[PATCH] D19211: Teach ValueTracking that compare-and-branch on poison is UB

[David Majnemer via llvm-commits]

majnemer added inline comments.

================
Comment at: lib/Analysis/ValueTracking.cpp:3413-3416
@@ -3407,1 +3412,6 @@
 
+    case Instruction::Br:
+      if (cast<BranchInst>(I)->isConditional())
+        return cast<BranchInst>(I)->getCondition();
+      return nullptr;
+
----------------
Our langref states:
> An instruction control-depends on a terminator instruction if the terminator instruction has multiple successors and the instruction is always executed when control transfers to one of the successors, and may not be executed when control is transferred to another.

The code, as written, sounds like it will do the wrong thing if the branch instruction branches to the same basic block regardless of the condition operand.  Maybe this is just a bug in the langref?  Equally possible is that I misunderstand the behavior that this function is trying to suss out.


http://reviews.llvm.org/D19211