[libc-commits] [libc] [libc][malloc] Ensure a minimum block alignment of 4 (PR #169447)

[Daniel Thornburgh via libc-commits]

https://github.com/mysterymath created https://github.com/llvm/llvm-project/pull/169447

Most platforms inherently have a size_t alignment of 4, but this isn't true on every platform LLVM has some degree of backend support for. Accordingly, it's simple enough to just set the min alignment of Block to 4 and lose the static_assert.

>From d74bdfab2b7f5885de3ae98965ee85bc0a393fd1 Mon Sep 17 00:00:00 2001
From: Daniel Thornburgh <mysterymath at gmail.com>
Date: Mon, 24 Nov 2025 10:51:01 -0800
Subject: [PATCH] [libc][malloc] Ensure a minimum block alignment of 4

Most platforms inherently have a size_t alignment of 4, but this isn't
true on every platform LLVM has some degree of backend support for.
Accordingly, it's simple enough to just set the min alignment of Block
to 4 and lose the static_assert.
---
 libc/src/__support/block.h | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/libc/src/__support/block.h b/libc/src/__support/block.h
index b0d6576093244..faab409166c6e 100644
--- a/libc/src/__support/block.h
+++ b/libc/src/__support/block.h
@@ -90,7 +90,10 @@ using cpp::optional;
 ///
 /// The next offset of a block matches the previous offset of its next block.
 /// The first block in a list is denoted by having a previous offset of `0`.
-class Block {
+///
+// Ensure a minimum alignment of 4, since at least 2 bits must be available in
+// block sizes for flags.
+class alignas(cpp::max(alignof(size_t), size_t{4})) Block {
   // Masks for the contents of the next_ field.
   static constexpr size_t PREV_FREE_MASK = 1 << 0;
   static constexpr size_t LAST_MASK = 1 << 1;
@@ -360,9 +363,6 @@ class Block {
   static constexpr size_t PREV_FIELD_SIZE = sizeof(prev_);
 };
 
-static_assert(alignof(Block) >= 4,
-              "at least 2 bits must be available in block sizes for flags");
-
 LIBC_INLINE
 optional<Block *> Block::init(ByteSpan region) {
   if (!region.data())