[LLVMdev] Fwd: Adding multiples-of-8 integer types to MVT
etherzhhb at gmail.com
Sat Dec 5 18:24:11 PST 2009
ya, when you targeting some backend like FPGA, you are able to meet a
integer with any bitwitdh.
On Sun, Dec 6, 2009 at 8:58 AM, OvermindDL1 <overminddl1 at gmail.com> wrote:
> ---------- Forwarded message ----------
> From: OvermindDL1 <overminddl1 at gmail.com>
> Date: Sat, Dec 5, 2009 at 5:58 PM
> Subject: Re: [LLVMdev] Adding multiples-of-8 integer types to MVT
> To: Duncan Sands <duncan.sands at math.u-psud.fr>
> On Sat, Dec 5, 2009 at 5:33 AM, Duncan Sands
> <duncan.sands at math.u-psud.fr> wrote:
>>>> 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.
> This makes the most sense to me as well. I have used some embedded
> chips (long time ago, but still...) that had a 7-bit wide bus and
> register setup, certainly not a power of 2, not even even.
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