[libcxx-commits] [libcxx] [libc++] Add assumption for align of begin and end pointers of vector. (PR #108961)

[Louis Dionne via libcxx-commits]

https://github.com/ldionne updated https://github.com/llvm/llvm-project/pull/108961

>From 94c2ede1d6289e973497d44db7f31d1fff14b0cb Mon Sep 17 00:00:00 2001
From: Florian Hahn <flo at fhahn.com>
Date: Tue, 17 Sep 2024 12:07:29 +0100
Subject: [PATCH 1/2] [libc++] Add assumption for align of begin and end
 pointers of vector.

Missing information about begin and end pointers of std::vector can lead
to missed optimizations in LLVM.

See https://github.com/llvm/llvm-project/issues/101372 for a discussion
of missed range check optimizations in hardened mode.

Once https://github.com/llvm/llvm-project/pull/108958 lands, the created
`llvm.assume` calls for the alignment should be folded into the `load`
instructions, resulting in no extra instructions after InstCombine.
---
 libcxx/include/__vector/vector.h | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/libcxx/include/__vector/vector.h b/libcxx/include/__vector/vector.h
index ddbf1235b90691..eaccb1710c766d 100644
--- a/libcxx/include/__vector/vector.h
+++ b/libcxx/include/__vector/vector.h
@@ -341,13 +341,17 @@ class _LIBCPP_TEMPLATE_VIS vector {
   //
   // Iterators
   //
-  _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI iterator begin() _NOEXCEPT { return __make_iter(this->__begin_); }
+  _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI iterator begin() _NOEXCEPT {
+    return __make_iter(__add_alignment_assumption(this->__begin_));
+  }
   _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI const_iterator begin() const _NOEXCEPT {
-    return __make_iter(this->__begin_);
+    return __make_iter(__add_alignment_assumption(this->__begin_));
+  }
+  _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI iterator end() _NOEXCEPT {
+    return __make_iter(__add_alignment_assumption(this->__end_));
   }
-  _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI iterator end() _NOEXCEPT { return __make_iter(this->__end_); }
   _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI const_iterator end() const _NOEXCEPT {
-    return __make_iter(this->__end_);
+    return __make_iter(__add_alignment_assumption(this->__end_));
   }
 
   _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI reverse_iterator rbegin() _NOEXCEPT {
@@ -775,6 +779,10 @@ class _LIBCPP_TEMPLATE_VIS vector {
   }
 
   _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI void __move_assign_alloc(vector&, false_type) _NOEXCEPT {}
+
+  _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI pointer __add_alignment_assumption(pointer __p) const _NOEXCEPT {
+    return static_cast<pointer>(__builtin_assume_aligned(__p, alignof(decltype(*__p))));
+  }
 };
 
 #if _LIBCPP_STD_VER >= 17

>From 3daa6c5661ab4794c4ae5f8a97615c704d92e6cf Mon Sep 17 00:00:00 2001
From: Louis Dionne <ldionne.2 at gmail.com>
Date: Mon, 13 Jan 2025 13:18:45 -0500
Subject: [PATCH 2/2] Address review comments

---
 libcxx/include/__vector/vector.h | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/libcxx/include/__vector/vector.h b/libcxx/include/__vector/vector.h
index eaccb1710c766d..9e8e2ee1a03cd9 100644
--- a/libcxx/include/__vector/vector.h
+++ b/libcxx/include/__vector/vector.h
@@ -780,8 +780,13 @@ class _LIBCPP_TEMPLATE_VIS vector {
 
   _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI void __move_assign_alloc(vector&, false_type) _NOEXCEPT {}
 
-  _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI pointer __add_alignment_assumption(pointer __p) const _NOEXCEPT {
-    return static_cast<pointer>(__builtin_assume_aligned(__p, alignof(decltype(*__p))));
+  static _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI pointer __add_alignment_assumption(pointer __p) _NOEXCEPT {
+    if constexpr (is_pointer<pointer>::value) {
+      if (!__libcpp_is_constant_evaluated()) {
+        return static_cast<pointer>(__builtin_assume_aligned(__p, alignof(decltype(*__p))));
+      }
+    }
+    return __p;
   }
 };