From e96d8395f457f3b8ac0266489f199b32db24c63e Mon Sep 17 00:00:00 2001
From: Craig Topper <craig.topper@gmail.com>
Date: Fri, 12 May 2017 21:45:50 +0000
Subject: [PATCH] [APInt] Add early outs for a division by 1 to
 udiv/urem/udivrem

We already counted the number of bits in the RHS so its pretty cheap to just check if the RHS is 1.

Differential Revision: https://reviews.llvm.org/D33154

llvm-svn: 302953
---
 lib/Support/APInt.cpp | 22 ++++++++++++++++++----
 1 file changed, 18 insertions(+), 4 deletions(-)

diff --git a/lib/Support/APInt.cpp b/lib/Support/APInt.cpp
index 25d39771a08..369ded7cf14 100644
--- a/lib/Support/APInt.cpp
+++ b/lib/Support/APInt.cpp
@@ -1565,14 +1565,18 @@ APInt APInt::udiv(const APInt& RHS) const {
   }
 
   // Get some facts about the LHS and RHS number of bits and words
-  unsigned rhsWords = getNumWords(RHS.getActiveBits());
-  assert(rhsWords && "Divided by zero???");
   unsigned lhsWords = getNumWords(getActiveBits());
+  unsigned rhsBits  = RHS.getActiveBits();
+  unsigned rhsWords = getNumWords(rhsBits);
+  assert(rhsWords && "Divided by zero???");
 
   // Deal with some degenerate cases
   if (!lhsWords)
     // 0 / X ===> 0
     return APInt(BitWidth, 0);
+  if (rhsBits == 1)
+    // X / 1 ===> X
+    return *this;
   if (lhsWords < rhsWords || this->ult(RHS))
     // X / Y ===> 0, iff X < Y
     return APInt(BitWidth, 0);
@@ -1611,13 +1615,17 @@ APInt APInt::urem(const APInt& RHS) const {
   unsigned lhsWords = getNumWords(getActiveBits());
 
   // Get some facts about the RHS
-  unsigned rhsWords = getNumWords(RHS.getActiveBits());
+  unsigned rhsBits = RHS.getActiveBits();
+  unsigned rhsWords = getNumWords(rhsBits);
   assert(rhsWords && "Performing remainder operation by zero ???");
 
   // Check the degenerate cases
   if (lhsWords == 0)
     // 0 % Y ===> 0
     return APInt(BitWidth, 0);
+  if (rhsBits == 1)
+    // X % 1 ===> 0
+    return APInt(BitWidth, 0);
   if (lhsWords < rhsWords || this->ult(RHS))
     // X % Y ===> X, iff X < Y
     return *this;
@@ -1662,7 +1670,8 @@ void APInt::udivrem(const APInt &LHS, const APInt &RHS,
 
   // Get some size facts about the dividend and divisor
   unsigned lhsWords = getNumWords(LHS.getActiveBits());
-  unsigned rhsWords = getNumWords(RHS.getActiveBits());
+  unsigned rhsBits  = RHS.getActiveBits();
+  unsigned rhsWords = getNumWords(rhsBits);
   assert(rhsWords && "Performing divrem operation by zero ???");
 
   // Check the degenerate cases
@@ -1672,6 +1681,11 @@ void APInt::udivrem(const APInt &LHS, const APInt &RHS,
     return;
   }
 
+  if (rhsBits == 1) {
+    Quotient = LHS;             // X / 1 ===> X
+    Remainder = 0;              // X % 1 ===> 0
+  }
+
   if (lhsWords < rhsWords || LHS.ult(RHS)) {
     Remainder = LHS;            // X % Y ===> X, iff X < Y
     Quotient = 0;               // X / Y ===> 0, iff X < Y