[LLVMdev] BITS_BIG_ENDIAN in llvm-gcc

[Jay Foad]

There are various functions in llvm-convert.cpp that depend on BITS_BIG_ENDIAN:

TreeToLLVM::EmitLoadOfLValue()
TreeToLLVM::EmitMODIFY_EXPR()
InsertBitFieldValue()
ProcessBitFieldInitialization()

The comments say things like:

    // If this target has bitfields laid out in big-endian order, invert the bit
    // in the word if needed.

        // If this is a big-endian bit-field, take the top NumBitsToInsert
        // bits from the bitfield value.

        // If this is little-endian bit-field, take the bottom NumBitsToInsert
        // bits from the bitfield value.

But as I understand it in GCC, BITS_BIG_ENDIAN has nothing to do with
the way bit-fields are laid out. All it affects is how the operands of
the sign_extract and zero_extract RTXs are interpreted. This is what
the GCC internals manual says, and I believe it's true from looking at
the GCC source code.

So can anyone explain why llvm-gcc depends on BITS_BIG_ENDIAN in this way?

Thanks,
Jay.