[llvm-commits] [llvm] r123372 - in /llvm/trunk: lib/Analysis/InstructionSimplify.cpp test/Transforms/InstSimplify/2010-12-20-I1Arithmetic.ll

[Frits van Bommel]

On Thu, Jan 13, 2011 at 9:56 AM, Duncan Sands <baldrick at free.fr> wrote:
> +  // icmp <global/alloca*/null>, <global/alloca*/null> - Global/Stack value
> +  // addresses never equal each other!  We already know that Op0 != Op1.
> +  if ((isa<GlobalValue>(LHS) || isa<AllocaInst>(LHS) ||
> +       isa<ConstantPointerNull>(LHS)) &&
> +      (isa<GlobalValue>(RHS) || isa<AllocaInst>(RHS) ||
> +       isa<ConstantPointerNull>(RHS)))
> +    return ConstantInt::get(ITy, CmpInst::isFalseWhenEqual(Pred));

This is pre-existing code you only moved, but it looks buggy: some
globals may be null. In particular, 'extern_weak' declarations may be
null at run time if no definition is ever linked in. I guess it
probably dates from before extern_weak was introduced (possibly by way
of -instcombine).

Luckily, after trying it out, it seems that those cases are caught by
constant folding (which seems to do the right thing) before this code
is reached. Still, a note in the comment that this code depends on
that might be in order.