[llvm] [ADT] Simplify getFirstEl (NFC) (PR #153127)

[Kazu Hirata via llvm-commits]

https://github.com/kazutakahirata created https://github.com/llvm/llvm-project/pull/153127

getFirstEl computes the address of FirstEl.  This patch computes the
address by first casting this to SmallVectorAlignmentAndSize<T> and
letting the C++ language compute the field address instead of adding
the offset to this on our own.


>From f7feffe174f595962d6a18ef4319eaca7e7ccdbb Mon Sep 17 00:00:00 2001
From: Kazu Hirata <kazu at google.com>
Date: Sat, 9 Aug 2025 21:08:41 -0700
Subject: [PATCH] [ADT] Simplify getFirstEl (NFC)

getFirstEl computes the address of FirstEl.  This patch computes the
address by first casting this to SmallVectorAlignmentAndSize<T> and
letting the C++ language compute the field address instead of adding
the offset to this on our own.
---
 llvm/include/llvm/ADT/SmallVector.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/llvm/include/llvm/ADT/SmallVector.h b/llvm/include/llvm/ADT/SmallVector.h
index 80f7734b86907..6da519adcee6f 100644
--- a/llvm/include/llvm/ADT/SmallVector.h
+++ b/llvm/include/llvm/ADT/SmallVector.h
@@ -128,8 +128,8 @@ class SmallVectorTemplateCommon
   /// SmallVectorStorage is properly-aligned even for small-size of 0.
   void *getFirstEl() const {
     return const_cast<void *>(reinterpret_cast<const void *>(
-        reinterpret_cast<const char *>(this) +
-        offsetof(SmallVectorAlignmentAndSize<T>, FirstEl)));
+        reinterpret_cast<const SmallVectorAlignmentAndSize<T> *>(this)
+            ->FirstEl));
   }
   // Space after 'FirstEl' is clobbered, do not add any instance vars after it.