[libc-commits] [libc] [libc][arm32] define argc type and stack alignment (PR #96367)

[Nick Desaulniers via libc-commits]

https://github.com/nickdesaulniers updated https://github.com/llvm/llvm-project/pull/96367

>From 2489c6cfbb5c81034a9103c991fc2f5c7d130107 Mon Sep 17 00:00:00 2001
From: Nick Desaulniers <ndesaulniers at google.com>
Date: Fri, 21 Jun 2024 15:59:17 -0700
Subject: [PATCH] [libc][arm32] define argc type and stack alignment

https://github.com/ARM-software/abi-aa/blob/main/aapcs32/aapcs32.rst#6212stack-constraints-at-a-public-interface
mentions that the stack on ARM32 is double word aligned.

Remove confused comments around ArgcType.  argc is always an int, passed on the
stack, so we need to store a pointer to it (regardless of ILP32 or LP64).
---
 libc/config/linux/app.h             | 24 +++---------------------
 libc/src/__support/threads/thread.h |  3 +++
 libc/startup/linux/do_start.cpp     |  4 ++--
 3 files changed, 8 insertions(+), 23 deletions(-)

diff --git a/libc/config/linux/app.h b/libc/config/linux/app.h
index 766cd49e88f6f..2a3b1560817b8 100644
--- a/libc/config/linux/app.h
+++ b/libc/config/linux/app.h
@@ -35,24 +35,6 @@ struct TLSImage {
   uintptr_t align;
 };
 
-#if defined(LIBC_TARGET_ARCH_IS_X86_64) ||                                     \
-    defined(LIBC_TARGET_ARCH_IS_AARCH64) ||                                    \
-    defined(LIBC_TARGET_ARCH_IS_ANY_RISCV)
-// At the language level, argc is an int. But we use uint64_t as the x86_64
-// ABI specifies it as an 8 byte value. Likewise, in the ARM64 ABI, arguments
-// are usually passed in registers.  x0 is a doubleword register, so this is
-// 64 bit for aarch64 as well.
-typedef uintptr_t ArgcType;
-
-// At the language level, argv is a char** value. However, we use uint64_t as
-// ABIs specify the argv vector be an |argc| long array of 8-byte values.
-typedef uintptr_t ArgVEntryType;
-
-typedef uintptr_t EnvironType;
-#else
-#error "argc and argv types are not defined for the target platform."
-#endif
-
 // Linux manpage on `proc(5)` says that the aux vector is an array of
 // unsigned long pairs.
 // (see: https://man7.org/linux/man-pages/man5/proc.5.html)
@@ -65,7 +47,7 @@ struct AuxEntry {
 };
 
 struct Args {
-  ArgcType argc;
+  uintptr_t argc;
 
   // A flexible length array would be more suitable here, but C++ doesn't have
   // flexible arrays: P1039 proposes to fix this. So, for now we just fake it.
@@ -73,7 +55,7 @@ struct Args {
   // (ISO C 5.1.2.2.1) so one is fine. Also, length of 1 is not really wrong as
   // |argc| is guaranteed to be atleast 1, and there is an 8-byte null entry at
   // the end of the argv array.
-  ArgVEntryType argv[1];
+  uintptr_t argv[1];
 };
 
 // Data structure which captures properties of a linux application.
@@ -87,7 +69,7 @@ struct AppProperties {
   TLSImage tls;
 
   // Environment data.
-  EnvironType *env_ptr;
+  uintptr_t *env_ptr;
 
   // Auxiliary vector data.
   AuxEntry *auxv_ptr;
diff --git a/libc/src/__support/threads/thread.h b/libc/src/__support/threads/thread.h
index acfe33879f878..f89c687eeaa19 100644
--- a/libc/src/__support/threads/thread.h
+++ b/libc/src/__support/threads/thread.h
@@ -43,6 +43,9 @@ union ThreadReturnValue {
      defined(LIBC_TARGET_ARCH_IS_X86_64) ||                                    \
      defined(LIBC_TARGET_ARCH_IS_ANY_RISCV))
 constexpr unsigned int STACK_ALIGNMENT = 16;
+#elif defined(LIBC_TARGET_ARCH_IS_ARM)
+// See Section 6.2.1.2 Stack constraints at a public interface of AAPCS32.
+constexpr unsigned int STACK_ALIGNMENT = 8;
 #endif
 // TODO: Provide stack alignment requirements for other architectures.
 
diff --git a/libc/startup/linux/do_start.cpp b/libc/startup/linux/do_start.cpp
index 55fd575f7ad0b..005ecbe991904 100644
--- a/libc/startup/linux/do_start.cpp
+++ b/libc/startup/linux/do_start.cpp
@@ -68,8 +68,8 @@ static ThreadAttributes main_thread_attrib;
   // After the argv array, is a 8-byte long NULL value before the array of env
   // values. The end of the env values is marked by another 8-byte long NULL
   // value. We step over it (the "+ 1" below) to get to the env values.
-  ArgVEntryType *env_ptr = app.args->argv + app.args->argc + 1;
-  ArgVEntryType *env_end_marker = env_ptr;
+  uintptr_t *env_ptr = app.args->argv + app.args->argc + 1;
+  uintptr_t *env_end_marker = env_ptr;
   app.env_ptr = env_ptr;
   while (*env_end_marker)
     ++env_end_marker;