Add subtraction support for setLimitsForBinOp

We can determine the range from a subtraction if it has nsw or nuw.

Author: AZero13

llvm/lib/Analysis/ValueTracking.cpp

@@ -9576,15 +9576,48 @@ static void setLimitsForBinOp(const BinaryOperator &BO, APInt &Lower,
   unsigned Width = Lower.getBitWidth();
   const APInt *C;
   switch (BO.getOpcode()) {
-  case Instruction::Add:
-    if (match(BO.getOperand(1), m_APInt(C)) && !C->isZero()) {
+  case Instruction::Sub:
+    if (match(BO.getOperand(0), m_APInt(C))) {
       bool HasNSW = IIQ.hasNoSignedWrap(&BO);
       bool HasNUW = IIQ.hasNoUnsignedWrap(&BO);
 
       // If the caller expects a signed compare, then try to use a signed range.
       // Otherwise if both no-wraps are set, use the unsigned range because it
       // is never larger than the signed range. Example:
-      // "add nuw nsw i8 X, -2" is unsigned [254,255] vs. signed [-128, 125].
+      // "sub nuw nsw i8 -2, x" is unsigned [0, 254] vs. signed [-128, 126].
+      // "sub nuw nsw i8 2, x" is unsigned [0, 2] vs. signed [-125, 127].
+      if (PreferSignedRange && HasNSW && HasNUW)
+        HasNUW = false;
+
+      if (HasNUW) {
+        // 'sub nuw c, x' produces [0, C].
+        Upper = *C + 1;
+      } else if (HasNSW) {
+        if (C->isNegative()) {
+          // 'sub nsw -C, x' produces [SINT_MIN, SINT_MAX - (C - 1)].
+          // Because to be negative, C must be - 1, and the highest result is
+          // INT_MIN, so -INT_MIN - 1 is INT_MAX, so it is SINT_MAX - (C - 1),
+          // or SINT_MAX - C + 1
+          Lower = APInt::getSignedMinValue(Width);
+          Upper = APInt::getSignedMaxValue(Width) + *C + 2;
+        } else {
+          // Note that sub 0, INT_MIN is not NSW. It techically is a signed wrap
+          // 'sub nsw C, x' produces [SINT_MIN + 1 + C, SINT_MAX].
+          Lower = APInt::getSignedMinValue(Width) + *C + 1;
+          Upper = APInt::getSignedMaxValue(Width) + 1;
+        }
+      }
+    }
+    break;
+  case Instruction::Add:
+    if (match(BO.getOperand(1), m_APInt(C)) && !C->isZero()) {
+      bool HasNSW = IIQ.hasNoSignedWrap(&BO);
+      bool HasNUW = IIQ.hasNoUnsignedWrap(&BO);
+
+      // If the caller expects a signed compare, then try to use a signed
+      // range. Otherwise if both no-wraps are set, use the unsigned range
+      // because it is never larger than the signed range. Example: "add nuw
+      // nsw i8 X, -2" is unsigned [254,255] vs. signed [-128, 125].
       if (PreferSignedRange && HasNSW && HasNUW)
         HasNUW = false;
 
@@ -9609,8 +9642,8 @@ static void setLimitsForBinOp(const BinaryOperator &BO, APInt &Lower,
     if (match(BO.getOperand(1), m_APInt(C)))
       // 'and x, C' produces [0, C].
       Upper = *C + 1;
-    // X & -X is a power of two or zero. So we can cap the value at max power of
-    // two.
+    // X & -X is a power of two or zero. So we can cap the value at max power
+    // of two.
     if (match(BO.getOperand(0), m_Neg(m_Specific(BO.getOperand(1)))) ||
         match(BO.getOperand(1), m_Neg(m_Specific(BO.getOperand(0)))))
       Upper = APInt::getSignedMinValue(Width) + 1;
@@ -9679,10 +9712,10 @@ static void setLimitsForBinOp(const BinaryOperator &BO, APInt &Lower,
         // If lowbit is set, value can never be zero.
         if ((*C)[0])
           Lower = APInt::getOneBitSet(Width, 0);
-        // If we are shifting a constant the largest it can be is if the longest
-        // sequence of consecutive ones is shifted to the highbits (breaking
-        // ties for which sequence is higher). At the moment we take a liberal
-        // upper bound on this by just popcounting the constant.
+        // If we are shifting a constant the largest it can be is if the
+        // longest sequence of consecutive ones is shifted to the highbits
+        // (breaking ties for which sequence is higher). At the moment we take
+        // a liberal upper bound on this by just popcounting the constant.
         // TODO: There may be a bitwise trick for it longest/highest
         // consecutative sequence of ones (naive method is O(Width) loop).
         Upper = APInt::getHighBitsSet(Width, C->popcount()) + 1;

llvm/test/Transforms/InstCombine/div.ll

@@ -494,9 +494,7 @@ define <2 x i8> @sdiv_exact_negated_dividend_constant_divisor_vec_splat(<2 x i8>
 
 define i8 @sdiv_negated_dividend_constant_divisor_smin(i8 %x) {
 ; CHECK-LABEL: @sdiv_negated_dividend_constant_divisor_smin(
-; CHECK-NEXT:    [[TMP1:%.*]] = icmp eq i8 [[X:%.*]], -128
-; CHECK-NEXT:    [[D:%.*]] = zext i1 [[TMP1]] to i8
-; CHECK-NEXT:    ret i8 [[D]]
+; CHECK-NEXT:    ret i8 0
 ;
   %neg = sub nsw i8 0, %x
   %d = sdiv i8 %neg, -128
@@ -505,9 +503,7 @@ define i8 @sdiv_negated_dividend_constant_divisor_smin(i8 %x) {
 
 define <2 x i8> @sdiv_negated_dividend_constant_divisor_vec_splat_smin(<2 x i8> %x) {
 ; CHECK-LABEL: @sdiv_negated_dividend_constant_divisor_vec_splat_smin(
-; CHECK-NEXT:    [[TMP1:%.*]] = icmp eq <2 x i8> [[X:%.*]], splat (i8 -128)
-; CHECK-NEXT:    [[D:%.*]] = zext <2 x i1> [[TMP1]] to <2 x i8>
-; CHECK-NEXT:    ret <2 x i8> [[D]]
+; CHECK-NEXT:    ret <2 x i8> zeroinitializer
 ;
   %neg = sub nsw <2 x i8> zeroinitializer, %x
   %d = sdiv <2 x i8> %neg, <i8 -128, i8 -128>

llvm/test/Transforms/InstCombine/icmp-sub.ll

@@ -290,8 +290,7 @@ define i1 @subC_nsw_ne(i32 %x) {
 ; CHECK-LABEL: @subC_nsw_ne(
 ; CHECK-NEXT:    [[SUBX:%.*]] = sub nsw i32 -2147483647, [[X:%.*]]
 ; CHECK-NEXT:    call void @use(i32 [[SUBX]])
-; CHECK-NEXT:    [[R:%.*]] = icmp ne i32 [[X]], 2147483603
-; CHECK-NEXT:    ret i1 [[R]]
+; CHECK-NEXT:    ret i1 true
 ;
   %subx = sub nsw i32 -2147483647, %x
   call void @use(i32 %subx)