[libc] Fix alignment issue for HermeticTestUtils.cpp.

[lntue]

Full build precommit bots were failing due to mis-alignment of atomics in hermetic tests. This PR enforces the alignment for the bump allocator of hermetic test framework.

Fixes #128185.

[llvmbot]

@llvm/pr-subscribers-libc

Author: None (lntue)

Changes

Full build precommit bots were failing due to mis-alignment of atomics in hermetic tests. This PR enforces the alignment for the bump allocator of hermetic test framework.

Fixes #128185.

---

Full diff: https://github.com/llvm/llvm-project/pull/128426.diff

1 Files Affected:

• (modified) libc/test/UnitTest/HermeticTestUtils.cpp (+5-3)

diff --git a/libc/test/UnitTest/HermeticTestUtils.cpp b/libc/test/UnitTest/HermeticTestUtils.cpp
index 47f813b0b7a4e..343b622fd4f30 100644
--- a/libc/test/UnitTest/HermeticTestUtils.cpp
+++ b/libc/test/UnitTest/HermeticTestUtils.cpp
@@ -33,6 +33,10 @@ int atexit(void (*func)(void));
 
 } // namespace LIBC_NAMESPACE_DECL
 
+extern "C" {
+constexpr uint64_t ALIGNMENT = alignof(uintptr_t);
+}
+
 namespace {
 
 // Integration tests cannot use the SCUDO standalone allocator as SCUDO pulls
@@ -42,7 +46,7 @@ namespace {
 // which just hands out continuous blocks from a statically allocated chunk of
 // memory.
 static constexpr uint64_t MEMORY_SIZE = 65336;
-static uint8_t memory[MEMORY_SIZE];
+alignas(ALIGNMENT) static uint8_t memory[MEMORY_SIZE];
 static uint8_t *ptr = memory;
 
 } // anonymous namespace
@@ -74,8 +78,6 @@ void *memset(void *ptr, int value, size_t count) {
 // This is needed if the test was compiled with '-fno-use-cxa-atexit'.
 int atexit(void (*func)(void)) { return LIBC_NAMESPACE::atexit(func); }
 
-constexpr uint64_t ALIGNMENT = alignof(uintptr_t);
-
 void *malloc(size_t s) {
   // Keep the bump pointer aligned on an eight byte boundary.
   s = ((s + ALIGNMENT - 1) / ALIGNMENT) * ALIGNMENT;

[jhuber6]

Why extern C? Constexpr has no linkage.

[lntue]

it was inside an extern "C" block before so I kept it the same way when moving up. Is it safe to remove extern "C" then?

[jhuber6]

Should be.

[lntue]

Done.