From 0c73079855de925bf08335fe39b0b112f3d9407a Mon Sep 17 00:00:00 2001
From: Nick Lewycky <nicholas@mxc.ca>
Date: Fri, 21 Nov 2008 07:33:58 +0000
Subject: [PATCH] Optimize (x/y)*y into x-(x%y) in general. Div and rem are
 about the same, and a subtract is cheaper than a multiply. This generalizes
 an existing transform.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@59800 91177308-0d34-0410-b5e6-96231b3b80d8
---
 .../Scalar/InstructionCombining.cpp           | 45 ++++++++++++++-----
 .../InstCombine/2008-11-20-DivMulRem.ll       | 34 ++++++++++++++
 2 files changed, 68 insertions(+), 11 deletions(-)
 create mode 100644 test/Transforms/InstCombine/2008-11-20-DivMulRem.ll

diff --git a/lib/Transforms/Scalar/InstructionCombining.cpp b/lib/Transforms/Scalar/InstructionCombining.cpp
index e7d596773f7..ee6b51cf296 100644
--- a/lib/Transforms/Scalar/InstructionCombining.cpp
+++ b/lib/Transforms/Scalar/InstructionCombining.cpp
@@ -2477,17 +2477,6 @@ Instruction *InstCombiner::visitSub(BinaryOperator &I) {
         Constant *CP1 = Subtract(ConstantInt::get(I.getType(), 1), C2);
         return BinaryOperator::CreateMul(Op0, CP1);
       }
-
-      // X - ((X / Y) * Y) --> X % Y
-      if (Op1I->getOpcode() == Instruction::Mul)
-        if (Instruction *I = dyn_cast<Instruction>(Op1I->getOperand(0)))
-          if (Op0 == I->getOperand(0) &&
-              Op1I->getOperand(1) == I->getOperand(1)) {
-            if (I->getOpcode() == Instruction::SDiv)
-              return BinaryOperator::CreateSRem(Op0, Op1I->getOperand(1));
-            if (I->getOpcode() == Instruction::UDiv)
-              return BinaryOperator::CreateURem(Op0, Op1I->getOperand(1));
-          }
     }
   }
 
@@ -2622,6 +2611,40 @@ Instruction *InstCombiner::visitMul(BinaryOperator &I) {
     if (Value *Op1v = dyn_castNegVal(I.getOperand(1)))
       return BinaryOperator::CreateMul(Op0v, Op1v);
 
+  // (X / Y) *  Y = X - (X % Y)
+  // (X / Y) * -Y = (X % Y) - X
+  {
+    Value *Op1 = I.getOperand(1);
+    BinaryOperator *BO = dyn_cast<BinaryOperator>(Op0);
+    if (!BO ||
+        (BO->getOpcode() != Instruction::UDiv && 
+         BO->getOpcode() != Instruction::SDiv)) {
+      Op1 = Op0;
+      BO = dyn_cast<BinaryOperator>(I.getOperand(1));
+    }
+    Value *Neg = dyn_castNegVal(Op1);
+    if (BO && BO->hasOneUse() &&
+        (BO->getOperand(1) == Op1 || BO->getOperand(1) == Neg) &&
+        (BO->getOpcode() == Instruction::UDiv ||
+         BO->getOpcode() == Instruction::SDiv)) {
+      Value *Op0BO = BO->getOperand(0), *Op1BO = BO->getOperand(1);
+
+      Instruction *Rem;
+      if (BO->getOpcode() == Instruction::UDiv)
+        Rem = BinaryOperator::CreateURem(Op0BO, Op1BO);
+      else
+        Rem = BinaryOperator::CreateSRem(Op0BO, Op1BO);
+
+      InsertNewInstBefore(Rem, I);
+      Rem->takeName(BO);
+
+      if (Op1BO == Op1)
+        return BinaryOperator::CreateSub(Op0BO, Rem);
+      else
+        return BinaryOperator::CreateSub(Rem, Op0BO);
+    }
+  }
+
   if (I.getType() == Type::Int1Ty)
     return BinaryOperator::CreateAnd(Op0, I.getOperand(1));
 
diff --git a/test/Transforms/InstCombine/2008-11-20-DivMulRem.ll b/test/Transforms/InstCombine/2008-11-20-DivMulRem.ll
new file mode 100644
index 00000000000..8c58a2ae7f7
--- /dev/null
+++ b/test/Transforms/InstCombine/2008-11-20-DivMulRem.ll
@@ -0,0 +1,34 @@
+; RUN: llvm-as < %s | opt -instcombine | llvm-dis > %t
+; RUN: grep urem %t | count 3
+; RUN: grep srem %t | count 1
+; RUN: grep sub %t | count 2
+; RUN: grep add %t | count 1
+; PR3103
+
+define i8 @test1(i8 %x, i8 %y) {
+  %A = udiv i8 %x, %y
+  %B = mul i8 %A, %y
+  %C = sub i8 %x, %B
+  ret i8 %C
+}
+
+define i8 @test2(i8 %x, i8 %y) {
+  %A = sdiv i8 %x, %y
+  %B = mul i8 %A, %y
+  %C = sub i8 %x, %B
+  ret i8 %C
+}
+
+define i8 @test3(i8 %x, i8 %y) {
+  %A = udiv i8 %x, %y
+  %B = mul i8 %A, %y
+  %C = sub i8 %B, %x
+  ret i8 %C
+}
+
+define i8 @test4(i8 %x) {
+  %A = udiv i8 %x, 3
+  %B = mul i8 %A, -3
+  %C = sub i8 %x, %B
+  ret i8 %C
+}