[libc] implement memalignment

[hulxv]

This patch adds the memalignment function to LLVM-libc, following its description in WG14 N3220, §7.24.2.1.

• Add the implementation of memalignment in /src/stdlib
• Add tests for memalignment in /test/src/stdlib
• Add memalignment to entrypoints.txt for at least x86_64 and whatever you're building on
• Add memalignment to include/stdlib.yaml

Closes #132300

[llvmbot]

@llvm/pr-subscribers-backend-amdgpu

@llvm/pr-subscribers-libc

Author: Mohamed Emad (hulxv)

Changes

This patch adds the memalignment function to LLVM-libc, following its description in WG14 N3220, §7.24.2.1.

Closes #132300

---

Full diff: https://github.com/llvm/llvm-project/pull/132493.diff

2 Files Affected:

• (added) libc/src/stdlib/memalignment.c (+19)
• (added) libc/src/stdlib/memalignment.h (+22)

diff --git a/libc/src/stdlib/memalignment.c b/libc/src/stdlib/memalignment.c
new file mode 100644
index 0000000000000..bf182f16ea138
--- /dev/null
+++ b/libc/src/stdlib/memalignment.c
@@ -0,0 +1,19 @@
+#include "src/stdlib/memalignment.h"
+#include "src/__support/macros/config.h"
+
+namespace LIBC_NAMESPACE_DECL {
+
+LLVM_LIBC_FUNCTION(size_t, memalignment, (const void *p)) {
+  if (p == NULL) {
+    return 0;
+  }
+
+  uintptr_t addr = (uintptr_t)p;
+
+  // Find the rightmost set bit, which represents the maximum alignment
+  // The alignment is a power of two, so we need to find the largest
+  // power of two that divides the address
+  return addr & (~addr + 1);
+}
+
+} // namespace LIBC_NAMESPACE_DECL
diff --git a/libc/src/stdlib/memalignment.h b/libc/src/stdlib/memalignment.h
new file mode 100644
index 0000000000000..8ed900122a153
--- /dev/null
+++ b/libc/src/stdlib/memalignment.h
@@ -0,0 +1,22 @@
+//===-- Implementation header for memalignment --------------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SRC_STDLIB_MEM_ALIGNMENT_H
+#define LLVM_LIBC_SRC_STDLIB_MEM_ALIGNMENT_H
+
+#include "src/__support/macros/config.h"
+#include <stddef.h>
+
+namespace LIBC_NAMESPACE_DECL {
+
+size_t memalignment(const void* p);
+
+} // namespace LIBC_NAMESPACE_DECL
+
+#endif // LLVM_LIBC_SRC_STDLIB_LDIV_H
+

[github-actions]

✅ With the latest revision this PR passed the C/C++ code formatter.

[hulxv]

@michaelrj-google Should I add it to stdlib.yaml or stdlib-malloc.yaml ?

[jhuber6]

Unrelated to this patch: I think this is also one of the freestanding functions, I wonder what the plan is to provide that? Normally those go in the clang resource directory headers but we don't have a freestanding stdlib.h there yet.

[jhuber6]

Suggested change

	return addr & (~addr + 1);
	return addr & (~addr + 1);

I can't decide if it would be better to just use ctz. They reduce to the same ASM https://godbolt.org/z/qTjxc6dPn.

[jhuber6]

Made up my mind, the bit twiddling is good here, while 1ull << cpp::count_trailing_zeroes(addr) is probably easier to read, I think it performs worse for 64-bit values.

[hulxv]

then going with __builtin_ctzg is better?

[jhuber6]

No, the opposite.

[hulxv]

does 1ull << cpp::count_trailing_zeroes(addr) exist? I don't find it anywhere. what about using countr_zeros?

[jhuber6]

Go ahead and add this to all the entrypoints, it's a freestanding function so I'd be amazed if it didn't work on those targets. You also need to add it to the yaml if you want it to actually show up when it's installed.

[michaelrj-google]

Looks like you're making good progress, just a bit more and this should be ready to land

[hulxv]

@michaelrj-google all done now?

[jhuber6]

FAILED: libc/test/src/stdlib/libc.test.src.stdlib.memalignment_test.__unit__.__build__ 
: && /usr/bin/clang++ -fvisibility-inlines-hidden -Werror=date-time -Werror=unguarded-availability-new -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wmissing-field-initializers -Wimplicit-fallthrough -Wcovered-switch-default -Wno-noexcept-type -Wnon-virtual-dtor -Wdelete-non-virtual-dtor -Wsuggest-override -Wstring-conversion -Wmisleading-indentation -Wctad-maybe-unsupported -fdiagnostics-color -g -fpie -DLIBC_FULL_BUILD -ffreestanding -fno-exceptions -fno-unwind-tables -fno-asynchronous-unwind-tables -fno-rtti -Wconversion -Wno-sign-conversion -Wimplicit-fallthrough -Wwrite-strings -Wno-c99-extensions -Wno-gnu-imaginary-constant -Wno-pedantic -Wstrict-prototypes -Wextra-semi -Wnewline-eof -Wnonportable-system-include-path -Wthread-safety -Xlinker --dependency-file=libc/test/src/stdlib/CMakeFiles/libc.test.src.stdlib.memalignment_test.__unit__.__build__.dir/link.d libc/test/src/stdlib/CMakeFiles/libc.test.src.stdlib.memalignment_test.__unit__.__build__.dir/memalignment_test.cpp.o -o libc/test/src/stdlib/libc.test.src.stdlib.memalignment_test.__unit__.__build__  libc/src/stdlib/CMakeFiles/libc.src.stdlib.memalignment.__internal__.dir/./memalignment.cpp.o  libc/src/errno/CMakeFiles/libc.src.errno.errno.__internal__.dir/./libc_errno.cpp.o  libc/src/__support/StringUtil/CMakeFiles/libc.src.__support.StringUtil.error_to_string.dir/./error_to_string.cpp.o  libc/src/__support/OSUtil/linux/CMakeFiles/libc.src.__support.OSUtil.linux.linux_util.dir/./exit.cpp.o  libc/src/__support/OSUtil/linux/CMakeFiles/libc.src.__support.OSUtil.linux.linux_util.dir/./fcntl.cpp.o  libc/test/UnitTest/libLibcDeathTestExecutors.unit.a  libc/test/UnitTest/libLibcTest.unit.a && :
/usr/bin/ld: libc/test/src/stdlib/CMakeFiles/libc.test.src.stdlib.memalignment_test.__unit__.__build__.dir/memalignment_test.cpp.o: in function `LlvmLibcMemAlignmentTest_MallocedPointers::Run()':
/home/runner/work/llvm-project/llvm-project/libc/test/src/stdlib/memalignment_test.cpp:22:(.text+0xb4): undefined reference to `__llvm_libc_21_0_0_git::malloc(unsigned long)'
/usr/bin/ld: /home/runner/work/llvm-project/llvm-project/libc/test/src/stdlib/memalignment_test.cpp:28:(.text+0x1a4): undefined reference to `__llvm_libc_21_0_0_git::free(void*)'
/usr/bin/ld: /home/runner/work/llvm-project/llvm-project/libc/test/src/stdlib/memalignment_test.cpp:32:(.text+0x1ac): undefined reference to `__llvm_libc_21_0_0_git::malloc(unsigned long)'
/usr/bin/ld: /home/runner/work/llvm-project/llvm-project/libc/test/src/stdlib/memalignment_test.cpp:40:(.text+0x2fc): undefined reference to `__llvm_libc_21_0_0_git::free(void*)'
/usr/bin/ld: libc/test/src/stdlib/libc.test.src.stdlib.memalignment_test.__unit__.__build__: hidden symbol `_ZN22__llvm_libc_21_0_0_git4freeEPv' isn't defined
/usr/bin/ld: final link failed: bad value

Bot fails tests. Honestly you should probably just remove the dependencies since this doesn't need malloc to be tested (we have alignas).

[arsenm]

This should probably be an intrinsic so we don't introduce the ptrtoint, it's the opposite of llvm.ptrmask

[jhuber6]

This should probably be an intrinsic so we don't introduce the ptrtoint, it's the opposite of llvm.ptrmask

Presumably we'd want a fallback for GCC, but that definitely sounds compelling considering that this is a freestanding function, so we could just implement it as a builitin in a header for that case.

[michaelrj-google]

One tweak, then this is good to land.

Do you have commit access or do you need me to merge it for you?

[hulxv]

Do you have commit access or do you need me to merge it for you?

No, I don't have access. Please merge it

[michaelrj-google]

Need to fix the buildbots before merging. One of the parts of the test is failing.

[hulxv]

It passed the tests

[michaelrj-google]

LGTM, thanks for getting this done!