[LLVMdev] question about Type::canLoslesslyBitCastTo

[Duncan Sands]

Hi Akira,

> This is the definition of BITCAST in include/llvm/CodeGen/ISDOpcodes.h:
>
> // BITCAST - This operator converts between integer, vector and FP
> // values, as if the value was stored to memory with one type and loaded
> // from the same address with the other type (or equivalently for vector
> // format conversions, etc).  The source and result are required to have
> // the same bit size (e.g.  f32 <-> i32).  This can also be used for
> // int-to-int or fp-to-fp conversions, but that is a noop, deleted by
> // getNode().
>
> If I can assume the bitcast instruction in LLVM IR has the same definition, does
> this mean that the following instruction
>
> %dst = bitcast i32 %src to v2i16
>
> is equivalent to the following sequence of instructions (which I think is lossless)?
>
> store i32 %src, i32* %ptr
> %ptr2 = bitcast i32* %ptr to v2i16*
> %dst = load v2i16* %ptr2

while there has been some discussion about changing the definition of bitcast,
yes, this is currently the definition.  That canLoslesslyBitCastTo returns
false for bitcast of i32 to <2 x i16> is probably just an oversight.  Please
feel free to send in a patch fixing it.

Ciao, Duncan.

>
>
> I am trying to change all vector types (v2i16 and v4i8) in
> include/llvm/IntrinsicsMips.td to i32, but I can't do that because the code in
> CodeGenFunction::EmitBuiltinExpr (in clang/CGBuiltin.cpp) raises an assertion
> when it calls Type::canLoslesslyBitCastTo to check whether conversion between
> v2i16 (or v4i8) and i32 is legal.
>
> To work around this problem, I can put back CodeGenFunction::EmitMipsBuiltinExpr
> which was removed in r159368 and add code to do type conversions, but I prefer
> not to do this.
>
>
> On Mon, Jul 2, 2012 at 5:40 PM, Akira Hatanaka <ahatanak at gmail.com
> <mailto:ahatanak at gmail.com>> wrote:
>
>     Type::canLoslesslyBitCastTo(Type *Ty) in lib/VMCore/Type.cpp always returns
>     false when it checks whether an integer can be bitcast to a vector or vice
>     versa.
>
>     For example, (i32 => v2i16) or (v2i16 => i32) is false.
>
>     But it seems that it returns true if it is checking conversion between two
>     vector types which have the same size.
>
>     For example, (v4i8 => v2i16) would return true.
>
>     What is the rationale behind this?
>
>
>
>
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