InstCombine: Combine select sequences into a single select

Normalize
select(C0, select(C1, a, b), b) -> select((C0 & C1), a, b)
select(C0, a, select(C1, a, b)) -> select((C0 | C1), a, b)

This normal form may enable further combines on the And/Or and shortens
paths for the values. Many targets prefer the other but can go back
easily in CodeGen.

Differential Revision: http://reviews.llvm.org/D7399

llvm-svn: 228409

1 files changed, 18 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
index 69380fc41d3..0debe63faf4 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
@@ -1178,20 +1178,38 @@ Instruction *InstCombiner::visitSelectInst(SelectInst &SI) {
         return NV;
 
   if (SelectInst *TrueSI = dyn_cast<SelectInst>(TrueVal)) {
+    // select(C, select(C, a, b), c) -> select(C, a, c)
     if (TrueSI->getCondition() == CondVal) {
       if (SI.getTrueValue() == TrueSI->getTrueValue())
         return nullptr;
       SI.setOperand(1, TrueSI->getTrueValue());
       return &SI;
     }
+    // select(C0, select(C1, a, b), b) -> select(C0&C1, a, b)
+    // We choose this as normal form to enable folding on the And and shortening
+    // paths for the values (this helps GetUnderlyingObjects() for example).
+    if (TrueSI->getFalseValue() == FalseVal && TrueSI->hasOneUse()) {
+      Value *And = Builder->CreateAnd(CondVal, TrueSI->getCondition());
+      SI.setOperand(0, And);
+      SI.setOperand(1, TrueSI->getTrueValue());
+      return &SI;
+    }
   }
   if (SelectInst *FalseSI = dyn_cast<SelectInst>(FalseVal)) {
+    // select(C, a, select(C, b, c)) -> select(C, a, c)
     if (FalseSI->getCondition() == CondVal) {
       if (SI.getFalseValue() == FalseSI->getFalseValue())
         return nullptr;
       SI.setOperand(2, FalseSI->getFalseValue());
       return &SI;
     }
+    // select(C0, a, select(C1, a, b)) -> select(C0|C1, a, b)
+    if (FalseSI->getTrueValue() == TrueVal && FalseSI->hasOneUse()) {
+      Value *Or = Builder->CreateOr(CondVal, FalseSI->getCondition());
+      SI.setOperand(0, Or);
+      SI.setOperand(2, FalseSI->getFalseValue());
+      return &SI;
+    }
   }
 
   if (BinaryOperator::isNot(CondVal)) {