[LLVMdev] Any extend

Greg Chadwick gac43 at cam.ac.uk
Tue Jan 26 06:22:29 PST 2010

Duncan Sands wrote:
> Hi Greg,
>> 1) What causes the Initial selection DAG code to choose an any_extend 
>> over a sign_extend (or zero_extend)?
> because it is more efficient: the backend gets more choice in how to do
> it, and at the same time it tells the optimizers that the extra bits
> contain rubbish, which gives them more freedom to reason.
Makes sense, though I was wondering why it would choose to sign_extend 
an 8-bit or 16-bit value, but any_extend a 32-bit value (These are all 
signed values).
>> 2) What does any_extend actually signify?  Presumably this indicates 
>> that either a sign extend or zero extend will suffice.
> Yes, or anything else.

Anyway thanks for the info, I'll just implement an any_extend load (The 
original MIPS backend doesn't seem to have any, which is why I was wary 
of doing it in the first place).



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