[InstSimplify] fold select with implied condition

This is an almost direct move of the functionality from InstCombine to 
InstSimplify. There's no reason not to do this in InstSimplify because 
we never create a new value with this transform.

(There's a question of whether any dominance-based transform belongs in
either of these passes, but that's a separate issue.)

I've changed 1 of the conditions for the fold (1 of the blocks for the 
branch must be the block we started with) into an assert because I'm not 
sure how that could ever be false.

We need 1 extra check to make sure that the instruction itself is in a
basic block because passes other than InstCombine may be using InstSimplify
as an analysis on values that are not wired up yet.

The 3-way compare changes show that InstCombine has some kind of 
phase-ordering hole. Otherwise, we would have already gotten the intended
final result that we now show here.

llvm-svn: 347896

1 files changed, 0 insertions, 18 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
index db75c9383a1..19858ae149a 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
@@ -2021,24 +2021,6 @@ Instruction *InstCombiner::visitSelectInst(SelectInst &SI) {
     }
   }
 
-  // See if we can determine the result of this select based on a dominating
-  // condition.
-  BasicBlock *Parent = SI.getParent();
-  if (BasicBlock *Dom = Parent->getSinglePredecessor()) {
-    auto *PBI = dyn_cast_or_null<BranchInst>(Dom->getTerminator());
-    if (PBI && PBI->isConditional() &&
-        PBI->getSuccessor(0) != PBI->getSuccessor(1) &&
-        (PBI->getSuccessor(0) == Parent || PBI->getSuccessor(1) == Parent)) {
-      bool CondIsTrue = PBI->getSuccessor(0) == Parent;
-      Optional<bool> Implication = isImpliedCondition(
-          PBI->getCondition(), SI.getCondition(), DL, CondIsTrue);
-      if (Implication) {
-        Value *V = *Implication ? TrueVal : FalseVal;
-        return replaceInstUsesWith(SI, V);
-      }
-    }
-  }
-
   // If we can compute the condition, there's no need for a select.
   // Like the above fold, we are attempting to reduce compile-time cost by
   // putting this fold here with limitations rather than in InstSimplify.