[LLVMdev] Custom lowering ADDE?

Duncan Sands duncan.sands at math.u-psud.fr
Fri Nov 7 06:16:39 PST 2008


> Why can't I custom lower ADDE operation for legal types?

probably because no-one needed it.  Feel free to add it!

> Our target does not have an add with carry instruction. We check the
> carry bit in status register and add 1 to the result if it is set.

It would also be nice (if less efficient) to have a generic code
expansion of ADDE that uses booleans (i1) rather than a flag.  A
variation is to eliminate flags at this level, using only booleans,
and have them be synthesized out of boolean later, eg: automatically
understanding that the carry bit can be used for the boolean.  Maybe
this is crazy, I don't know!  If it worked it means that your custom
expansion would no longer be useful, since it would happen 
automatically if you set the operation for ADDE to expand.



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