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

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-adt

Author: Kazu Hirata (kazutakahirata)

<details>
<summary>Changes</summary>

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.


---
Full diff: https://github.com/llvm/llvm-project/pull/153127.diff


1 Files Affected:

- (modified) llvm/include/llvm/ADT/SmallVector.h (+2-2) 


``````````diff
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.
 

``````````

</details>


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