[llvm-dev] Understanding SlotIndexes

via llvm-dev llvm-dev at lists.llvm.org
Thu Feb 23 19:01:45 PST 2017

Krzysztof Parzyszek via llvm-dev <llvm-dev at lists.llvm.org> writes:

>> 4B  A = ...
>> 10B D = use A
>> This seems straightforward.  The instruction at slot 10 uses A at 10r
>> and the VNInfo for 10r should have a def at 4r.
>> But that's not right!  I am staring at the following:
>> LiveInterval: %vreg163 [5404r,6432B:0)[6480B,6688r:0)  0 at 5404r
>> Instruction:  6688B %RDX<def> = COPY %vreg163
> An instruction by itself doesn't have any meaningful slot.  It shows
> as B, but only because B is the default slot. For a given register you
> get the actual slot from the live interval.

Which slot?  In this case I'm staring at a use of vreg163 and want to
find the def of the value reaching that use.  6688r is *not* in the
interval.  Thus I can't look up 6688r in the interval and get the def
from the VNInfo.  If I look up 6688B I get the expected answer.  Thus
the question.

Again, LVQuery seems to do what I want.  But the comments in
LiveInterval.h seem to be misleading at best.


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