From 5f6b81405737a9a056fc8259c5f032c2e1791583 Mon Sep 17 00:00:00 2001
From: Leonard Hecker <lhecker@microsoft.com>
Date: Wed, 17 Dec 2025 17:49:01 +0100
Subject: [PATCH] Improve performance of til::equals (#19653)
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As explained in the added comment. Perf 📈.
---
 src/inc/til/string.h | 27 ++++++++++++++++++++-------
 1 file changed, 20 insertions(+), 7 deletions(-)

diff --git a/src/inc/til/string.h b/src/inc/til/string.h
index 824df3212b..2fb0328273 100644
--- a/src/inc/til/string.h
+++ b/src/inc/til/string.h
@@ -159,16 +159,29 @@ namespace til // Terminal Implementation Library. Also: "Today I Learned"
 
     // Just like std::wstring_view::operator==().
     //
-    // At the time of writing wmemcmp() is not an intrinsic for MSVC,
-    // but the STL uses it to implement wide string comparisons.
-    // This produces 3x the assembly _per_ comparison and increases
-    // runtime by 2-3x for strings of medium length (16 characters)
-    // and 5x or more for long strings (128 characters or more).
+    // NOTE: If you pass a literal, ALWAYS pass it as the 2nd argument.
+    // This is because MSVC does not understand that `lhs.size() == rhs.size()` implies
+    // that a constant size on one side implies a constant size on the other side.
+    // As such the size argument to memcmp() must be the constant size, which is `rhs`.
+    //
+    // The STL uses wmemcmp() to implement wide string comparisons.
+    // Compared to memcmp() this results 3x the assembly per comparison,
+    // while running 2-3x slower for 16 chars and >5x slower for >128 chars.
     // See: https://github.com/microsoft/STL/issues/2289
     template<typename T, typename Traits>
-    bool equals(const std::basic_string_view<T, Traits>& lhs, const std::basic_string_view<T, Traits>& rhs) noexcept
+    constexpr bool equals(const std::basic_string_view<T, Traits>& lhs, const std::basic_string_view<T, Traits>& rhs) noexcept
     {
-        return lhs.size() == rhs.size() && __builtin_memcmp(lhs.data(), rhs.data(), lhs.size() * sizeof(T)) == 0;
+        return lhs.size() == rhs.size() && __builtin_memcmp(lhs.data(), rhs.data(), rhs.size() * sizeof(T)) == 0;
+    }
+
+    constexpr bool equals(const std::string_view& lhs, const std::string_view& rhs) noexcept
+    {
+        return equals<>(lhs, rhs);
+    }
+
+    constexpr bool equals(const std::wstring_view& lhs, const std::wstring_view& rhs) noexcept
+    {
+        return equals<>(lhs, rhs);
     }
 
     // Just like _memicmp, but without annoying locales.