fbdev: Improve performance of sys_fillrect()

Improve the performance of sys_fillrect() by using word-aligned
32/64-bit mov instructions. While the code tried to implement this,
the compiler failed to create fast instructions. The resulting
binary instructions were even slower than cfb_fillrect(), which
uses the same algorithm, but operates on I/O memory.

A microbenchmark measures the average number of CPU cycles
for sys_fillrect() after a stabilizing period of a few minutes
(i7-4790, FullHD, simpledrm, kernel with debugging). The value
for CFB is given as a reference.

sys_fillrect(), new: 26586 cycles
sys_fillrect(), old: 166603 cycles
cfb_fillrect(): 41012 cycles

In the optimized case, sys_fillrect() is now ~6x faster than before
and ~1.5x faster than the CFB implementation.

Signed-off-by: Thomas Zimmermann <[e-mail]>
Reviewed-by: Javier Martinez Canillas <[e-mail]>
Reviewed-by: Sam Ravnborg <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/video/fbdev/core/sysfillrect.c

···
 
 		/* Main chunk */
 		n /= bits;
-		while (n >= 8) {
-			*dst++ = pat;
-			*dst++ = pat;
-			*dst++ = pat;
-			*dst++ = pat;
-			*dst++ = pat;
-			*dst++ = pat;
-			*dst++ = pat;
-			*dst++ = pat;
-			n -= 8;
-		}
-		while (n--)
-			*dst++ = pat;
+		memset_l(dst, pat, n);
+		dst += n;
+
 		/* Trailing bits */
 		if (last)
 			*dst = comp(pat, *dst, last);