[LLVMbugs] [Bug 4920] New: Document undef semantics

bugzilla-daemon at cs.uiuc.edu bugzilla-daemon at cs.uiuc.edu
Mon Sep 7 14:00:48 PDT 2009


           Summary: Document undef semantics
           Product: Documentation
           Version: trunk
          Platform: PC
        OS/Version: Linux
            Status: NEW
          Severity: normal
          Priority: P2
         Component: General docs
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: edwintorok at gmail.com
                CC: llvmbugs at cs.uiuc.edu

LangRef should document undef better, some thoughts from a discussion on IRC:

x = undef
a = xor x, x
What is the value of x?

x = undef
i1 a = icmp x, 4
i2 b = icmp x,5

Both a and b can be true here

select undef,a,b -> either a or b
br undef, L1, L2 -> choose either br L1 or br L2, not unreachable

undef is a value in the range of the type, but it can change its value during
its lifetime.

Does using an 'undef' value destroy SSA properties of the program when you RAUW
a value with undef? (previously it was a single value, and optimizers could
rely on this, after the replace it can become different values at different

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