Fix rotate_{left, right} for multiple of bitsize rotation amounts #15324
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Could you add tests for this as well? They should go in libcoretest and there's some macro infrastructure in there to generate cases for all of the integer types. |
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I believe these should cover it. |
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Replaced unsigned negation by subtraction, to appease the compiler. |
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Nice catch! Could you squash the commits together as well? |
Add additional rotation tests
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OK, done. |
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The current implementation of `rotate_left` and `rotate_right` are incorrect when the rotation amount is 0, or a multiple of the input's bitsize. When `n = 0`, the expression
(self >> n) | (self << ($BITS - n))
results in a shift left by `$BITS` bits, which is undefined behavior (see #10183), and currently results in a hardcoded `-1` value, instead of the original input value. Reducing `($BITS - n)` modulo `$BITS`, simplified to `(-n % $BITS)`, fixes this problem.
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The current implementation of
rotate_leftandrotate_rightare incorrect when the rotation amount is 0, or a multiple of the input's bitsize. Whenn = 0, the expressionresults in a shift left by
$BITSbits, which is undefined behavior (see #10183), and currently results in a hardcoded-1value, instead of the original input value. Reducing($BITS - n)modulo$BITS, simplified to(-n % $BITS), fixes this problem.