From a40a3a59818d1fe394f23cda1df940762e30ea65 Mon Sep 17 00:00:00 2001
From: David Majnemer <david.majnemer@gmail.com>
Date: Thu, 18 Apr 2013 07:30:07 +0000
Subject: Combine bit test + conditional or into simple math

Simplify:
(select (icmp eq (and X, C1), 0), Y, (or Y, C2))

Into:
(or (shl (and X, C1), C3), y)

Where:
C3 = Log(C2) - Log(C1)

If:
C1 and C2 are both powers of two


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179748 91177308-0d34-0410-b5e6-96231b3b80d8
---
 lib/Transforms/InstCombine/InstCombineSelect.cpp | 61 ++++++++++++++++++++++++
 1 file changed, 61 insertions(+)

(limited to 'lib/Transforms/InstCombine')

diff --git a/lib/Transforms/InstCombine/InstCombineSelect.cpp b/lib/Transforms/InstCombine/InstCombineSelect.cpp
index f0738b48cb..f5a33bad29 100644
--- a/lib/Transforms/InstCombine/InstCombineSelect.cpp
+++ b/lib/Transforms/InstCombine/InstCombineSelect.cpp
@@ -350,6 +350,64 @@ static Value *SimplifyWithOpReplaced(Value *V, Value *Op, Value *RepOp,
   return 0;
 }
 
+/// foldSelectICmpAndOr - We want to turn:
+///   (select (icmp eq (and X, C1), 0), Y, (or Y, C2))
+/// into:
+///   (or (shl (and X, C1), C3), y)
+/// iff:
+///   C1 and C2 are both powers of 2
+/// where:
+///   C3 = Log(C2) - Log(C1)
+///
+/// This transform handles cases where:
+/// 1. The icmp predicate is inverted
+/// 2. The select operands are reversed
+/// 3. The magnitude of C2 and C1 are flipped
+static Value *foldSelectICmpAndOr(const SelectInst &SI, Value *TrueVal,
+                                  Value *FalseVal,
+                                  InstCombiner::BuilderTy *Builder) {
+  const ICmpInst *IC = dyn_cast<ICmpInst>(SI.getCondition());
+  if (!IC || !IC->isEquality())
+    return 0;
+
+  Value *CmpLHS = IC->getOperand(0);
+  Value *CmpRHS = IC->getOperand(1);
+
+  if (!match(CmpRHS, m_Zero()))
+    return 0;
+
+  Value *X;
+  const APInt *C1;
+  if (!match(CmpLHS, m_And(m_Value(X), m_Power2(C1))))
+    return 0;
+
+  const APInt *C2;
+  bool OrOnTrueVal = false;
+  bool OrOnFalseVal = match(FalseVal, m_Or(m_Specific(TrueVal), m_Power2(C2)));
+  if (!OrOnFalseVal)
+    OrOnTrueVal = match(TrueVal, m_Or(m_Specific(FalseVal), m_Power2(C2)));
+
+  if (!OrOnFalseVal && !OrOnTrueVal)
+    return 0;
+
+  Value *V = CmpLHS;
+
+  unsigned C1Log = C1->logBase2();
+  unsigned C2Log = C2->logBase2();
+  if (C2Log > C1Log)
+    V = Builder->CreateShl(V, C2Log - C1Log);
+  else if (C1Log > C2Log)
+    V = Builder->CreateLShr(V, C1Log - C2Log);
+
+  ICmpInst::Predicate Pred = IC->getPredicate();
+  if ((Pred == ICmpInst::ICMP_NE && OrOnFalseVal) ||
+      (Pred == ICmpInst::ICMP_EQ && OrOnTrueVal))
+    V = Builder->CreateXor(V, *C2);
+
+  Value *Y = OrOnFalseVal ? TrueVal : FalseVal;
+  return Builder->CreateOr(V, Y);
+}
+
 /// visitSelectInstWithICmp - Visit a SelectInst that has an
 /// ICmpInst as its first operand.
 ///
@@ -521,6 +579,9 @@ Instruction *InstCombiner::visitSelectInstWithICmp(SelectInst &SI,
     }
   }
 
+  if (Value *V = foldSelectICmpAndOr(SI, TrueVal, FalseVal, Builder))
+    return ReplaceInstUsesWith(SI, V);
+
   return Changed ? &SI : 0;
 }
 
-- 
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