[llvm-dev] Is a PHI use of another PHI in the same block valid?

Philip Reames via llvm-dev llvm-dev at lists.llvm.org
Fri Feb 26 09:02:26 PST 2016

Over in pr26718, we ran across a case where input IR had one PHI within 
a basic block using the value of another PHI within the same basic block 
(without a backedge).  There has been some disagreement as to whether 
this is valid IR.  I believe it is not.

The verifier currently accepts the following code without error:

define void @f() {
   br label %next

   %y = phi i32 [ 0, %entry ]
   %x = phi i32 [ %y, %entry ]
   ret void

Looking at the code, this may be an oversight - due to a special casing 
of the dominance relation within a single basic block - but that's not 
/obviously/ true.  Thus, we need to clarify what the intended semantics are.

The lang ref seems pretty clear about this:
"For the purposes of the SSA form, the use of each incoming value is 
deemed to occur on the edge from the corresponding predecessor block to 
the current block (but after any definition of an ‘invoke‘ instruction’s 
return value on the same edge)."

But David pointed out that various bits of the optimizer appear to have 
code and test cases covering exactly this case.

So, is this legal IR?  Or not?


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