InstCombine: Don't miscompile (x lshr C1) udiv C2

We have a transform that changes:
  (x lshr C1) udiv C2
into:
  x udiv (C2 << C1)

However, it is unsafe to do so if C2 << C1 discards any of C2's bits.

This fixes PR21255.

llvm-svn: 219634

4 files changed, 40 insertions, 13 deletions

diff --git a/llvm/include/llvm/ADT/APInt.h b/llvm/include/llvm/ADT/APInt.h
index f815628f30c..4d19bab13f4 100644
--- a/llvm/include/llvm/ADT/APInt.h
+++ b/llvm/include/llvm/ADT/APInt.h
@@ -945,7 +945,8 @@ public:
   APInt sdiv_ov(const APInt &RHS, bool &Overflow) const;
   APInt smul_ov(const APInt &RHS, bool &Overflow) const;
   APInt umul_ov(const APInt &RHS, bool &Overflow) const;
-  APInt sshl_ov(unsigned Amt, bool &Overflow) const;
+  APInt sshl_ov(const APInt &Amt, bool &Overflow) const;
+  APInt ushl_ov(const APInt &Amt, bool &Overflow) const;
 
   /// \brief Array-indexing support.
   ///
diff --git a/llvm/lib/Support/APInt.cpp b/llvm/lib/Support/APInt.cpp
index 02778b2fc7c..c20eeb26948 100644
--- a/llvm/lib/Support/APInt.cpp
+++ b/llvm/lib/Support/APInt.cpp
@@ -2064,19 +2064,29 @@ APInt APInt::umul_ov(const APInt &RHS, bool &Overflow) const {
   return Res;
 }
 
-APInt APInt::sshl_ov(unsigned ShAmt, bool &Overflow) const {
-  Overflow = ShAmt >= getBitWidth();
+APInt APInt::sshl_ov(const APInt &ShAmt, bool &Overflow) const {
+  Overflow = ShAmt.uge(getBitWidth());
   if (Overflow)
-    ShAmt = getBitWidth()-1;
+    return APInt(BitWidth, 0);
 
   if (isNonNegative()) // Don't allow sign change.
-    Overflow = ShAmt >= countLeadingZeros();
+    Overflow = ShAmt.uge(countLeadingZeros());
   else
-    Overflow = ShAmt >= countLeadingOnes();
+    Overflow = ShAmt.uge(countLeadingOnes());
   
   return *this << ShAmt;
 }
 
+APInt APInt::ushl_ov(const APInt &ShAmt, bool &Overflow) const {
+  Overflow = ShAmt.uge(getBitWidth());
+  if (Overflow)
+    return APInt(BitWidth, 0);
+
+  Overflow = ShAmt.ugt(countLeadingZeros());
+
+  return *this << ShAmt;
+}
+
 
 
 
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp b/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
index dad2c2d256d..846a3640930 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
@@ -965,11 +965,17 @@ Instruction *InstCombiner::visitUDiv(BinaryOperator &I) {
     return Common;
 
   // (x lshr C1) udiv C2 --> x udiv (C2 << C1)
-  if (Constant *C2 = dyn_cast<Constant>(Op1)) {
+  {
     Value *X;
-    Constant *C1;
-    if (match(Op0, m_LShr(m_Value(X), m_Constant(C1))))
-      return BinaryOperator::CreateUDiv(X, ConstantExpr::getShl(C2, C1));
+    const APInt *C1, *C2;
+    if (match(Op0, m_LShr(m_Value(X), m_APInt(C1))) &&
+        match(Op1, m_APInt(C2))) {
+      bool Overflow;
+      APInt C2ShlC1 = C2->ushl_ov(*C1, Overflow);
+      if (!Overflow)
+        return BinaryOperator::CreateUDiv(
+            X, ConstantInt::get(X->getType(), C2ShlC1));
+    }
   }
 
   // (zext A) udiv (zext B) --> zext (A udiv B)
diff --git a/llvm/test/Transforms/InstCombine/div.ll b/llvm/test/Transforms/InstCombine/div.ll
index 5a884ac671d..f2a70fd0f0d 100644
--- a/llvm/test/Transforms/InstCombine/div.ll
+++ b/llvm/test/Transforms/InstCombine/div.ll
@@ -132,11 +132,11 @@ define i32 @test15(i32 %a, i32 %b) nounwind {
 }
 
 define <2 x i64> @test16(<2 x i64> %x) nounwind {
-  %shr = lshr <2 x i64> %x, <i64 3, i64 5>
-  %div = udiv <2 x i64> %shr, <i64 4, i64 6>
+  %shr = lshr <2 x i64> %x, <i64 5, i64 5>
+  %div = udiv <2 x i64> %shr, <i64 6, i64 6>
   ret <2 x i64> %div
 ; CHECK-LABEL: @test16(
-; CHECK-NEXT: udiv <2 x i64> %x, <i64 32, i64 192>
+; CHECK-NEXT: udiv <2 x i64> %x, <i64 192, i64 192>
 ; CHECK-NEXT: ret <2 x i64>
 }
 
@@ -264,3 +264,13 @@ define i32 @test30(i32 %a) {
 ; CHECK-LABEL: @test30(
 ; CHECK-NEXT: ret i32 %a
 }
+
+define <2 x i32> @test31(<2 x i32> %x) nounwind {
+  %shr = lshr <2 x i32> %x, <i32 31, i32 31>
+  %div = udiv <2 x i32> %shr, <i32 2147483647, i32 2147483647>
+  ret <2 x i32> %div
+; CHECK-LABEL: @test31(
+; CHECK-NEXT: %[[shr:.*]] = lshr <2 x i32> %x, <i32 31, i32 31>
+; CHECK-NEXT: udiv <2 x i32> %[[shr]], <i32 2147483647, i32 2147483647>
+; CHECK-NEXT: ret <2 x i32>
+}