Improve lookThroughCast function.

Summary:
When we have the following case:

  %cond = cmp iN %x, CmpConst
  %tr = trunc iN %x to iK
  %narrowsel = select i1 %cond, iK %t, iK C

We could possibly match only min/max pattern after looking through cast.
So it is more profitable if widened C constant will be equal CmpConst.
That is why just set widened C constant equal to CmpConst, because there
is a further check in this function that trunc CmpConst == C.

Also description for lookTroughCast function was added.

Reviewers: spatel

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D38536

Patch by: Artur Gainullin <artur.gainullin@intel.com>

llvm-svn: 316070

1 files changed, 41 insertions, 1 deletions

diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index f70f2d1..55a7a35 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -4320,6 +4320,20 @@ static SelectPatternResult matchSelectPattern(CmpInst::Predicate Pred,
   return matchFastFloatClamp(Pred, CmpLHS, CmpRHS, TrueVal, FalseVal, LHS, RHS);
 }
 
+/// Helps to match a select pattern in case of a type mismatch.
+///
+/// The function processes the case when type of true and false values of a
+/// select instruction differs from type of the cmp instruction operands because
+/// of a cast instructon. The function checks if it is legal to move the cast
+/// operation after "select". If yes, it returns the new second value of
+/// "select" (with the assumption that cast is moved):
+/// 1. As operand of cast instruction when both values of "select" are same cast
+/// instructions.
+/// 2. As restored constant (by applying reverse cast operation) when the first
+/// value of the "select" is a cast operation and the second value is a
+/// constant.
+/// NOTE: We return only the new second value because the first value could be
+/// accessed as operand of cast instruction.
 static Value *lookThroughCast(CmpInst *CmpI, Value *V1, Value *V2,
                               Instruction::CastOps *CastOp) {
   auto *Cast1 = dyn_cast<CastInst>(V1);
@@ -4350,7 +4364,33 @@ static Value *lookThroughCast(CmpInst *CmpI, Value *V1, Value *V2,
       CastedTo = ConstantExpr::getTrunc(C, SrcTy, true);
     break;
   case Instruction::Trunc:
-    CastedTo = ConstantExpr::getIntegerCast(C, SrcTy, CmpI->isSigned());
+    Constant *CmpConst;
+    if (match(CmpI->getOperand(1), m_Constant(CmpConst))) {
+      // Here we have the following case:
+      //
+      //   %cond = cmp iN %x, CmpConst
+      //   %tr = trunc iN %x to iK
+      //   %narrowsel = select i1 %cond, iK %t, iK C
+      //
+      // We can always move trunc after select operation:
+      //
+      //   %cond = cmp iN %x, CmpConst
+      //   %widesel = select i1 %cond, iN %x, iN CmpConst
+      //   %tr = trunc iN %widesel to iK
+      //
+      // Note that C could be extended in any way because we don't care about
+      // upper bits after truncation. It can't be abs pattern, because it would
+      // look like:
+      //
+      //   select i1 %cond, x, -x.
+      //
+      // So only min/max pattern could be matched. Such match requires widened C
+      // == CmpConst. That is why set widened C = CmpConst, condition trunc
+      // CmpConst == C is checked below.
+      CastedTo = CmpConst;
+    } else {
+      CastedTo = ConstantExpr::getIntegerCast(C, SrcTy, CmpI->isSigned());
+    }
     break;
   case Instruction::FPTrunc:
     CastedTo = ConstantExpr::getFPExtend(C, SrcTy, true);