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Overshift is implementation defined in C because different ISEs (that are too widely used to ignore handle) it differently. For us, the easy case is the excepting form “shiftlx”, which will get you an overshift NaR. The saturating form “shiftls” is also simple, as it saturates rather than overflows. Nearly as simple is the widening form “shiftlw” which will give you a double width result if the count is within that double width. However, if it is also not within the double width then you will get a non-NaR zero. And plain “shiftl” also overshifts to a non-NaR zero too.
There is no single Mill op that uses the approach in which the count is modulo-divided to the operand size such that shift by 32 and shift by zero are synonymous (for word-width data). If you want those semantics you must explicitly mask the shift by an AND.