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Speed up console framebuffer imageblit function
Especially on platforms with a slower CPU but a relatively high framebuffer fill bandwidth, like current ARM devices, the existing console monochrome imageblit function used to draw console text is suboptimal for common pixel depths such as 16bpp and 32bpp. The existing code is quite general and can deal with several pixel depths. By creating special case functions for 16bpp and 32bpp, by far the most common pixel formats used on modern systems, a significant speed-up is attained which can be readily felt on ARM-based devices like the Raspberry Pi and the Allwinner platform, but should help any platform using the fb layer. The special case functions allow constant folding, eliminating a number of instructions including divide operations, and allow the use of an unrolled loop, eliminating instructions with a variable shift size, reducing source memory access instructions, and eliminating excessive branching. These unrolled loops also allow much better code optimization by the C compiler. The code that selects which optimized variant is used is also simplified, eliminating integer divide instructions. The speed-up, measured by timing 'cat file.txt' in the console, varies between 40% and 70%, when testing on the Raspberry Pi and Allwinner ARM-based platforms, depending on font size and the pixel depth, with the greater benefit for 32bpp. Signed-off-by: Harm Hanemaaijer <fgenfb@yahoo.com>
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