[LLVMdev] question about Type::canLoslesslyBitCastTo (original) (raw)

Duncan Sands baldrick at free.fr
Tue Jul 10 00:21:35 PDT 2012

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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