[llvm] [KnownBits] Optimize isConstant (NFC) (PR #191919)

[Max Graey via llvm-commits]

https://github.com/MaxGraey updated https://github.com/llvm/llvm-project/pull/191919

>From 9d46fe170eb503419bb2d02fdd50325c4f375f60 Mon Sep 17 00:00:00 2001
From: MaxGraey <maxgraey at gmail.com>
Date: Tue, 14 Apr 2026 04:18:19 +0300
Subject: [PATCH 1/5] simplify and optimize

---
 llvm/include/llvm/Support/KnownBits.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/llvm/include/llvm/Support/KnownBits.h b/llvm/include/llvm/Support/KnownBits.h
index ecedccc9afd95..ee2477efa153f 100644
--- a/llvm/include/llvm/Support/KnownBits.h
+++ b/llvm/include/llvm/Support/KnownBits.h
@@ -52,7 +52,7 @@ struct KnownBits {
 
   /// Returns true if we know the value of all bits.
   bool isConstant() const {
-    return Zero.popcount() + One.popcount() == getBitWidth();
+    return (Zero | One).isAllOnes();
   }
 
   /// Returns the value when all bits have a known value. This just returns One

>From 11194c63a83f743af5f4ba105c5c4dc8d425ccfc Mon Sep 17 00:00:00 2001
From: MaxGraey <maxgraey at gmail.com>
Date: Tue, 14 Apr 2026 04:23:51 +0300
Subject: [PATCH 2/5] format

---
 llvm/include/llvm/Support/KnownBits.h | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/llvm/include/llvm/Support/KnownBits.h b/llvm/include/llvm/Support/KnownBits.h
index ee2477efa153f..96de46d821e8a 100644
--- a/llvm/include/llvm/Support/KnownBits.h
+++ b/llvm/include/llvm/Support/KnownBits.h
@@ -51,9 +51,7 @@ struct KnownBits {
   bool hasConflict() const { return Zero.intersects(One); }
 
   /// Returns true if we know the value of all bits.
-  bool isConstant() const {
-    return (Zero | One).isAllOnes();
-  }
+  bool isConstant() const { return (Zero | One).isAllOnes(); }
 
   /// Returns the value when all bits have a known value. This just returns One
   /// with a protective assertion.

>From 7819d75d6eeb17e381398d5d86cb28410e7d07fe Mon Sep 17 00:00:00 2001
From: MaxGraey <maxgraey at gmail.com>
Date: Tue, 14 Apr 2026 11:40:01 +0300
Subject: [PATCH 3/5] use hybrid approach

---
 llvm/include/llvm/Support/KnownBits.h | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/llvm/include/llvm/Support/KnownBits.h b/llvm/include/llvm/Support/KnownBits.h
index 96de46d821e8a..7f56149be6631 100644
--- a/llvm/include/llvm/Support/KnownBits.h
+++ b/llvm/include/llvm/Support/KnownBits.h
@@ -51,7 +51,13 @@ struct KnownBits {
   bool hasConflict() const { return Zero.intersects(One); }
 
   /// Returns true if we know the value of all bits.
-  bool isConstant() const { return (Zero | One).isAllOnes(); }
+  bool isConstant() const {
+    if (Zero.isSingleWord()) {
+      return (Zero.getZExtValue() | One.getZExtValue()) ==
+          llvm::maskTrailingOnes<uint64_t>(getBitWidth());
+    }
+    return Zero.popcount() + One.popcount() == getBitWidth();
+  }
 
   /// Returns the value when all bits have a known value. This just returns One
   /// with a protective assertion.

>From d8b3b367c68b051d4fc4983b18ad306b700276c5 Mon Sep 17 00:00:00 2001
From: MaxGraey <maxgraey at gmail.com>
Date: Tue, 14 Apr 2026 11:43:51 +0300
Subject: [PATCH 4/5] format

---
 llvm/include/llvm/Support/KnownBits.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/llvm/include/llvm/Support/KnownBits.h b/llvm/include/llvm/Support/KnownBits.h
index 7f56149be6631..8936a42246ea8 100644
--- a/llvm/include/llvm/Support/KnownBits.h
+++ b/llvm/include/llvm/Support/KnownBits.h
@@ -54,7 +54,7 @@ struct KnownBits {
   bool isConstant() const {
     if (Zero.isSingleWord()) {
       return (Zero.getZExtValue() | One.getZExtValue()) ==
-          llvm::maskTrailingOnes<uint64_t>(getBitWidth());
+             llvm::maskTrailingOnes<uint64_t>(getBitWidth());
     }
     return Zero.popcount() + One.popcount() == getBitWidth();
   }

>From 807153f3304a4d704fc9b96db977566f0a13c1fc Mon Sep 17 00:00:00 2001
From: MaxGraey <maxgraey at gmail.com>
Date: Tue, 14 Apr 2026 12:15:10 +0300
Subject: [PATCH 5/5] add faster slow fallback

---
 llvm/include/llvm/Support/KnownBits.h | 10 ++++++----
 llvm/lib/Support/KnownBits.cpp        | 14 ++++++++++++++
 2 files changed, 20 insertions(+), 4 deletions(-)

diff --git a/llvm/include/llvm/Support/KnownBits.h b/llvm/include/llvm/Support/KnownBits.h
index 8936a42246ea8..ffc71073bb554 100644
--- a/llvm/include/llvm/Support/KnownBits.h
+++ b/llvm/include/llvm/Support/KnownBits.h
@@ -52,11 +52,10 @@ struct KnownBits {
 
   /// Returns true if we know the value of all bits.
   bool isConstant() const {
-    if (Zero.isSingleWord()) {
+    if (Zero.isSingleWord())
       return (Zero.getZExtValue() | One.getZExtValue()) ==
-             llvm::maskTrailingOnes<uint64_t>(getBitWidth());
-    }
-    return Zero.popcount() + One.popcount() == getBitWidth();
+              llvm::maskTrailingOnes<uint64_t>(getBitWidth());
+    return isConstantSlowCase();
   }
 
   /// Returns the value when all bits have a known value. This just returns One
@@ -580,6 +579,9 @@ struct KnownBits {
   // Internal helper for getting the initial KnownBits for an `srem` or `urem`
   // operation with the low-bits set.
   static KnownBits remGetLowBits(const KnownBits &LHS, const KnownBits &RHS);
+
+  /// Multi-word case helper for isConstant().
+  bool isConstantSlowCase() const;
 };
 
 inline KnownBits operator&(KnownBits LHS, const KnownBits &RHS) {
diff --git a/llvm/lib/Support/KnownBits.cpp b/llvm/lib/Support/KnownBits.cpp
index 07e7781d0839d..fc84f7b4100a7 100644
--- a/llvm/lib/Support/KnownBits.cpp
+++ b/llvm/lib/Support/KnownBits.cpp
@@ -19,6 +19,20 @@
 
 using namespace llvm;
 
+bool KnownBits::isConstantSlowCase() const {
+  const unsigned LastWord = Zero.getNumWords() - 1;
+  const uint64_t *Zeros = Zero.getRawData();
+  const uint64_t *Ones = One.getRawData();
+
+  for (unsigned I = 0; I != LastWord; ++I)
+    if ((Zeros[I] | Ones[I]) != UINT64_MAX)
+      return false;
+
+  unsigned TailBits = getBitWidth() - LastWord * APInt::APINT_BITS_PER_WORD;
+  uint64_t TailMask = llvm::maskTrailingOnes<uint64_t>(TailBits);
+  return ((Zeros[LastWord] | Ones[LastWord]) & TailMask) == TailMask;
+}
+
 KnownBits KnownBits::flipSignBit(const KnownBits &Val) {
   unsigned SignBitPosition = Val.getBitWidth() - 1;
   APInt Zero = Val.Zero;