[llvm-commits] [LLVMdev] question about Type::canLoslesslyBitCastTo

[Akira Hatanaka]

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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> > LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> >
>
>
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