[LLVMdev] question about Type::canLoslesslyBitCastTo

[Akira Hatanaka]

Ping?

Any comments or suggestions?

On Mon, Jul 16, 2012 at 4:52 PM, Akira Hatanaka <ahatanak at gmail.com> wrote:

> Hi Duncan and llvm developers,
>
> The attached patch makes the function return true if the conversion is
> between a vector and an integer of the same size.
>
> Please review when you have time.
>
>
> On Tue, Jul 10, 2012 at 12:21 AM, Duncan Sands <baldrick at free.fr> wrote:
>
>> 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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>> >
>>
>>
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>
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