[libc] implement memalignment

libc/config/linux/x86_64/entrypoints.txt

@@ -214,6 +214,7 @@ set(TARGET_LIBC_ENTRYPOINTS
     libc.src.stdlib.free
     libc.src.stdlib.malloc
     libc.src.stdlib.realloc
+    libc.src.stdlib.memalignment
 
     # stdio.h entrypoints
     libc.src.stdio.fprintf

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

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

libc/test/src/stdlib/CMakeLists.txt

@@ -457,4 +457,16 @@ if(LLVM_LIBC_FULL_BUILD)
         libc.src.stdlib.free
     )
   endif()
+  add_libc_test(
+    memalignment_test
+    # The EXPECT_EXITS test is only availible for unit tests.
+    UNIT_TEST_ONLY
+    SUITE
+      libc-stdlib-tests
+    SRCS
+      memalignment_test.cpp
+    DEPENDS
+      libc.include.stdlib
+      libc.src.stdlib.memalignment
+  )
 endif()

libc/test/src/stdlib/memalignment_test.cpp

@@ -0,0 +1,67 @@
+//===-- Unittests for memalignment ---------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/stdlib/free.h"
+#include "src/stdlib/malloc.h"
+#include "src/stdlib/memalignment.h"
+#include "test/UnitTest/Test.h"
+
+#include <stdint.h>
+
+TEST(LlvmLibcMemAlignmentTest, NullPointer) {
+  void *ptr = nullptr;
+  EXPECT_EQ(LIBC_NAMESPACE::memalignment(ptr), static_cast<size_t>(0));
+}
+
+TEST(LlvmLibcMemAlignmentTest, MallocedPointers) {
+  int *int_ptr = reinterpret_cast<int *>(LIBC_NAMESPACE::malloc(sizeof(int)));
+  EXPECT_NE(reinterpret_cast<void *>(int_ptr), static_cast<void *>(nullptr));
+
+  size_t int_alignment = LIBC_NAMESPACE::memalignment(int_ptr);
+  EXPECT_GE(int_alignment, alignof(int));
+
+  LIBC_NAMESPACE::free(int_ptr);
+
+  // Allocate a double (typically 8-byte aligned)
+  double *double_ptr =
+      reinterpret_cast<double *>(LIBC_NAMESPACE::malloc(sizeof(double)));
+  EXPECT_NE(reinterpret_cast<void *>(double_ptr), static_cast<void *>(nullptr));
+
+  size_t double_alignment = LIBC_NAMESPACE::memalignment(double_ptr);
+  EXPECT_GE(double_alignment, alignof(double));
+
+  EXPECT_EQ(double_alignment & (double_alignment - 1), static_cast<size_t>(0));
+
+  LIBC_NAMESPACE::free(double_ptr);
+}
+
+TEST(LlvmLibcMemAlignmentTest, SpecificAlignment) {
+
+  // These addresses have known alignment patterns - if we can construct them
+  uintptr_t addr_align2 = 0x2;   // 2-byte aligned
+  uintptr_t addr_align4 = 0x4;   // 4-byte aligned
+  uintptr_t addr_align8 = 0x8;   // 8-byte aligned
+  uintptr_t addr_align16 = 0x10; // 16-byte aligned
+  uintptr_t addr_align32 = 0x20; // 32-byte aligned
+
+  void *ptr_align2 = reinterpret_cast<void *>(addr_align2);
+  void *ptr_align4 = reinterpret_cast<void *>(addr_align4);
+  void *ptr_align8 = reinterpret_cast<void *>(addr_align8);
+  void *ptr_align16 = reinterpret_cast<void *>(addr_align16);
+  void *ptr_align32 = reinterpret_cast<void *>(addr_align32);
+
+  EXPECT_EQ(LIBC_NAMESPACE::memalignment(ptr_align2), static_cast<size_t>(2));
+  EXPECT_EQ(LIBC_NAMESPACE::memalignment(ptr_align4), static_cast<size_t>(4));
+  EXPECT_EQ(LIBC_NAMESPACE::memalignment(ptr_align8), static_cast<size_t>(8));
+  EXPECT_EQ(LIBC_NAMESPACE::memalignment(ptr_align16), static_cast<size_t>(16));
+  EXPECT_EQ(LIBC_NAMESPACE::memalignment(ptr_align32), static_cast<size_t>(32));
+
+  uintptr_t addr_complex = 0x1234560; // 16-byte aligned (ends in 0)
+  void *ptr_complex = reinterpret_cast<void *>(addr_complex);
+  EXPECT_EQ(LIBC_NAMESPACE::memalignment(ptr_complex), static_cast<size_t>(16));
+}