[LLVMdev] Question about documentation of BitCast CExps.

[Nick Lewycky]

Jesper Louis Andersen wrote:
> In the documentation at [http://llvm.org/docs/LangRef.html], we have
> the section [#aggregateconstants] telling us that the aggregate
> constants are:
> 
>   * Structure constants
>   * Array constants
>   * Vector constants
>   * Zeroinitialization
> 
> However, in [#constantexprs] we have a bitcast(CST, TYPE) operator
> with the documentation (emphasis by me):
> 
> Convert a constant, CST, to another TYPE. The size of CST and TYPE
> must be identical (same number of bits). The conversion is done as if
> the CST value was stored to memory and read back as TYPE. In other
> words, no bits change with this operator, just the type. {{{This can
> be used for conversion of vector types to any other type, as long as
> they have the same bit width}}}. For pointers it is only valid to cast
> to another pointer type. {{{It is not valid to bitcast to or from an
> aggregate type.}}}

Thanks for the report. I've updated this text to read:

"Convert a constant, CST, to another TYPE. The size of CST and TYPE must 
be identical (same number of bits). The conversion is done as if the CST 
value was stored to memory and read back as TYPE. In other words, no 
bits change with this operator, just the type.  This can be used for 
conversion of aggregate types to any aggregate type, as long as they 
have the same bit width. Vector types may also be casted to and from any 
other type as long as they have the same bit width. For pointers it is 
only valid to cast to another pointer type."

This should make it clear that aggregates may be bitcasted to other 
aggregates, and that additionally vectors may be bitcasted to and from 
other types, but not pointers or labels or functions or void, because 
those will never have the same width as a vector (tricky!).

Nick

> The first emphasis states that it is possible to use the bitcast
> operation for conversion of vector types. However, the second emphasis
> says that this is not valid (A vector is an aggregate type). So my
> questions are:
> 
> What is right?
> Is this a mistake?
> Am I reading this wrong in any way? If so, where?
> 
> The reason I am asking is because I am in the process of implementing
> a rudimentary type-checker phase for LLVM in Standard ML. It is mostly
> done because it is a tremendous way to learn about the finer details
> of the language. I am forced to think and know details about LLVM
> because of the process, so hence my question.
>