[InstSimplify] simplify add instruction if two operands are negative

[shchenz]

Summary

This is from D48754, use isKnownNegation() function to simplify more add instruction patterns. such as:

define i8 @knownnegation(i8 %x, i8 %y) {
  %xy = sub i8 %x, %y
  %yx = sub i8 %y, %x
  %r = add i8 %xy, %yx
  ret i8 %r
}

--->

define i8 @knownnegation(i8 %x, i8 %y) {
  ret i8 0
}

Test cases are added at rL336822

[spatel]

LGTM

llvm/include/llvm/Analysis/ValueTracking.h

class Value;
   /// specified, perform context-sensitive analysis and return true if the
   /// pointer couldn't possibly be null at the specified instruction.
   /// Supports values with integer or pointer type and vectors of integers.
   bool isKnownNonZero(const Value *V, const DataLayout &DL, unsigned Depth = 0,
                       AssumptionCache *AC = nullptr,
                       const Instruction *CxtI = nullptr,
                       const DominatorTree *DT = nullptr);
 
+  /// Return true if the two given values are negation.
+  bool isKnownNegation(const Value *X, const Value *Y);
+
   /// Returns true if the give value is known to be non-negative.
   bool isKnownNonNegative(const Value *V, const DataLayout &DL,
                           unsigned Depth = 0,
                           AssumptionCache *AC = nullptr,
                           const Instruction *CxtI = nullptr,
                           const DominatorTree *DT = nullptr);
 
   /// Returns true if the given value is known be positive (i.e. non-negative

llvm/lib/Analysis/InstructionSimplify.cpp

static Value *SimplifyAddInst(Value *Op0, Value *Op1, bool IsNSW, bool IsNUW,
   // X + undef -> undef
   if (match(Op1, m_Undef()))
     return Op1;
 
   // X + 0 -> X
   if (match(Op1, m_Zero()))
     return Op0;
 
+  // If two operands are negative, return 0.
+  if (isKnownNegation(Op0, Op1))
+    return Constant::getNullValue(Op0->getType());
+
   // X + (Y - X) -> Y
   // (Y - X) + X -> Y
   // Eg: X + -X -> 0
   Value *Y = nullptr;
   if (match(Op1, m_Sub(m_Value(Y), m_Specific(Op0))) ||
       match(Op0, m_Sub(m_Value(Y), m_Specific(Op1))))
     return Y;
 

llvm/lib/Analysis/ValueTracking.cpp

static SelectPatternResult matchMinMax(CmpInst::Predicate Pred,
   // (X <s C) ? ~C : ~X ==> (~X >s ~C) ? ~C : ~X ==> SMIN(~C, ~X)
   if (match(FalseVal, m_Not(m_Specific(CmpLHS))) &&
       match(TrueVal, m_APInt(C2)) && ~(*C1) == *C2)
     return {Pred == CmpInst::ICMP_SGT ? SPF_SMAX : SPF_SMIN, SPNB_NA, false};
 
   return {SPF_UNKNOWN, SPNB_NA, false};
 }
 
+bool llvm::isKnownNegation(const Value *X, const Value *Y) {
+  assert(X && Y && "Invalid operand");
+
+  // X = sub (0, Y)
+  if (match(X, m_Neg(m_Specific(Y))))
+    return true;
+
+  // Y = sub (0, X)
+  if (match(Y, m_Neg(m_Specific(X))))
+    return true;
+
+  // X = sub (A, B), Y = sub (B, A)
+  Value *A, *B;
+  if (match(X, m_Sub(m_Value(A), m_Value(B))) &&
+      match(Y, m_Sub(m_Specific(B), m_Specific(A))))
+    return true;
+
+  return false;
+}
+
 static SelectPatternResult matchSelectPattern(CmpInst::Predicate Pred,
                                               FastMathFlags FMF,
                                               Value *CmpLHS, Value *CmpRHS,
                                               Value *TrueVal, Value *FalseVal,
                                               Value *&LHS, Value *&RHS,
                                               unsigned Depth) {
   LHS = CmpLHS;
   RHS = CmpRHS;

llvm/test/Transforms/InstSimplify/add.ll

 ;
   %negx = sub <2 x i32> zeroinitializer, %x
   %r = add <2 x i32> %x, %negx
   ret <2 x i32> %r
 }
 
 define i8 @knownnegation(i8 %x, i8 %y) {
 ; CHECK-LABEL: @knownnegation(
-; CHECK-NEXT:    [[XY:%.*]] = sub i8 [[X:%.*]], [[Y:%.*]]
-; CHECK-NEXT:    [[YX:%.*]] = sub i8 [[Y]], [[X]]
-; CHECK-NEXT:    [[R:%.*]] = add i8 [[XY]], [[YX]]
-; CHECK-NEXT:    ret i8 [[R]]
+; CHECK-NEXT:    ret i8 0 
 ;
   %xy = sub i8 %x, %y
   %yx = sub i8 %y, %x
   %r = add i8 %xy, %yx
   ret i8 %r
 }
 
 define <2 x i8> @knownnegation_commute_vec(<2 x i8> %x, <2 x i8> %y) {
 ; CHECK-LABEL: @knownnegation_commute_vec(
-; CHECK-NEXT:    [[XY:%.*]] = sub <2 x i8> [[X:%.*]], [[Y:%.*]]
-; CHECK-NEXT:    [[YX:%.*]] = sub <2 x i8> [[Y]], [[X]]
-; CHECK-NEXT:    [[R:%.*]] = add <2 x i8> [[YX]], [[XY]]
-; CHECK-NEXT:    ret <2 x i8> [[R]]
+; CHECK-NEXT:    ret <2 x i8> zeroinitializer
 ;
   %xy = sub <2 x i8> %x, %y
   %yx = sub <2 x i8> %y, %x
   %r = add <2 x i8> %yx, %xy
   ret <2 x i8> %r
 }