[llvm-dev] Semantics of udef values in PHI instructions

[Charitha Saumya via llvm-dev]

Hi,

Consider the following LLVM IR snippet. (also attached)

the value of %unify.phi is undefined if the loop body (while.body9)  is
executed, otherwise it has some specific value (%c.1 is a defined value).

However when I execute this IR (using lli), the value of %c.2 is always the
value of %c.1, even if the loop is executed multiple times.

What is the reason for this behavior?  how undefined values are handled in
LLVM and is this behaviour architecture dependent (depending on how undef
is handled in Codegen) ?

Thanks
Charitha

[image: Screen Shot 2021-08-14 at 9.36.28 PM.png]
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