[llvm] r321402 - [SCCP] Manually fold branches on undef.

[Davide Italiano via llvm-commits]

The problem is likely elsewhere, I'll revert this and commit a proper
fix once I get access to my workstation (in ~24 hours).

On Sat, Dec 23, 2017 at 4:06 PM, Davide Italiano via llvm-commits
<llvm-commits at lists.llvm.org> wrote:
> Author: davide
> Date: Sat Dec 23 07:06:30 2017
> New Revision: 321402
>
> URL: http://llvm.org/viewvc/llvm-project?rev=321402&view=rev
> Log:
> [SCCP] Manually fold branches on undef.
>
> This code was originally removed and replace with an assertion
> because believed unnecessary. It turns out there was simply
> no test coverage for this case, and the constant folder doesn't
> yet know about patterns like `br undef %label1, %label2`.
> Presumably at some point the constant folder might learn about
> these patterns, but it's a broader change.
> A testcase will be added to make sure this doesn't regress again
> in the future.
>
> Fixes PR35723.
>
> Modified:
>     llvm/trunk/lib/Transforms/Scalar/SCCP.cpp
>
> Modified: llvm/trunk/lib/Transforms/Scalar/SCCP.cpp
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/SCCP.cpp?rev=321402&r1=321401&r2=321402&view=diff
> ==============================================================================
> --- llvm/trunk/lib/Transforms/Scalar/SCCP.cpp (original)
> +++ llvm/trunk/lib/Transforms/Scalar/SCCP.cpp Sat Dec 23 07:06:30 2017
> @@ -1929,9 +1929,32 @@ static bool runIPSCCP(Module &M, const D
>          if (!I) continue;
>
>          bool Folded = ConstantFoldTerminator(I->getParent());
> -        assert(Folded &&
> -              "Expect TermInst on constantint or blockaddress to be folded");
> -        (void) Folded;
> +        if (!Folded) {
> +          // The constant folder may not have been able to fold the terminator
> +          // if this is a branch or switch on undef.  Fold it manually as a
> +          // branch to the first successor.
> +#ifndef NDEBUG
> +          if (auto *BI = dyn_cast<BranchInst>(I)) {
> +            assert(BI->isConditional() && isa<UndefValue>(BI->getCondition()) &&
> +                   "Branch should be foldable!");
> +          } else if (auto *SI = dyn_cast<SwitchInst>(I)) {
> +            assert(isa<UndefValue>(SI->getCondition()) && "Switch should fold");
> +          } else {
> +            llvm_unreachable("Didn't fold away reference to block!");
> +          }
> +#endif
> +
> +          // Make this an uncond branch to the first successor.
> +          TerminatorInst *TI = I->getParent()->getTerminator();
> +          BranchInst::Create(TI->getSuccessor(0), TI);
> +
> +          // Remove entries in successor phi nodes to remove edges.
> +          for (unsigned i = 1, e = TI->getNumSuccessors(); i != e; ++i)
> +            TI->getSuccessor(i)->removePredecessor(TI->getParent());
> +
> +          // Remove the old terminator.
> +          TI->eraseFromParent();
> +        }
>        }
>
>        // Finally, delete the basic block.
>
>
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-- 
Davide

"There are no solved problems; there are only problems that are more
or less solved" -- Henri Poincare