This patch fixes pr48832 by correctly generating the mask when a poison value is involved.
Consider this CFG (which is a part of the input):
for.body: ; preds = %for.cond br i1 true, label %cond.false, label %land.rhs land.rhs: ; preds = %for.body br i1 poison, label %cond.end, label %cond.false cond.false: ; preds = %for.body, %land.rhs br label %cond.end cond.end: ; preds = %land.rhs, %cond.false %cond = phi i32 [ 0, %cond.false ], [ 1, %land.rhs ]
The path for.body -> land.rhs -> cond.end should be taken when 'select i1 false, i1 poison, i1 false' holds (which means it's never taken); but VPRecipeBuilder::createEdgeMask was emitting 'and i1 false, poison' instead.
The former one successfully blocks poison propagation whereas the latter one doesn't, making the condition poison and thus causing the miscompilation.
SimplifyCFG has a similar bug (which didn't expose a real-world bug yet), and a patch for this is also ongoing (see https://reviews.llvm.org/D95026).