linux-kernel - [RFC v2 1/3] documentation: Discourage alignment assumptions

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Message-ID: <53a8eeef7227391c1d4298b547386a027f23fb46.1757810729.git.fthain@linux-m68k.org>
Date: Sun, 14 Sep 2025 10:45:29 +1000
From: Finn Thain <fthain@...ux-m68k.org>
To: Peter Zijlstra <peterz@...radead.org>,
    Will Deacon <will@...nel.org>,
    Jonathan Corbet <corbet@....net>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
    Boqun Feng <boqun.feng@...il.com>,
    Mark Rutland <mark.rutland@....com>,
    Arnd Bergmann <arnd@...db.de>,
    linux-kernel@...r.kernel.org,
    linux-arch@...r.kernel.org,
    Geert Uytterhoeven <geert@...ux-m68k.org>,
    linux-m68k@...r.kernel.org,
    linux-doc@...r.kernel.org
Subject: [RFC v2 1/3] documentation: Discourage alignment assumptions

Discourage assumptions that simply don't hold for all Linux ABIs.
Exceptions to the natural alignment rule for scalar types include
long long on i386 and sh.
---
 Documentation/core-api/unaligned-memory-access.rst | 7 -------
 1 file changed, 7 deletions(-)

diff --git a/Documentation/core-api/unaligned-memory-access.rst b/Documentation/core-api/unaligned-memory-access.rst
index 5ceeb80eb539..1390ce2b7291 100644
--- a/Documentation/core-api/unaligned-memory-access.rst
+++ b/Documentation/core-api/unaligned-memory-access.rst
@@ -40,9 +40,6 @@ The rule mentioned above forms what we refer to as natural alignment:
 When accessing N bytes of memory, the base memory address must be evenly
 divisible by N, i.e. addr % N == 0.
 
-When writing code, assume the target architecture has natural alignment
-requirements.
-
 In reality, only a few architectures require natural alignment on all sizes
 of memory access. However, we must consider ALL supported architectures;
 writing code that satisfies natural alignment requirements is the easiest way
@@ -103,10 +100,6 @@ Therefore, for standard structure types you can always rely on the compiler
 to pad structures so that accesses to fields are suitably aligned (assuming
 you do not cast the field to a type of different length).
 
-Similarly, you can also rely on the compiler to align variables and function
-parameters to a naturally aligned scheme, based on the size of the type of
-the variable.
-
 At this point, it should be clear that accessing a single byte (u8 or char)
 will never cause an unaligned access, because all memory addresses are evenly
 divisible by one.
-- 
2.49.1