 2;
+		}
+	}
+	else
+	{
+		while (w--)
+		{
+			dp[0] = FZ_BLEND(color[0], dp[0], sa);
+			dp[1] = FZ_BLEND(255, dp[1], sa);
+			dp += 2;
+		}
+	}
+}
+
+#if 0
+/* This should be a #ifdef ARCH_ARM section, but in tests this seems to perform
+ * more slowly than the C. I have no idea why, but I am leaving it disabled
+ * for now. Once it has been in git, I may remove it. */
+static void
+fz_paint_solid_color_4(byte * restrict dp, int w, byte *color)
+  __attribute__((naked));
+
+static void
+fz_paint_solid_color_4(byte * restrict dp, int w, byte *color)
 {
+	asm volatile(
+	ENTER_ARM
+	"stmfd  r13!,{r4,r10-r11,r14}	@ r0 = dp, r1 = w, r2 = color	\n"
+	"ldr    r2, [r2]		@ r2 = aabbggrr = color[0,1,2,3]\n"
+	"mov	r14,#0xFF00		@ r14= 0xFF00			\n"
+	"orr    r14,r14,#0xFF000000	@ r14= 0xff00ff00 = mask	\n"
+	"mov    r3, r2, LSR #24		@ r3 = sa = color[3]		\n"
+	"orr    r2, r2, #0xFF000000	@ r2 = ffbbggrr			\n"
+	"adds   r3, r3, r3, LSR #7	@ r3 = sa = FZ_EXPAND(sa)	\n"
+	"cmpne  r1, #0			@ if (sa == 0 || w == 0)	\n"
+	"beq    9f			@ 	return			\n"
+	"cmp	r3, #256		@ if (sa == 256) {		\n"
+	"bne	5f			@				\n"
+	"1:				@ do {				\n"
+	"str	r2, [r0], #4		@ *(int *)dp = rgba, dp+=4	\n"
+	"subs	r1, r1, #1		@ w--;				\n"
+	"bne	1b			@ } while (w);			\n"
+	"b	9f			@ } else {			\n"
+	"5:				@				\n"
+	"and	r4, r2, r14		@ r4 = ga = ff00gg00		\n"
+	"bic    r2, r2, r14		@ r2 = rb = 00bb00rr		\n"
+	"mov	r4, r4, LSR #8		@ r4 = ga = 00ff00gg		\n"
+	"6:				@ do {				\n"
+	"ldr	r12,[r0]		@ r12 = *(int *)dp = AABBGGRR	\n"
+	"and    r11,r14,r12		@ r11= AA00GG00			\n"
+	"sub    r10,r4, r11,LSR #8	@ r10= aa00gg - AA00GG		\n"
+	"and    r12,r14,r12,LSL #8	@ r12= BB00RR00			\n"
+	"mla	r11,r3, r10,r11		@ r11= sa*r10+r11      		\n"
+	"sub    r10,r2, r12,LSR #8	@ r9 = bb00rr - BB00RR		\n"
+	"mla	r12,r3, r10,r12		@ r12= sa*r10+r12		\n"
+	"and    r11,r11,r14		@ r11= Aa00Gg00			\n"
+	"and    r12,r12,r14		@ r12= Bb00Rr00			\n"
+	"orr	r12,r11,r12,LSR #8	@ r12= AaBbGgRr			\n"
+	"str	r12,[r0, #-4]		@ dp[-4] = r12			\n"
+	"subs   r1, r1, #1		@ w--				\n"
+	"bne	6b			@ } while (w != 0);		\n"
+	"9:				@ }				\n"
+	"ldmfd  r13!,{r4,r10-r11,PC}					\n"
+	ENTER_THUMB
+	);
+}
+#else
+#ifndef AVOID_SWAR
+static inline void
+fz_paint_solid_color_4(byte * restrict dp, int w, byte *color)
+{
+	unsigned int rgba = *(int *)color;
+	int sa = FZ_EXPAND(color[3]);
+	if (sa == 0)
+		return;
+	rgba |= 0xFF000000;
+	if (sa == 256)
+	{
+		while (w--)
+		{
+			*(unsigned int *)dp = rgba;
+			dp += 4;
+		}
+	}
+	else
+	{
+		unsigned int mask = 0xFF00FF00;
+		unsigned int rb = rgba & (mask>>8);
+		unsigned int ga = (rgba & mask)>>8;
+		while (w--)
+		{
+			unsigned int RGBA = *(unsigned int *)dp;
+			unsigned int RB = (RGBA<<8) & mask;
+			unsigned int GA = RGBA & mask;
+			RB += (rb-(RB>>8))*sa;
+			GA += (ga-(GA>>8))*sa;
+			RB &= mask;
+			GA &= mask;
+			*(unsigned int *)dp = (RB>>8) | GA;
+			dp += 4;
+		}
+	}
+}
+#else
+static inline void
+fz_paint_solid_color_4(byte * restrict dp, int w, byte *color)
+{
+	int sa = FZ_EXPAND(color[3]);
+	if (sa == 0)
+		return;
+	if (sa == 256)
+	{
+		while (w--)
+		{
+			dp[0] = color[0];
+			dp[1] = color[1];
+			dp[2] = color[2];
+			dp[3] = 255;
+			dp += 4;
+		}
+	}
+	else
+	{
+		while (w--)
+		{
+			dp[0] = FZ_BLEND(color[0], dp[0], sa);
+			dp[1] = FZ_BLEND(color[1], dp[1], sa);
+			dp[2] = FZ_BLEND(color[2], dp[2], sa);
+			dp[3] = FZ_BLEND(255, dp[3], sa);
+			dp += 4;
+		}
+	}
+}
+#endif
+#endif
+
+static inline void
+fz_paint_solid_color_N(byte * restrict dp, int n, int w, byte *color)
+{
+	int k;
 	int n1 = n - 1;
 	int sa = FZ_EXPAND(color[n1]);
-	int k;
-	while (w--)
+	if (sa == 0)
+		return;
+	if (sa == 256)
 	{
-		int ma = FZ_COMBINE(FZ_EXPAND(255), sa);
-		for (k = 0; k < n1; k++)
-			dp[k] = FZ_BLEND(color[k], dp[k], ma);
-		dp[k] = FZ_BLEND(255, dp[k], ma);
-		dp += n;
+		while (w--)
+		{
+			for (k = 0; k < n1; k++)
+				dp[k] = color[k];
+			dp[k] = 255;
+			dp += n;
+		}
+	}
+	else
+	{
+		while (w--)
+		{
+			for (k = 0; k < n1; k++)
+				dp[k] = FZ_BLEND(color[k], dp[k], sa);
+			dp[k] = FZ_BLEND(255, dp[k], sa);
+			dp += n;
+		}
+	}
+}
+
+void
+fz_paint_solid_color(byte * restrict dp, int n, int w, byte *color)
+{
+	switch (n)
+	{
+	case 2: fz_paint_solid_color_2(dp, w, color); break;
+	case 4: fz_paint_solid_color_4(dp, w, color); break;
+	default: fz_paint_solid_color_N(dp, n, w, color); break;
 	}
 }
 
@@ -119,39 +292,274 @@ fz_paint_span_with_color_2(byte * restrict dp, byte * restrict mp, int w, byte *
 	}
 }
 
+#if 0
+/* This should be a #ifdef ARCH_ARM section, but in tests this seems to perform
+ * more slowly than the C. I have no idea why, but I am leaving it disabled
+ * for now. Once it has been in git, I may remove it. */
+static void
+fz_paint_span_with_color_4(byte * restrict dp, byte * restrict mp, int w, byte *color)
+  __attribute__((naked));
+
+static void
+fz_paint_span_with_color_4(byte * restrict dp, byte * restrict mp, int w, byte *color)
+{
+	asm volatile(
+	ENTER_ARM
+	"stmfd  r13!,{r4-r11,r14}	@ r0 = dp, r1 = mp, r2 = w	\n"
+	"				@ r3 = color			\n"
+	"ldr    r3, [r3]		@ r3 = aabbggrr = color[0,1,2,3]\n"
+	"mov	r14,#0xFF00		@ r14= 0xFF00			\n"
+	"orr    r14,r14,#0xFF000000	@ r14= 0xff00ff00 = mask	\n"
+	"mov    r7, r3, LSR #24		@ r7 = sa = color[3]		\n"
+	"orr    r3, r3, #0xFF000000	@ r3 = ffbbggrr			\n"
+	"and	r5, r3, r14		@ r5 = ff00gg00			\n"
+	"bic    r4, r3, r14		@ r4 = 00bb00rr			\n"
+	"mov	r5, r5, LSR #8		@ r5 = 00ff00gg			\n"
+	"adds   r7, r7, r7, LSR #7	@ r7 = sa = FZ_EXPAND(sa)	\n"
+	"cmpne  r2, #0			@ if (sa == 0 || w == 0)	\n"
+	"beq    9f			@ 	return			\n"
+	"cmp    r7, #256		@ if (sa == 256)		\n"
+	"beq	1f			@    fast_loop			\n"
+	"b	8f			@ else slow_loop		\n"
+	"@ fast 'solid' loop (biased towards 0 or 256)			\n"
+	"0:								\n"
+	"subs   r2, r2, #1		@ w--				\n"
+	"beq	9f			@ } while (w != 0);		\n"
+	"1:				@ do {				\n"
+	"ldrb	r12,[r1], #1		@ r12 = ma = *mp++		\n"
+	"add	r0, r0, #4		@ Advance r0 without loading	\n"
+	"@stall								\n"
+	"adds	r12,r12,r12,LSR #7	@ r12 = FZ_EXPAND(ma)		\n"
+	"beq    0b			@ if (r12 = 0) skip		\n"
+	"cmp    r12,#256		@ if (r12 != 256)		\n"
+	"bne    3f			@    use non-solid loop         \n"
+	"2:				@				\n"
+	"str	r3,[r0, #-4]		@				\n"
+	"subs   r2, r2, #1		@ w--				\n"
+	"bne	1b			@ } while (w != 0);		\n"
+	"b	9f			@ } else			\n"
+	"3:				@				\n"
+	"ldr	r11,[r0, #-4]		@				\n"
+	"b	6f			@				\n"
+	"@ Non-solid loop (biased towards 0 or non-256)			\n"
+	"4:								\n"
+	"subs   r2, r2, #1		@ w--				\n"
+	"beq	9f			@ } while (w != 0);		\n"
+	"5:				@ do {				\n"
+	"ldrb	r12,[r1], #1		@ r12 = ma = *mp++		\n"
+	"ldr	r11,[r0], #4		@ r11 = AABBGGRR = dp[0123]	\n"
+	"@stall								\n"
+	"adds	r12,r12,r12,LSR #7	@ r12 = FZ_EXPAND(ma)		\n"
+	"beq    4b			@ if (r12 = 0) skip		\n"
+	"cmp    r12,#256		@ if (r12 == 256)		\n"
+	"beq    2b			@    use solid loop		\n"
+	"6:				@				\n"
+	"and    r10,r14,r11		@ r10= AA00GG00			\n"
+	"sub    r9, r5, r10,LSR #8	@ r9 = aa00gg - AA00GG		\n"
+	"and    r11,r14,r11,LSL #8	@ r11= BB00RR00			\n"
+	"mla	r10,r12,r9, r10		@ r10= ma*r9+r10       		\n"
+	"sub    r9, r4, r11,LSR #8	@ r9 = bb00rr - BB00RR		\n"
+	"mla	r11,r12,r9, r11		@ r11= ma*r9+r11		\n"
+	"and    r10,r10,r14		@ r10= Aa00Gg00			\n"
+	"and    r11,r11,r14		@ r11= Bb00Rr00			\n"
+	"orr	r11,r10,r11,LSR #8	@ r11= AaBbGgRr			\n"
+	"str	r11,[r0, #-4]		@ dp[-4] = r11			\n"
+	"subs   r2, r2, #1		@ w--				\n"
+	"bne	5b			@ } while (w != 0);		\n"
+	"b	9f			@ } else			\n"
+	"7:								\n"
+	"subs   r2, r2, #1		@ w--				\n"
+	"beq	9f			@ } while (w != 0);		\n"
+	"8:				@ do {				\n"
+	"ldrb	r12,[r1], #1		@ r12 = ma = *mp++		\n"
+	"ldr	r11,[r0], #4		@ r11 = AABBGGRR = dp[0123]	\n"
+	"@stall								\n"
+	"adds	r12,r12,r12,LSR #7	@ r12 = FZ_EXPAND(ma)		\n"
+	"beq    7b			@ if (r12 = 0) skip		\n"
+	"mul	r12,r7, r12		@				\n"
+	"and    r10,r14,r11		@ r10= AA00GG00			\n"
+	"mov    r12,r12,LSR #8		@ r12 = ma = FZ_COMBINE(r12, sa)\n"
+	"sub    r9, r5, r10,LSR #8	@ r9 = aa00gg - AA00GG		\n"
+	"and    r11,r14,r11,LSL #8	@ r11= BB00RR00			\n"
+	"mla	r10,r12,r9, r10		@ r10= ma*r9+r10       		\n"
+	"sub    r9, r4, r11,LSR #8	@ r9 = bb00rr - BB00RR		\n"
+	"mla	r11,r12,r9, r11		@ r11= ma*r9+r11		\n"
+	"and    r10,r10,r14		@ r10= Aa00Gg00			\n"
+	"and    r11,r11,r14		@ r11= Bb00Rr00			\n"
+	"orr	r11,r10,r11,LSR #8	@ r11= AaBbGgRr			\n"
+	"str	r11,[r0, #-4]		@ dp[-4] = r11			\n"
+	"subs   r2, r2, #1		@ w--				\n"
+	"bne	8b			@ } while (w != 0);		\n"
+	"9:				@ }				\n"
+	"ldmfd  r13!,{r4-r11,PC}					\n"
+	ENTER_THUMB
+	);
+}
+#else
+#ifndef AVOID_SWAR
 static inline void
 fz_paint_span_with_color_4(byte * restrict dp, byte * restrict mp, int w, byte *color)
 {
+	unsigned int rgba = *((unsigned int *)color);
+	unsigned int mask, rb, ga;
 	int sa = FZ_EXPAND(color[3]);
+	if (sa == 0)
+		return;
+	rgba |= 0xFF000000;
+	mask = 0xFF00FF00;
+	rb = rgba & (mask>>8);
+	ga = (rgba & mask)>>8;
+	if (sa == 256)
+	{
+		while (w--)
+		{
+			unsigned int ma = *mp++;
+			dp += 4;
+			ma = FZ_EXPAND(ma);
+			if (ma == 0)
+			{
+			}
+			else if (ma == 256)
+			{
+				((unsigned int *)dp)[-1] = rgba;
+			}
+			else
+			{
+				unsigned int RGBA = ((unsigned int *)dp)[-1];
+				unsigned int RB = (RGBA<<8) & mask;
+				unsigned int GA = RGBA & mask;
+				RB += (rb-(RB>>8))*ma;
+				GA += (ga-(GA>>8))*ma;
+				RB &= mask;
+				GA &= mask;
+				((unsigned int *)dp)[-1] = (RB>>8) | GA;
+			}
+		}
+	}
+	else
+	{
+		while (w--)
+		{
+			unsigned int ma = *mp++;
+			ma = FZ_COMBINE(FZ_EXPAND(ma), sa);
+			dp += 4;
+			if (ma != 0)
+ 			{
+				unsigned int RGBA = ((unsigned int*)dp)[-1];
+				unsigned int RB = (RGBA<<8) & mask;
+				unsigned int GA = RGBA & mask;
+				RB += (rb-(RB>>8))*ma;
+				GA += (ga-(GA>>8))*ma;
+				RB &= mask;
+				GA &= mask;
+				((unsigned int *)dp)[-1] = (RB>>8) | GA;
+			}
+		}
+	}
+}
+#else
+static inline void
+fz_paint_span_with_color_4(byte * restrict dp, byte * restrict mp, int w, byte *color)
+{
 	int r = color[0];
 	int g = color[1];
 	int b = color[2];
-	while (w--)
+	int sa = FZ_EXPAND(color[3]);
+	if (sa == 0)
+		return;
+	if (sa == 256)
 	{
-		int ma = *mp++;
-		ma = FZ_COMBINE(FZ_EXPAND(ma), sa);
-		dp[0] = FZ_BLEND(r, dp[0], ma);
-		dp[1] = FZ_BLEND(g, dp[1], ma);
-		dp[2] = FZ_BLEND(b, dp[2], ma);
-		dp[3] = FZ_BLEND(255, dp[3], ma);
-		dp += 4;
+		int ival;
+		union {
+			char c[4];
+			unsigned int i;
+		} rgba;
+		rgba.c[0] = r;
+		rgba.c[1] = g;
+		rgba.c[2] = b;
+		rgba.c[3] = 255;
+		ival = rgba.i;
+		while (w--)
+		{
+			int ma = *mp++;
+			ma = FZ_EXPAND(ma);
+			if (ma == 0)
+			{
+			}
+			else if (ma == 256)
+			{
+				((int *)dp)[0] = ival;
+			}
+			else
+			{
+				dp[0] = FZ_BLEND(r, dp[0], ma);
+				dp[1] = FZ_BLEND(g, dp[1], ma);
+				dp[2] = FZ_BLEND(b, dp[2], ma);
+				dp[3] = FZ_BLEND(255, dp[3], ma);
+			}
+			dp += 4;
+		}
+	}
+	else
+	{
+		while (w--)
+		{
+			int ma = *mp++;
+			ma = FZ_COMBINE(FZ_EXPAND(ma), sa);
+			dp[0] = FZ_BLEND(r, dp[0], ma);
+			dp[1] = FZ_BLEND(g, dp[1], ma);
+			dp[2] = FZ_BLEND(b, dp[2], ma);
+			dp[3] = FZ_BLEND(255, dp[3], ma);
+			dp += 4;
+		}
 	}
 }
+#endif
+#endif
 
 static inline void
 fz_paint_span_with_color_N(byte * restrict dp, byte * restrict mp, int n, int w, byte *color)
 {
+	int k;
 	int n1 = n - 1;
 	int sa = FZ_EXPAND(color[n1]);
-	int k;
-	while (w--)
+	if (sa == 0)
+		return;
+	if (sa == 256)
 	{
-		int ma = *mp++;
-		ma = FZ_COMBINE(FZ_EXPAND(ma), sa);
-		for (k = 0; k < n1; k++)
-			dp[k] = FZ_BLEND(color[k], dp[k], ma);
-		dp[k] = FZ_BLEND(255, dp[k], ma);
-		dp += n;
+		while (w--)
+		{
+			int ma = *mp++;
+			ma = FZ_EXPAND(ma);
+			if (ma == 0)
+			{
+			}
+			else if (ma == 256)
+			{
+				for (k = 0; k < n1; k++)
+					dp[k] = color[k];
+				dp[k] = 255;
+			}
+			else
+			{
+				for (k = 0; k < n1; k++)
+					dp[k] = FZ_BLEND(color[k], dp[k], ma);
+				dp[k] = FZ_BLEND(255, dp[k], ma);
+			}
+			dp += n;
+		}
+	}
+	else
+	{
+		while (w--)
+		{
+			int ma = *mp++;
+			ma = FZ_COMBINE(FZ_EXPAND(ma), sa);
+			for (k = 0; k < n1; k++)
+				dp[k] = FZ_BLEND(color[k], dp[k], ma);
+			dp[k] = FZ_BLEND(255, dp[k], ma);
+			dp += n;
+		}
 	}
 }
 
@@ -168,6 +576,7 @@ fz_paint_span_with_color(byte * restrict dp, byte * restrict mp, int n, int w, b
 
 /* Blend source in mask over destination */
 
+/* FIXME: There is potential for SWAR optimisation here */
 static inline void
 fz_paint_span_with_mask_2(byte * restrict dp, byte * restrict sp, byte * restrict mp, int w)
 {
@@ -176,16 +585,42 @@ fz_paint_span_with_mask_2(byte * restrict dp, byte * restrict sp, byte * restric
 		int masa;
 		int ma = *mp++;
 		ma = FZ_EXPAND(ma);
-		masa = FZ_COMBINE(sp[1], ma);
-		masa = 255 - masa;
-		masa = FZ_EXPAND(masa);
-		*dp = FZ_COMBINE2(*sp, ma, *dp, masa);
-		sp++; dp++;
-		*dp = FZ_COMBINE2(*sp, ma, *dp, masa);
-		sp++; dp++;
+		if (ma == 0)
+		{
+			dp += 2;
+			sp += 2;
+		}
+		else if (ma == 256)
+		{
+			masa = 255 - sp[1];
+			if (masa == 0)
+			{
+				*dp++ = *sp++;
+				*dp++ = *sp++;
+			}
+			else
+			{
+				masa = FZ_EXPAND(masa);
+				*dp = *sp + FZ_COMBINE(*dp, masa);
+				sp++; dp++;
+				*dp = *sp + FZ_COMBINE(*dp, masa);
+				sp++; dp++;
+			}
+		}
+		else
+		{
+			masa = FZ_COMBINE(sp[1], ma);
+			masa = 255 - masa;
+			masa = FZ_EXPAND(masa);
+			*dp = FZ_COMBINE2(*sp, ma, *dp, masa);
+			sp++; dp++;
+			*dp = FZ_COMBINE2(*sp, ma, *dp, masa);
+			sp++; dp++;
+		}
 	}
 }
 
+/* FIXME: There is potential for SWAR optimisation here */
 static inline void
 fz_paint_span_with_mask_4(byte * restrict dp, byte * restrict sp, byte * restrict mp, int w)
 {
@@ -194,17 +629,47 @@ fz_paint_span_with_mask_4(byte * restrict dp, byte * restrict sp, byte * restric
 		int masa;
 		int ma = *mp++;
 		ma = FZ_EXPAND(ma);
-		masa = FZ_COMBINE(sp[3], ma);
-		masa = 255 - masa;
-		masa = FZ_EXPAND(masa);
-		*dp = FZ_COMBINE2(*sp, ma, *dp, masa);
-		sp++; dp++;
-		*dp = FZ_COMBINE2(*sp, ma, *dp, masa);
-		sp++; dp++;
-		*dp = FZ_COMBINE2(*sp, ma, *dp, masa);
-		sp++; dp++;
-		*dp = FZ_COMBINE2(*sp, ma, *dp, masa);
-		sp++; dp++;
+		if (ma == 0)
+		{
+			dp += 4;
+			sp += 4;
+		}
+		else if (ma == 256)
+		{
+			masa = 255 - sp[3];
+			if (masa == 0)
+			{
+				*(int*)dp = *(int *)sp;
+				sp += 4; dp += 4;
+			}
+			else
+			{
+				masa = FZ_EXPAND(masa);
+				*dp = *sp + FZ_COMBINE(*dp, masa);
+				sp++; dp++;
+				*dp = *sp + FZ_COMBINE(*dp, masa);
+				sp++; dp++;
+				*dp = *sp + FZ_COMBINE(*dp, masa);
+				sp++; dp++;
+				*dp = *sp + FZ_COMBINE(*dp, masa);
+				sp++; dp++;
+			}
+		}
+		else
+		{
+			/* FIXME: There is potential for SWAR optimisation here */
+			masa = FZ_COMBINE(sp[3], ma);
+			masa = 255 - masa;
+			masa = FZ_EXPAND(masa);
+			*dp = FZ_COMBINE2(*sp, ma, *dp, masa);
+			sp++; dp++;
+			*dp = FZ_COMBINE2(*sp, ma, *dp, masa);
+			sp++; dp++;
+			*dp = FZ_COMBINE2(*sp, ma, *dp, masa);
+			sp++; dp++;
+			*dp = FZ_COMBINE2(*sp, ma, *dp, masa);
+			sp++; dp++;
+		}
 	}
 }
 
@@ -213,17 +678,45 @@ fz_paint_span_with_mask_N(byte * restrict dp, byte * restrict sp, byte * restric
 {
 	while (w--)
 	{
-		int k = n;
-		int masa;
 		int ma = *mp++;
 		ma = FZ_EXPAND(ma);
-		masa = FZ_COMBINE(sp[n-1], ma);
-		masa = 255-masa;
-		masa = FZ_EXPAND(masa);
-		while (k--)
+		if (ma == 0)
 		{
-			*dp = FZ_COMBINE2(*sp, ma, *dp, masa);
-			sp++; dp++;
+			dp += n;
+			sp += n;
+		}
+		else if (ma == 256)
+		{
+			int k = n;
+			int masa = 255 - sp[n-1];
+			if (masa == 0)
+			{
+				while (k--)
+				{
+					*dp++ = *sp++;
+				}
+			}
+			else
+			{
+				masa = FZ_EXPAND(masa);
+				while (k--)
+				{
+					*dp = *sp + FZ_COMBINE(*dp, masa);
+					sp++; dp++;
+				}
+			}
+		}
+		else
+		{
+			int k = n;
+			int masa = FZ_COMBINE(sp[n-1], ma);
+			masa = 255-masa;
+			masa = FZ_EXPAND(masa);
+			while (k--)
+			{
+				*dp = FZ_COMBINE2(*sp, ma, *dp, masa);
+				sp++; dp++;
+			}
 		}
 	}
 }