[LLVMdev] Definition of the bitcast instruction for vectors

[Eli Friedman]

On Fri, Aug 16, 2013 at 7:51 AM, Daniel Sanders
<Daniel.Sanders at imgtec.com>wrote:

>  Hi,****
>
> ** **
>
> From http://llvm.org/docs/LangRef.html#bitcast-to-instruction:****
>
> ** **
>
> The ‘bitcast‘ instruction converts value to type ty2. It is always a no-op
> cast because no bits change with this conversion. The conversion is done as
> if the value had been stored to memory and read back as type ty2. Pointer
> (or vector of pointers) types may only be converted to other pointer (or
> vector of pointers) types with this instruction if the pointer sizes are
> equal. To convert pointers to other types, use the inttoptr or ptrtoint
> instructions first.****
>
> ** **
>
> If I understand the definition correctly, then:****
>
> %1 = bitcast <8 x i16> %0 to <4 x i32>****
>
> Should be equivalent to:****
>
>                 store <8 x i16> %0, <8 x i16>* %ptr****
>
> %1 = load <4 x i32>* %ptr****
>
> Which the definition above asserts is a no-op cast. However, the
> store/load sequence is not a no-op cast for vector types when the target
> does not treat the whole vector as either little or big-endian. MIPS MSA
> stores vector elements in big or little endian order according to a
> configuration bit, but irrespective of this bit the lanes are stored in
> ascending order.  For example, <4 x i32> <i32 1, i32 2, i32 3, i32 4> would
> be stored as this in big-endian mode:****
>
> %ptr   : 00 00 00 01****
>
> %ptr+4 : 00 00 00 02****
>
> %ptr+8 : 00 00 00 03****
>
> %ptr+12: 00 00 00 04****
>
> And this in little-endian mode:****
>
> %ptr   : 01 00 00 00****
>
> %ptr+4 : 02 00 00 00****
>
> %ptr+8 : 03 00 00 00****
>
> %ptr+12: 04 00 00 00****
>
> ** **
>
> How should I resolve this conflicting definition?****
>
>
>
>
>
It's a "no-op" cast in the sense that there isn't any math involved; it's
not a statement about the number of necessary machine instructions.

-Eli
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