[libc-commits] [libc] [libc] Fix malloc Block alignment issue on riscv32 (PR #117815)

[Nick Desaulniers via libc-commits]

================
@@ -186,11 +174,19 @@ class Block {
   }
 
   /// @returns A pointer to the usable space inside this block.
+  ///
+  /// Aligned to some multiple of max_align_t.
   LIBC_INLINE cpp::byte *usable_space() {
-    return reinterpret_cast<cpp::byte *>(this) + BLOCK_OVERHEAD;
+    auto *s = reinterpret_cast<cpp::byte *>(this) + BLOCK_OVERHEAD;
+    LIBC_ASSERT(reinterpret_cast<uintptr_t>(s) % alignof(max_align_t) == 0 &&
+                "usable space must be aligned to a multiple of max_align_t");
+    return s;
   }
   LIBC_INLINE const cpp::byte *usable_space() const {
-    return reinterpret_cast<const cpp::byte *>(this) + BLOCK_OVERHEAD;
+    const auto *s = reinterpret_cast<const cpp::byte *>(this) + BLOCK_OVERHEAD;
+    LIBC_ASSERT(reinterpret_cast<uintptr_t>(s) % alignof(max_align_t) == 0 &&
+                "usable space must be aligned to a multiple of max_align_t");
----------------
nickdesaulniers wrote:

```
llvm-project/libc/src/__support/block.h:188: Assertion failed: 'reinterpret_cast<uintptr_t>(s) % alignof(max_align_t) == 0 && "usable space must be aligned to a multiple of max_align_t"' in function: 'const cpp::byte *__llvm_libc_20_0_0_git::Block::usable_space() const'
```

https://github.com/llvm/llvm-project/pull/117815