[llvm-dev] how LLVM deals with 'undef'

Xiangyang Guo via llvm-dev llvm-dev at lists.llvm.org
Tue Aug 25 12:09:28 PDT 2015


I'm using LLVM-3.8 (current SVN version) to optimize my code. After loop
unrolling, I find there is 'undef' in the IR. For example, "*%.lcssa21.unr
= phi double [ undef, %.lr.ph9 ], [ %13, %7 ]*". I have two questions:

(1) Why this 'undef' is generated?  Because before 'loop unrolling', there
is no such 'undef'.
(2) How LLVM backend deals with this 'undef'?

Any suggestion is appreciable.


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