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

[David Chisnall]

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.  

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).

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
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