[LLVMdev] Adding multiples-of-8 integer types to MVT

Duncan Sands duncan.sands at math.u-psud.fr
Sat Dec 5 04:33:38 PST 2009

>> Would there be any interest/opposition to extending the set of simple
>> integer types in MVT to include the missing multiples of 8 (up to 64
>> bits)? That is: i24, i40, i48, i56?

By the way, the integer type legalization logic should probably go like
this: let T be an integer type.

(1) If T is legal, do nothing.
(2) If there is a legal integer type which is bigger (in bitwidth) than T,
then promote T to the smallest legal type which is bigger than T.
(3) In the remaining case, T is necessarily bigger than the largest legal
integer type (call this type L).  Take the smallest positive N such that
  (bitwidth of T) <= (bitwidth of L) * 2^N
If you have equality in the equation, i.e. if the bitwidth of T is a power
of two multiple of the bitwidth of L, then expand T into two equal integer
types of half the size.  Otherwise promote T to the type with bitwidth equal
to the right-hand-side of the equation, i.e. (bitwidth of L) * 2^N.

If all legal integer types have a power of two size, then this coincides
with what we have today.  If some legal types do not have a power of 2
size then finding the type to promote to in (2) requires more computation
than in the power-of-two case.  For (3), you need to know the largest legal
type L, which currently isn't exposed in a convenient way for this.  For
simple integer types everything can of course be pre-calculated in tables,
like now.  For extended integer types it would be good to have an efficient
algorithm for calculating this on the fly.  At worst, values can be cached.



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