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

[via llvm-commits]

https://github.com/AZero13 created https://github.com/llvm/llvm-project/pull/143618

None

>From ccb3601ecc5bfa932b94cc5efaf59e011e20e8d2 Mon Sep 17 00:00:00 2001
From: Rose <gfunni234 at gmail.com>
Date: Tue, 10 Jun 2025 16:44:34 -0400
Subject: [PATCH] Add subtraction support for setLimitsForBinOp

---
 llvm/lib/Analysis/ValueTracking.cpp | 54 +++++++++++++++++++++++++++++
 1 file changed, 54 insertions(+)

diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index d8c1096049dce..88e8ea4878fe1 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -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].
+      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].
+          Lower = APInt::getSignedMinValue(Width);
+          Upper = APInt::getSignedMaxValue(Width) - *C + 1;
+        } 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);