[LLVMdev] Clobbering llvm.eh.selector return

Duncan Sands baldrick at free.fr
Fri Feb 5 16:35:53 PST 2010

Hi Garrison,

> Funny I remember that code (DwarfEHPrepare), wondering at the time why it was doing this. Thanks for your response,
> as I can now stop trying figure out how I am introducing this error, and more importantly now will be able to go back to
> DwarfEHPrepare, and understand it better knowing one of its purposes. However the result of my tests indicates that
> the underlying register used by the exception/unwind (get/setGR(built in reg. index 1)), mechanism is shared for other uses.
> If this is true, what criteria is used to identify whether this register is going to be used. I can image future efforts directed toward
> changing or adding to the exception mechanism needing to know what to avoid before the passes that actually lower the graphs 
> into machine/assembly code have been executed.

it doesn't matter what else it is used for, since at the start of every landing
pad you can store the register to a stack variable (or, for bonus points, place
it in a virtual register), and replace each call to eh.exception with a load of
the stack variable.  It then doesn't matter if the register gets used for other
things too.  This is an essence what DwarfEHPrepare does for eh.exception.



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