From 1895e98ef38afbe011575cc25f4889d96e37421b Mon Sep 17 00:00:00 2001
From: Duncan Sands <baldrick@free.fr>
Date: Sun, 30 Jan 2011 18:03:50 +0000
Subject: [PATCH] Transform (X/Y)*Y into X if the division is exact. 
 Instcombine already knows how to do this and more, but would only do it if
 X/Y had only one use.  Spotted as the most common missed simplification in
 SPEC by my auto-simplifier, now that it knows about nuw/nsw/exact flags. 
 This removes a bunch of multiplications from 447.dealII and 483.xalancbmk. 
 It also removes a lot from tramp3d-v4, which results in much more inlining.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@124560 91177308-0d34-0410-b5e6-96231b3b80d8
---
 lib/Analysis/InstructionSimplify.cpp           |  9 +++++++++
 .../InstSimplify/2010-12-20-Reassociate.ll     | 18 ++++++++++++++++++
 2 files changed, 27 insertions(+)

diff --git a/lib/Analysis/InstructionSimplify.cpp b/lib/Analysis/InstructionSimplify.cpp
index cf50668cb01..fcbc6d50eeb 100644
--- a/lib/Analysis/InstructionSimplify.cpp
+++ b/lib/Analysis/InstructionSimplify.cpp
@@ -710,6 +710,15 @@ static Value *SimplifyMulInst(Value *Op0, Value *Op1, const TargetData *TD,
   if (match(Op1, m_One()))
     return Op0;
 
+  // (X / Y) * Y -> X if the division is exact.
+  Value *X = 0, *Y = 0;
+  if ((match(Op0, m_SDiv(m_Value(X), m_Value(Y))) && Y == Op1) || // (X / Y) * Y
+      (match(Op1, m_SDiv(m_Value(X), m_Value(Y))) && Y == Op0)) { // Y * (X / Y)
+    BinaryOperator *SDiv = cast<BinaryOperator>(Y == Op1 ? Op0 : Op1);
+    if (SDiv->isExact())
+      return X;
+  }
+
   // i1 mul -> and.
   if (MaxRecurse && Op0->getType()->isIntegerTy(1))
     if (Value *V = SimplifyAndInst(Op0, Op1, TD, DT, MaxRecurse-1))
diff --git a/test/Transforms/InstSimplify/2010-12-20-Reassociate.ll b/test/Transforms/InstSimplify/2010-12-20-Reassociate.ll
index 13724669d7f..928442ac56c 100644
--- a/test/Transforms/InstSimplify/2010-12-20-Reassociate.ll
+++ b/test/Transforms/InstSimplify/2010-12-20-Reassociate.ll
@@ -119,6 +119,24 @@ define i32 @sdiv3(i32 %x, i32 %y) {
 ; CHECK: ret i32 0
 }
 
+define i32 @sdiv4(i32 %x, i32 %y) {
+; CHECK: @sdiv4
+; (X / Y) * Y -> X if the division is exact
+  %div = sdiv exact i32 %x, %y
+  %mul = mul i32 %div, %y
+  ret i32 %mul
+; CHECK: ret i32 %x
+}
+
+define i32 @sdiv5(i32 %x, i32 %y) {
+; CHECK: @sdiv5
+; Y * (X / Y) -> X if the division is exact
+  %div = sdiv exact i32 %x, %y
+  %mul = mul i32 %y, %div
+  ret i32 %mul
+; CHECK: ret i32 %x
+}
+
 define i32 @udiv1(i32 %x, i32 %y) {
 ; CHECK: @udiv1
 ; (no overflow X * Y) / Y -> X