Re: Rationale for an old TRUNCATE patch

[Adam Nemet <anemet@caviumnetworks.com>]

Ian Lance Taylor writes:
> I agree that this patch looks wrong in todays compiler.  There should be
> no need to call TRULY_NOOP_TRUNCATION if you are in a TRUNCATE anyhow.

Thanks.

Do you think we can assume this for TRUNCATEs in general or only for MIPS-like
TRUNCATEs?

I can't think of why it would be useful to represent a mode so that bits
outside the mode mask don't either depent on bits inside the mask or are
don't-care bits.  IOW, can we assume that for any TRUNCATE from wider mode W
to narrower mode N the following holds:

  (truncate:N expr:W) == (truncate:N (and:W expr:W GET_MODE_MASK(Nmode)))

?  Where == is not necessarily identical bit representation of the object
holding the value (e.g. QI HI values in MIPS) but that they are
indistinguishable in the operations that are defined on them.

Adam