[InstSimplify] refactor udiv/urem code and add tests; NFCI

This removes some duplicated code and makes it easier to support signed div/rem
in a similar way if we want to do that. Note that the existing comments were not
accurate - we don't need a constant divisor to simplify; icmp simplification does
more than that. But as the added tests show, it could go even further.

llvm-svn: 312885

1 files changed, 31 insertions, 18 deletions

diff --git a/llvm/lib/Analysis/InstructionSimplify.cpp b/llvm/lib/Analysis/InstructionSimplify.cpp
index 0fb0b7d2449..dbcc7e13f21 100644
--- a/llvm/lib/Analysis/InstructionSimplify.cpp
+++ b/llvm/lib/Analysis/InstructionSimplify.cpp
@@ -1075,6 +1075,33 @@ Value *llvm::SimplifySDivInst(Value *Op0, Value *Op1, const SimplifyQuery &Q) {
   return ::SimplifySDivInst(Op0, Op1, Q, RecursionLimit);
 }
 
+/// Given a predicate and two operands, return true if the comparison is true.
+/// This is a helper for div/rem simplification where we return some other value
+/// when we can prove a relationship between the operands.
+static bool isICmpTrue(ICmpInst::Predicate Pred, Value *LHS, Value *RHS,
+                       const SimplifyQuery &Q, unsigned MaxRecurse) {
+  Value *V = SimplifyICmpInst(Pred, LHS, RHS, Q, MaxRecurse);
+  Constant *C = dyn_cast_or_null<Constant>(V);
+  return (C && C->isAllOnesValue());
+}
+
+static Value *simplifyUnsignedDivRem(Value *Op0, Value *Op1,
+                                     const SimplifyQuery &Q,
+                                     unsigned MaxRecurse, bool IsDiv) {
+  // Recursion is always used, so bail out at once if we already hit the limit.
+  if (!MaxRecurse--)
+    return nullptr;
+
+  // If we can prove that the quotient is unsigned less than the divisor, then
+  // we know the answer:
+  // X / Y --> 0
+  // X % Y --> X
+  if (isICmpTrue(ICmpInst::ICMP_ULT, Op0, Op1, Q, MaxRecurse))
+    return IsDiv ? Constant::getNullValue(Op0->getType()) : Op0;
+
+  return nullptr;
+}
+
 /// Given operands for a UDiv, see if we can fold the result.
 /// If not, this returns null.
 static Value *SimplifyUDivInst(Value *Op0, Value *Op1, const SimplifyQuery &Q,
@@ -1082,15 +1109,8 @@ static Value *SimplifyUDivInst(Value *Op0, Value *Op1, const SimplifyQuery &Q,
   if (Value *V = SimplifyDiv(Instruction::UDiv, Op0, Op1, Q, MaxRecurse))
     return V;
 
-  // udiv %V, C -> 0 if %V < C
-  if (MaxRecurse) {
-    if (Constant *C = dyn_cast_or_null<Constant>(SimplifyICmpInst(
-            ICmpInst::ICMP_ULT, Op0, Op1, Q, MaxRecurse - 1))) {
-      if (C->isAllOnesValue()) {
-        return Constant::getNullValue(Op0->getType());
-      }
-    }
-  }
+  if (Value *V = simplifyUnsignedDivRem(Op0, Op1, Q, MaxRecurse, true))
+    return V;
 
   return nullptr;
 }
@@ -1198,15 +1218,8 @@ static Value *SimplifyURemInst(Value *Op0, Value *Op1, const SimplifyQuery &Q,
   if (Value *V = SimplifyRem(Instruction::URem, Op0, Op1, Q, MaxRecurse))
     return V;
 
-  // urem %V, C -> %V if %V < C
-  if (MaxRecurse) {
-    if (Constant *C = dyn_cast_or_null<Constant>(SimplifyICmpInst(
-            ICmpInst::ICMP_ULT, Op0, Op1, Q, MaxRecurse - 1))) {
-      if (C->isAllOnesValue()) {
-        return Op0;
-      }
-    }
-  }
+  if (Value *V = simplifyUnsignedDivRem(Op0, Op1, Q, MaxRecurse, false))
+    return V;
 
   return nullptr;
 }