[llvm] [KnownBits] Add `force{Non}Negative` API (PR #110389)

[via llvm-commits]

https://github.com/goldsteinn created https://github.com/llvm/llvm-project/pull/110389

These modification patterns are desired enough to justify having an
API for them.


>From c5c8233c535f54418ddd4e18790a8a1cef418e12 Mon Sep 17 00:00:00 2001
From: Noah Goldstein <goldstein.w.n at gmail.com>
Date: Sat, 28 Sep 2024 17:35:44 -0500
Subject: [PATCH] [KnownBits] Add `force{Non}Negative` API

These modification patterns are desired enough to justify having an
API for them.
---
 llvm/include/llvm/Support/KnownBits.h | 14 ++++++++++++
 llvm/lib/Support/KnownBits.cpp        | 32 +++++++++------------------
 2 files changed, 25 insertions(+), 21 deletions(-)

diff --git a/llvm/include/llvm/Support/KnownBits.h b/llvm/include/llvm/Support/KnownBits.h
index e4ec202f36aae0..751f7759fb2f20 100644
--- a/llvm/include/llvm/Support/KnownBits.h
+++ b/llvm/include/llvm/Support/KnownBits.h
@@ -114,6 +114,20 @@ struct KnownBits {
     Zero.setSignBit();
   }
 
+  /// Force this value to be negative. Unlike `makeNegative`, this will clear
+  /// any existing signbit.
+  void forceNegative() {
+    Zero.clearSignBit();
+    One.setSignBit();
+  }
+
+  /// Force this value to be non-negative. Unlike `makeNonNegative`, this will
+  /// clear any existing signbit.
+  void forceNonNegative() {
+    One.clearSignBit();
+    Zero.setSignBit();
+  }
+
   /// Return the minimal unsigned value possible given these KnownBits.
   APInt getMinValue() const {
     // Assume that all bits that aren't known-ones are zeros.
diff --git a/llvm/lib/Support/KnownBits.cpp b/llvm/lib/Support/KnownBits.cpp
index 6863c5c0af5dca..9ae912d64d0a1e 100644
--- a/llvm/lib/Support/KnownBits.cpp
+++ b/llvm/lib/Support/KnownBits.cpp
@@ -578,8 +578,7 @@ KnownBits KnownBits::abs(bool IntMinIsPoison) const {
     // is poison anyways.
     if (IntMinIsPoison && Tmp.countMinPopulation() == 1 &&
         Tmp.countMaxPopulation() != 1) {
-      Tmp.One.clearSignBit();
-      Tmp.Zero.setSignBit();
+      Tmp.forceNonNegative();
       KnownAbs.One.setBits(getBitWidth() - Tmp.countMinLeadingZeros(),
                            getBitWidth() - 1);
     }
@@ -596,10 +595,8 @@ KnownBits KnownBits::abs(bool IntMinIsPoison) const {
     // We only know that the absolute values's MSB will be zero if INT_MIN is
     // poison, or there is a set bit that isn't the sign bit (otherwise it could
     // be INT_MIN).
-    if (IntMinIsPoison || (!One.isZero() && !One.isMinSignedValue())) {
-      KnownAbs.One.clearSignBit();
-      KnownAbs.Zero.setSignBit();
-    }
+    if (IntMinIsPoison || (!One.isZero() && !One.isMinSignedValue()))
+      KnownAbs.forceNonNegative();
   }
 
   return KnownAbs;
@@ -658,26 +655,19 @@ static KnownBits computeForSatAddSub(bool Add, bool Signed,
 
   if (Signed) {
     if (Add) {
-      if (LHS.isNonNegative() && RHS.isNonNegative()) {
+      if (LHS.isNonNegative() && RHS.isNonNegative())
         // Pos + Pos -> Pos
-        Res.One.clearSignBit();
-        Res.Zero.setSignBit();
-      }
-      if (LHS.isNegative() && RHS.isNegative()) {
+        Res.forceNonNegative();
+      if (LHS.isNegative() && RHS.isNegative())
         // Neg + Neg -> Neg
-        Res.One.setSignBit();
-        Res.Zero.clearSignBit();
-      }
+        Res.forceNegative();
     } else {
-      if (LHS.isNegative() && RHS.isNonNegative()) {
+      if (LHS.isNegative() && RHS.isNonNegative())
         // Neg - Pos -> Neg
-        Res.One.setSignBit();
-        Res.Zero.clearSignBit();
-      } else if (LHS.isNonNegative() && RHS.isNegative()) {
+        Res.forceNegative();
+      else if (LHS.isNonNegative() && RHS.isNegative())
         // Pos - Neg -> Pos
-        Res.One.clearSignBit();
-        Res.Zero.setSignBit();
-      }
+        Res.forceNonNegative();
     }
   } else {
     // Add: Leading ones of either operand are preserved.