[libc][startup]: create header for ElfW and use in startup

[nickdesaulniers]

This is necessary for 32b platforms such as ARM and i386.

Link: #94128

[llvmbot]

@llvm/pr-subscribers-libc

Author: Nick Desaulniers (paternity leave) (nickdesaulniers)

Changes

This is necessary for 32b platforms such as ARM and i386.

Link: #94128

---

Full diff: https://github.com/llvm/llvm-project/pull/96510.diff

2 Files Affected:

• (added) libc/include/llvm-libc-macros/link-macros.h (+13)
• (modified) libc/startup/linux/do_start.cpp (+5-4)

diff --git a/libc/include/llvm-libc-macros/link-macros.h b/libc/include/llvm-libc-macros/link-macros.h
new file mode 100644
index 0000000000000..5c8cadab8e71c
--- /dev/null
+++ b/libc/include/llvm-libc-macros/link-macros.h
@@ -0,0 +1,13 @@
+//===-- Definition of macros to for extra dynamic linker functionality ----===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifdef __LP64__
+#define ElfW(type) Elf64_ ## type
+#else
+#define ElfW(type) Elf32_ ## type
+#endif
diff --git a/libc/startup/linux/do_start.cpp b/libc/startup/linux/do_start.cpp
index 55fd575f7ad0b..3d7d32aead4fc 100644
--- a/libc/startup/linux/do_start.cpp
+++ b/libc/startup/linux/do_start.cpp
@@ -6,6 +6,7 @@
 //
 //===----------------------------------------------------------------------===//
 #include "startup/linux/do_start.h"
+#include "include/llvm-libc-macros/link-macros.h"
 #include "src/__support/OSUtil/syscall.h"
 #include "src/__support/threads/thread.h"
 #include "src/stdlib/atexit.h"
@@ -79,13 +80,13 @@ static ThreadAttributes main_thread_attrib;
 
   // After the env array, is the aux-vector. The end of the aux-vector is
   // denoted by an AT_NULL entry.
-  Elf64_Phdr *program_hdr_table = nullptr;
+  ElfW(Phdr) *program_hdr_table = nullptr;
   uintptr_t program_hdr_count = 0;
   app.auxv_ptr = reinterpret_cast<AuxEntry *>(env_end_marker + 1);
   for (auto *aux_entry = app.auxv_ptr; aux_entry->id != AT_NULL; ++aux_entry) {
     switch (aux_entry->id) {
     case AT_PHDR:
-      program_hdr_table = reinterpret_cast<Elf64_Phdr *>(aux_entry->value);
+      program_hdr_table = reinterpret_cast<ElfW(Phdr) *>(aux_entry->value);
       break;
     case AT_PHNUM:
       program_hdr_count = aux_entry->value;
@@ -100,10 +101,10 @@ static ThreadAttributes main_thread_attrib;
 
   ptrdiff_t base = 0;
   app.tls.size = 0;
-  Elf64_Phdr *tls_phdr = nullptr;
+  ElfW(Phdr) *tls_phdr = nullptr;
 
   for (uintptr_t i = 0; i < program_hdr_count; ++i) {
-    Elf64_Phdr &phdr = program_hdr_table[i];
+    ElfW(Phdr) &phdr = program_hdr_table[i];
     if (phdr.p_type == PT_PHDR)
       base = reinterpret_cast<ptrdiff_t>(program_hdr_table) - phdr.p_vaddr;
     if (phdr.p_type == PT_DYNAMIC && _DYNAMIC)

[nickdesaulniers]

probably need to update cmake