[llvm] Add subtraction support for setLimitsForBinOp (PR #143618)

[Yingwei Zheng via llvm-commits]

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@@ -9576,6 +9576,60 @@ static void setLimitsForBinOp(const BinaryOperator &BO, APInt &Lower,
   unsigned Width = Lower.getBitWidth();
   const APInt *C;
   switch (BO.getOpcode()) {
+  case Instruction::Sub:
+    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:
+      // "sub nuw nsw i8 X, -2" is unsigned [0, 127] vs. signed [-128, 126].
+      if (PreferSignedRange && HasNSW && HasNUW)
+        HasNUW = false;
+
+      if (HasNUW) {
+        // 'sub nuw x, C' produces [0, UINT_MAX - C].
+        Upper = APInt::getAllOnes(Width) - *C + 1;
+      } else if (HasNSW) {
+        if (C->isNegative()) {
+          // 'sub nsw x, -C' produces [SINT_MIN + C, SINT_MAX].
+          Lower = APInt::getSignedMinValue(Width) + *C;
+          Upper = APInt::getSignedMaxValue(Width) + 1;
+        } else {
+          // 'sub nsw x, +C' produces [SINT_MIN, SINT_MAX - C].
+          Lower = APInt::getSignedMinValue(Width);
+          Upper = APInt::getSignedMaxValue(Width) - *C + 1;
+        }
+      }
+    } else 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:
+      // "sub nuw nsw i8 X, -2" is unsigned [0, 127] vs. signed [-128, 126].
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dtcxzyw wrote:

Is this still true? The constant is on the LHS. Please update the comment.


https://github.com/llvm/llvm-project/pull/143618