[llvm] r212164 - InstCombine: Don't turn -(x/INT_MIN) -> x/INT_MIN

[David Majnemer]

Author: majnemer
Date: Wed Jul  2 01:07:09 2014
New Revision: 212164

URL: http://llvm.org/viewvc/llvm-project?rev=212164&view=rev
Log:
InstCombine: Don't turn -(x/INT_MIN) -> x/INT_MIN

It is not safe to negate the smallest signed integer, doing so yields
the same number back.

This fixes PR20186.

Modified:
    llvm/trunk/include/llvm/IR/Constant.h
    llvm/trunk/lib/IR/Constants.cpp
    llvm/trunk/lib/Transforms/InstCombine/InstCombineAddSub.cpp
    llvm/trunk/test/Transforms/InstCombine/sub.ll

Modified: llvm/trunk/include/llvm/IR/Constant.h
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/IR/Constant.h?rev=212164&r1=212163&r2=212164&view=diff
==============================================================================
--- llvm/trunk/include/llvm/IR/Constant.h (original)
+++ llvm/trunk/include/llvm/IR/Constant.h Wed Jul  2 01:07:09 2014
@@ -64,6 +64,9 @@ public:
   /// Return true if the value is negative zero or null value.
   bool isZeroValue() const;
 
+  /// \brief Return true if the value is the smallest signed value.
+  bool isMinSignedValue() const;
+
   /// canTrap - Return true if evaluation of this constant could trap.  This is
   /// true for things like constant expressions that could divide by zero.
   bool canTrap() const;

Modified: llvm/trunk/lib/IR/Constants.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/IR/Constants.cpp?rev=212164&r1=212163&r2=212164&view=diff
==============================================================================
--- llvm/trunk/lib/IR/Constants.cpp (original)
+++ llvm/trunk/lib/IR/Constants.cpp Wed Jul  2 01:07:09 2014
@@ -107,6 +107,28 @@ bool Constant::isAllOnesValue() const {
   return false;
 }
 
+bool Constant::isMinSignedValue() const {
+  // Check for INT_MIN integers
+  if (const ConstantInt *CI = dyn_cast<ConstantInt>(this))
+    return CI->isMinValue(/*isSigned=*/true);
+
+  // Check for FP which are bitcasted from INT_MIN integers
+  if (const ConstantFP *CFP = dyn_cast<ConstantFP>(this))
+    return CFP->getValueAPF().bitcastToAPInt().isMinSignedValue();
+
+  // Check for constant vectors which are splats of INT_MIN values.
+  if (const ConstantVector *CV = dyn_cast<ConstantVector>(this))
+    if (Constant *Splat = CV->getSplatValue())
+      return Splat->isMinSignedValue();
+
+  // Check for constant vectors which are splats of INT_MIN values.
+  if (const ConstantDataVector *CV = dyn_cast<ConstantDataVector>(this))
+    if (Constant *Splat = CV->getSplatValue())
+      return Splat->isMinSignedValue();
+
+  return false;
+}
+
 // Constructor to create a '0' constant of arbitrary type...
 Constant *Constant::getNullValue(Type *Ty) {
   switch (Ty->getTypeID()) {

Modified: llvm/trunk/lib/Transforms/InstCombine/InstCombineAddSub.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/InstCombine/InstCombineAddSub.cpp?rev=212164&r1=212163&r2=212164&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/InstCombine/InstCombineAddSub.cpp (original)
+++ llvm/trunk/lib/Transforms/InstCombine/InstCombineAddSub.cpp Wed Jul  2 01:07:09 2014
@@ -1553,9 +1553,9 @@ Instruction *InstCombiner::visitSub(Bina
       return BinaryOperator::CreateAnd(Op0,
                                   Builder->CreateNot(Y, Y->getName() + ".not"));
 
-    // 0 - (X sdiv C)  -> (X sdiv -C)
-    if (match(Op1, m_SDiv(m_Value(X), m_Constant(C))) &&
-        match(Op0, m_Zero()))
+    // 0 - (X sdiv C)  -> (X sdiv -C)  provided the negation doesn't overflow.
+    if (match(Op1, m_SDiv(m_Value(X), m_Constant(C))) && match(Op0, m_Zero()) &&
+        !C->isMinSignedValue())
       return BinaryOperator::CreateSDiv(X, ConstantExpr::getNeg(C));
 
     // 0 - (X << Y)  -> (-X << Y)   when X is freely negatable.

Modified: llvm/trunk/test/Transforms/InstCombine/sub.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstCombine/sub.ll?rev=212164&r1=212163&r2=212164&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/InstCombine/sub.ll (original)
+++ llvm/trunk/test/Transforms/InstCombine/sub.ll Wed Jul  2 01:07:09 2014
@@ -444,3 +444,22 @@ define <2 x i64> @test36(<2 x i64> %A) {
 ; CHECK-NEXT: %sub = mul <2 x i64> %A, <i64 7, i64 15>
 ; CHECK-NEXT: ret <2 x i64> %sub
 }
+
+define <2 x i64> @test37(<2 x i64> %A) {
+  %shl = shl <2 x i64> %A, <i64 3, i64 4>
+  %sub = sub <2 x i64> %shl, %A
+  ret <2 x i64> %sub
+; CHECK-LABEL: @test37(
+; CHECK-NEXT: %sub = mul <2 x i64> %A, <i64 7, i64 15>
+; CHECK-NEXT: ret <2 x i64> %sub
+}
+
+define i32 @test38(i32 %A) {
+  %div = sdiv i32 %A, -2147483648
+  %sub = sub nsw i32 0, %div
+  ret i32 %sub
+; CHECK-LABEL: @test38(
+; CHECK-NEXT: [[DIV:%.*]] = sdiv i32 %A, -2147483648
+; CHECK-NEXT: [[SUB:%.*]] = sub nsw i32 0, [[DIV]]
+; CHECK-NEXT: ret i32 [[SUB]]
+}