On Dec 11, 2007, at 1:25 AM, Duncan Sands wrote:
> Hi Chris,
>
>> Duncan, this patch gets llvm-gcc to build, but it still doesn't
>> answer the big question: why does TargetData contain information
>> about the host?
>
> well, it needed to go somewhere. Any suggestions for a better place?
> Another possibility is to have a function isWrongEndian in target
> data,
> which tells you if the target has opposite endianness to the host.
How about a static function in ExecutionEngine.cpp that takes a
TargetData& ?
>> I don't think this is the right approach. Also, you
>> can get the host endianness without autoconf by using something
>> simple like:
>>
>> bool islittleendian() {
>> union {
>> int i;
>> char c;
>> };
>>
>> i = 1;
>> return c;
>> }
>
> I have no particular opinion on this (except that it's amazing to me
> that there's no standard way to query the compiler for this in C+
> + :) ).
Ah, but no "well defined" code should depend on endianness. :) Ah
the differences in reality between standards and actual usage.
> Presumably the compiler will reduce this function to: return true;
> (or false)
> at compile time, so there's no cost to your approach.
Right.
> I suppose that plenty
> of things that autoconf does could also be done in a special
> function like this,
> yet they are left to autoconf - I have no idea what the criteria
> are for doing
> something via autoconf or via a runtime test.
A lot of things can't be done that way, for example finding the path
to an external tool, checking to see if your assembler has some
feature (e.g. .cfi support), etc.
> Having the function inline in a
> header would have the advantage that the compiler would eliminate a
> bunch of code
> in ExecutionEngine at runtime. Since it seems that the autoconf
> solution is not
> compatible with being exposed in a header, that suggests your
> solution is the way
> to go.
Thanks!
-Chris
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