Optimize DF representation #3469
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Signed-off-by: Alyssa Rosenzweig <[email protected]>
Signed-off-by: Alyssa Rosenzweig <[email protected]>
Signed-off-by: Alyssa Rosenzweig <[email protected]>
Use the 1/-1 representation instead of 0/1. This will be better by the end of the series. Signed-off-by: Alyssa Rosenzweig <[email protected]>
Signed-off-by: Alyssa Rosenzweig <[email protected]>
fuse the shift the right way Signed-off-by: Alyssa Rosenzweig <[email protected]>
Signed-off-by: Alyssa Rosenzweig <[email protected]>
Signed-off-by: Alyssa Rosenzweig <[email protected]>
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Sonicadvance1
approved these changes
Mar 14, 2024
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Currently we store the direction flag unencoded (0/1). To actually use DF we end up transforming. With this PR, we instead store the direction flag encoded as (1/-1). The "raw" DF value is given in the sign bit of the 8-bit flag byte. When we need the "raw" value we can just do a sign bit test, but when we want the transformed value it's available directly with a sign-extension. (The sign-extension in turn folds into the load with a single
ldrsbop.)Another way to think about the new encoding is as a sign-magnitude decomposition. A direction is a vector, and DF is indeed a 1-bit vector.