[llvm-commits] [llvm] r125319 - in /llvm/trunk: lib/Analysis/ValueTracking.cpp test/Transforms/JumpThreading/degenerate-phi.ll

[Duncan Sands]

Hi Nick,

> --- llvm/trunk/lib/Analysis/ValueTracking.cpp (original)
> +++ llvm/trunk/lib/Analysis/ValueTracking.cpp Thu Feb 10 17:54:10 2011
> @@ -590,6 +590,10 @@
>         }
>       }
>
> +    // Unreachable blocks may have zero-operand PHI nodes.
> +    if (P->getNumIncomingValues() == 0)
> +      return;

the LangRef says that zero operand phis are not allowed.  I'm pretty sure that
there is some code in InstructionSimplify (which is calling ValueTracking here)
which assumes that phi nodes have at least one operand.  Who knows where else
this assumption is made?  I think it would be better to have jump-threading zap
any zero operand phis when it makes them, rather than changing all analyses to
support them.

Ciao, Duncan.

> +
>       // Otherwise take the unions of the known bit sets of the operands,
>       // taking conservative care to avoid excessive recursion.
>       if (Depth<  MaxDepth - 1&&  !KnownZero&&  !KnownOne) {
>
> Added: llvm/trunk/test/Transforms/JumpThreading/degenerate-phi.ll
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/JumpThreading/degenerate-phi.ll?rev=125319&view=auto
> ==============================================================================
> --- llvm/trunk/test/Transforms/JumpThreading/degenerate-phi.ll (added)
> +++ llvm/trunk/test/Transforms/JumpThreading/degenerate-phi.ll Thu Feb 10 17:54:10 2011
> @@ -0,0 +1,24 @@
> +; RUN: opt -jump-threading -disable-output %s
> +; PR9112
> +
> +; This is actually a test for value tracking. Jump threading produces
> +; "%phi = phi i16" when it removes all edges leading to %unreachable.
> +; The .ll parser won't let us write that directly since it's invalid code.
> +
> +define void @func() nounwind {
> +entry:
> +  br label %bb
> +
> +bb:
> +  br label %bb
> +
> +unreachable:
> +  %phi = phi i16 [ %add, %unreachable ], [ 0, %next ]
> +  %add = add i16 0, %phi
> +  %cmp = icmp slt i16 %phi, 0
> +  br i1 %cmp, label %unreachable, label %next
> +
> +next:
> +  br label %unreachable
> +}
> +
>
>
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