[LLVMdev] Memcpy / Memset for address spaces >= 256

[Manuel Jacob]

Hi David,

sorry for sending you the mail two times, I forgot to send to the list 
the first time.

On 2014-03-12 09:48, David Chisnall wrote:
> I have some patches that automatically expand all memcpy and similar
> if the operands are not in AS 0.  I think this is probably not quite
> the right approach though, and we should be asking the back end for
> the function that does a memcpy / memset / whatever in a non-0 address
> space, and expand automatically if it doesn't provide one.

Can you share these patches?  This would be a tentative solution for the 
reporter of the bug I linked in the original post.

> In an ideal world, I'd rather have the memcpy / memset lowering moved
> entirely out of SelectionDAG and into a FunctionPass, where it would
> be much easier to debug.  I'd also want to do the same for lowering of
> unaligned loads / stores, so by the time you get to the back end every
> load and store is something that can map trivially to a single
> instruction (assuming an adequate addressing mode exists).

While I agree that the memcpy lowering pass could be done as an IR pass 
because it involves loops, I don't think you should do that for lowering 
of unaligned loads / stores.  But that's mostly unrelated to this thread 
and should be discussed separately.

There are still some advantages of lowering the memcpy / memset in 
SelectionDAGBuilder.  The infrastructure (e.g. target hooks for 
determining the right register class for memory operations) is already 
there.  I don't know how hard it is to generate loops in 
SelectionDAGBuilder, though.

-Manuel

> David
> 
> On 11 Mar 2014, at 22:23, Manuel Jacob <me at manueljacob.de> wrote:
> 
>> Hi,
>> 
>> SelectionDAGBuilder doesn't know how to lower a Memcpy and Memset if 
>> one of the pointer operands have an address space >= 256.  This is 
>> understandable since the libc's memcpy / memset don't work for these 
>> address spaces.  However, both Clang (when copying a struct) and some 
>> optimization passes (LoopIdiomRecognize, MemCpyOpt) can emit memcpy / 
>> memset for these address spaces.  This triggers an assert in 
>> SelectionDAGBuilder.  The optimization passes could be modified to 
>> give up when they encounter an address space >= 256, but I think clang 
>> would need some new code that emits a struct copy member-by-member.  I 
>> think it's better to extend the code generator to be able to emit code 
>> for that.  What do you think?
>> 
>> The problem is also described here: 
>> http://llvm.org/bugs/show_bug.cgi?id=18549
>> 
>> -Manuel