[llvm] [Support] Adding fast-path for KnownBits's udiv when RHS is contant (PR #189779)

[Max Graey via llvm-commits]

https://github.com/MaxGraey updated https://github.com/llvm/llvm-project/pull/189779

>From bc71364b702b686d437e63bd8fb043ff3d001a6b Mon Sep 17 00:00:00 2001
From: MaxGraey <maxgraey at gmail.com>
Date: Wed, 1 Apr 2026 03:48:37 +0300
Subject: [PATCH 1/2] add fast-path for KnownBits::udiv when RHS is contant

---
 llvm/lib/Support/KnownBits.cpp | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/llvm/lib/Support/KnownBits.cpp b/llvm/lib/Support/KnownBits.cpp
index 07e7781d0839d..5809fff3ad6ab 100644
--- a/llvm/lib/Support/KnownBits.cpp
+++ b/llvm/lib/Support/KnownBits.cpp
@@ -1209,6 +1209,20 @@ KnownBits KnownBits::udiv(const KnownBits &LHS, const KnownBits &RHS,
     return Known;
   }
 
+  if (RHS.isConstant()) {
+    const APInt Divisor = RHS.getConstant();
+    if (Divisor.isOne())
+      return LHS;
+
+    if (Divisor.isPowerOf2()) {
+      unsigned Shift = Divisor.logBase2();
+      KnownBits Known = LHS;
+      Known.One.lshrInPlace(Shift);
+      Known.Zero.lshrInPlace(Shift);
+      return Known;
+    }
+  }
+
   // We can figure out the minimum number of upper zero bits by doing
   // MaxNumerator / MinDenominator. If the Numerator gets smaller or Denominator
   // gets larger, the number of upper zero bits increases.

>From 98fbc0b1f97150ca44a9fe1b4ca9152665c75868 Mon Sep 17 00:00:00 2001
From: MaxGraey <maxgraey at gmail.com>
Date: Wed, 1 Apr 2026 03:56:48 +0300
Subject: [PATCH 2/2] refactor

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

diff --git a/llvm/lib/Support/KnownBits.cpp b/llvm/lib/Support/KnownBits.cpp
index 5809fff3ad6ab..25530d7e46727 100644
--- a/llvm/lib/Support/KnownBits.cpp
+++ b/llvm/lib/Support/KnownBits.cpp
@@ -1216,7 +1216,7 @@ KnownBits KnownBits::udiv(const KnownBits &LHS, const KnownBits &RHS,
 
     if (Divisor.isPowerOf2()) {
       unsigned Shift = Divisor.logBase2();
-      KnownBits Known = LHS;
+      Known = LHS;
       Known.One.lshrInPlace(Shift);
       Known.Zero.lshrInPlace(Shift);
       return Known;