[libc-commits] [libc] [libc] Make use of 32-bit time_t a config option (PR #102012)

Mon Aug 5 09:37:03 PDT 2024

https://github.com/statham-arm created https://github.com/llvm/llvm-project/pull/102012

The 32-bit Arm builds of libc define time_t to be `__INTPTR_TYPE__`, i.e. a 32-bit integer. This is commented in the commit introducing it (75398f28ebdb600) as being for compatibility with glibc. But in the near future not even every AArch32 build of glibc will have a 32-bit time_t: Debian is planning that their next release (trixie) will have switched to 64-bit. And non-Linux builds of this libc (e.g. baremetal) have no reason to need glibc compatibility in the first place – and every reason _not_ to want to start using a 32-bit time_t in 2024 or later.

So I've replaced the `#ifdef` in `llvm-libc-types/time_t.h` with two versions of the header file, chosen in `CMakeLists.txt` via a new configuration option. This involved adding an extra parameter to the cmake `add_header` function to specify different names for the header file in the source and destination directories.

>From fad0d6a3043680273512e4716d6c8a0b45dcde5a Mon Sep 17 00:00:00 2001
From: Simon Tatham <simon.tatham at arm.com>
Date: Mon, 5 Aug 2024 16:35:41 +0100
Subject: [PATCH] [libc] Make use of 32-bit time_t a config option
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The 32-bit Arm builds of libc define time_t to be `__INTPTR_TYPE__`,
i.e. a 32-bit integer. This is commented in the commit introducing it
(75398f28ebdb600) as being for compatibility with glibc. But in the
near future not even every AArch32 build of glibc will have a 32-bit
time_t: Debian is planning that their next release (trixie) will have
switched to 64-bit. And non-Linux builds of this libc (e.g. baremetal)
have no reason to need glibc compatibility in the first place – and
every reason _not_ to want to start using a 32-bit time_t in 2024 or
later.

So I've replaced the `#ifdef` in `llvm-libc-types/time_t.h` with two
versions of the header file, chosen in `CMakeLists.txt` via a new
configuration option. This involved adding an extra parameter to the
cmake `add_header` function to specify different names for the header
file in the source and destination directories.
---
 libc/cmake/modules/LLVMLibCHeaderRules.cmake       |  8 ++++++--
 libc/config/config.json                            |  6 ++++++
 libc/docs/configure.rst                            |  2 ++
 libc/include/llvm-libc-types/CMakeLists.txt        | 11 ++++++++++-
 libc/include/llvm-libc-types/time_t_32.h           | 14 ++++++++++++++
 .../llvm-libc-types/{time_t.h => time_t_64.h}      |  4 ----
 6 files changed, 38 insertions(+), 7 deletions(-)
 create mode 100644 libc/include/llvm-libc-types/time_t_32.h
 rename libc/include/llvm-libc-types/{time_t.h => time_t_64.h} (85%)

diff --git a/libc/cmake/modules/LLVMLibCHeaderRules.cmake b/libc/cmake/modules/LLVMLibCHeaderRules.cmake
index 3049f4db7301f..5b29fa7897e1c 100644
--- a/libc/cmake/modules/LLVMLibCHeaderRules.cmake
+++ b/libc/cmake/modules/LLVMLibCHeaderRules.cmake
@@ -10,7 +10,7 @@ function(add_header target_name)
   cmake_parse_arguments(
     "ADD_HEADER"
     ""    # No optional arguments
-    "HDR" # Single value arguments
+    "HDR;SRCHDR" # Single value arguments
     "DEPENDS"
     ${ARGN}
   )
@@ -21,7 +21,11 @@ function(add_header target_name)
   set(absolute_path ${CMAKE_CURRENT_SOURCE_DIR}/${ADD_HEADER_HDR})
   file(RELATIVE_PATH relative_path ${LIBC_INCLUDE_SOURCE_DIR} ${absolute_path})
   set(dest_file ${LIBC_INCLUDE_DIR}/${relative_path})
-  set(src_file ${CMAKE_CURRENT_SOURCE_DIR}/${ADD_HEADER_HDR})
+  if(ADD_HEADER_SRCHDR)
+    set(src_file ${CMAKE_CURRENT_SOURCE_DIR}/${ADD_HEADER_SRCHDR})
+  else()
+    set(src_file ${CMAKE_CURRENT_SOURCE_DIR}/${ADD_HEADER_HDR})
+  endif()
 
   add_custom_command(
     OUTPUT ${dest_file}
diff --git a/libc/config/config.json b/libc/config/config.json
index 538fea53cc704..2e72c0a3fd1d6 100644
--- a/libc/config/config.json
+++ b/libc/config/config.json
@@ -88,5 +88,11 @@
       "value": true,
       "doc": "Make setjmp save the value of x18, and longjmp restore it. The AArch64 ABI delegates this register to platform ABIs, which can choose whether to make it caller-saved."
     }
+  },
+  "time": {
+    "LIBC_CONF_TIME_64BIT": {
+      "value": false,
+      "doc": "Force the size of time_t to 64 bits, even on platforms where compatibility considerations would otherwise make it 32-bit."
+    }
   }
 }
diff --git a/libc/docs/configure.rst b/libc/docs/configure.rst
index 950de0eee4c05..54ca5d55d7b24 100644
--- a/libc/docs/configure.rst
+++ b/libc/docs/configure.rst
@@ -52,3 +52,5 @@ to learn about the defaults for your platform and target.
 * **"string" options**
     - ``LIBC_CONF_MEMSET_X86_USE_SOFTWARE_PREFETCHING``: Inserts prefetch for write instructions (PREFETCHW) for memset on x86 to recover performance when hardware prefetcher is disabled.
     - ``LIBC_CONF_STRING_UNSAFE_WIDE_READ``: Read more than a byte at a time to perform byte-string operations like strlen.
+* **"time" options**
+    - ``LIBC_CONF_TIME_64BIT``: Force the size of time_t to 64 bits, even on platforms where compatibility considerations would otherwise make it 32-bit.
diff --git a/libc/include/llvm-libc-types/CMakeLists.txt b/libc/include/llvm-libc-types/CMakeLists.txt
index d8b975572e0dd..3dfa39b9b132b 100644
--- a/libc/include/llvm-libc-types/CMakeLists.txt
+++ b/libc/include/llvm-libc-types/CMakeLists.txt
@@ -58,7 +58,16 @@ add_header(pthread_rwlock_t HDR pthread_rwlock_t.h DEPENDS .__futex_word .pid_t)
 add_header(pthread_rwlockattr_t HDR pthread_rwlockattr_t.h)
 add_header(pthread_t HDR pthread_t.h DEPENDS .__thread_type)
 add_header(rlim_t HDR rlim_t.h)
-add_header(time_t HDR time_t.h)
+if(LIBC_CONF_TIME_64BIT)
+  # Set time_t to 64 bit as requested in configuration
+  add_header(time_t HDR time_t.h SRCHDR time_t_64.h)
+elseif(LIBC_TARGET_ARCHITECTURE_IS_ARM)
+  # Set time_t to 32 bit for compatibility with glibc
+  add_header(time_t HDR time_t.h SRCHDR time_t_32.h)
+else()
+  # Other platforms default to 64-bit time_t
+  add_header(time_t HDR time_t.h SRCHDR time_t_64.h)
+endif()
 add_header(stack_t HDR stack_t.h DEPENDS .size_t)
 add_header(suseconds_t HDR suseconds_t.h)
 add_header(struct_flock HDR struct_flock.h DEPENDS .off_t .pid_t)
diff --git a/libc/include/llvm-libc-types/time_t_32.h b/libc/include/llvm-libc-types/time_t_32.h
new file mode 100644
index 0000000000000..ce5a3e8b73463
--- /dev/null
+++ b/libc/include/llvm-libc-types/time_t_32.h
@@ -0,0 +1,14 @@
+//===-- Definition of the type time_t -------------------------------------===//
+//
+// 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_TYPES_TIME_T_H
+#define LLVM_LIBC_TYPES_TIME_T_H
+
+typedef __INT32_TYPE__ time_t;
+
+#endif // LLVM_LIBC_TYPES_TIME_T_H
diff --git a/libc/include/llvm-libc-types/time_t.h b/libc/include/llvm-libc-types/time_t_64.h
similarity index 85%
rename from libc/include/llvm-libc-types/time_t.h
rename to libc/include/llvm-libc-types/time_t_64.h
index 59953b343ba96..c2c909e226144 100644
--- a/libc/include/llvm-libc-types/time_t.h
+++ b/libc/include/llvm-libc-types/time_t_64.h
@@ -9,10 +9,6 @@
 #ifndef LLVM_LIBC_TYPES_TIME_T_H
 #define LLVM_LIBC_TYPES_TIME_T_H
 
-#if (defined(__arm__) || defined(_M_ARM))
-typedef __INTPTR_TYPE__ time_t;
-#else
 typedef __INT64_TYPE__ time_t;
-#endif
 
 #endif // LLVM_LIBC_TYPES_TIME_T_H

