About the partial update clearence / dependency breaking mechanism


I am currently looking into the advantages of using the
partial update clearance / dependency breaking mechanism
for some ARM cores.

It seems that the ARM specific code for this will always
return a clearance of 0 for VLD1LNd32 because of the following
code in getPartialRegUpdateClearance:

if (UseOp != -1 && MI->getOperand(UseOp).readsReg())
   return 0;

so essentially VLD1LNd32 (and potentially other instruction)
will never be affected by this. Was this intended or is there
a bug here?

I'm confused why the dependency breaking code is correct.
Why would the dependency breaking mechanism apply only
when the register is dead?


It's breaking *false* dependencies.


Hi Jakob,

Thanks for the reply. I misunderstood the code there, it now
makes sense.