[llvm] [KnownBits] Simplify and 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 1/3] 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 2/3] 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 3/3] 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.