[libc] Make use of 32-bit time_t a config option (#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
(75398f2) 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.

Author: statham-arm

libc/cmake/modules/LLVMLibCCompileOptionRules.cmake

@@ -71,6 +71,10 @@ function(_get_compile_options_from_config output_var)
     list(APPEND config_options "-DLIBC_QSORT_IMPL=${LIBC_CONF_QSORT_IMPL}")
   endif()
 
+  if(LIBC_TYPES_TIME_T_IS_32_BIT AND LLVM_LIBC_FULL_BUILD)
+    list(APPEND config_options "-DLIBC_TYPES_TIME_T_IS_32_BIT")
+  endif()
+
   set(${output_var} ${config_options} PARENT_SCOPE)
 endfunction(_get_compile_options_from_config)
 

libc/cmake/modules/LLVMLibCFlagRules.cmake

@@ -286,3 +286,16 @@ if(NOT((LIBC_TARGET_ARCHITECTURE_IS_X86 AND (LIBC_CPU_FEATURES MATCHES "SSE4_2")
        LIBC_TARGET_ARCHITECTURE_IS_AARCH64 OR LIBC_TARGET_OS_IS_GPU))
   set(SKIP_FLAG_EXPANSION_ROUND_OPT TRUE)
 endif()
+
+# Choose whether time_t is 32- or 64-bit, based on target architecture
+# and config options. This will be used to set a #define during the
+# library build, and also to select the right version of time_t.h for
+# the output headers.
+if(LIBC_TARGET_ARCHITECTURE_IS_ARM AND NOT (LIBC_CONF_TIME_64BIT))
+  # Set time_t to 32 bit for compatibility with glibc, unless
+  # configuration says otherwise
+  set(LIBC_TYPES_TIME_T_IS_32_BIT TRUE)
+else()
+  # Other platforms default to 64-bit time_t
+  set(LIBC_TYPES_TIME_T_IS_32_BIT FALSE)
+endif()

libc/cmake/modules/LLVMLibCHeaderRules.cmake

@@ -9,16 +9,21 @@
 function(add_header target_name)
   cmake_parse_arguments(
     "ADD_HEADER"
-    ""    # No optional arguments
-    "HDR" # Single value arguments
+    ""             # No optional arguments
+    "HDR;DEST_HDR" # Single value arguments
     "DEPENDS"
     ${ARGN}
   )
   if(NOT ADD_HEADER_HDR)
     message(FATAL_ERROR "'add_header' rules requires the HDR argument specifying a headef file.")
   endif()
 
-  set(absolute_path ${CMAKE_CURRENT_SOURCE_DIR}/${ADD_HEADER_HDR})
+  if(ADD_HEADER_DEST_HDR)
+    set(dest_leaf_filename ${ADD_HEADER_DEST_HDR})
+  else()
+    set(dest_leaf_filename ${ADD_HEADER_HDR})
+  endif()
+  set(absolute_path ${CMAKE_CURRENT_SOURCE_DIR}/${dest_leaf_filename})
   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})

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."
+    }
   }
 }

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.

libc/include/llvm-libc-types/CMakeLists.txt

@@ -59,7 +59,11 @@ add_header(pthread_rwlockattr_t HDR pthread_rwlockattr_t.h)
 add_header(pthread_spinlock_t HDR pthread_spinlock_t.h DEPENDS .pid_t)
 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_TYPES_TIME_T_IS_32_BIT)
+  add_header(time_t HDR time_t_32.h DEST_HDR time_t.h)
+else()
+  add_header(time_t HDR time_t_64.h DEST_HDR time_t.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)

libc/include/llvm-libc-types/time_t.h

@@ -1,4 +1,4 @@
-//===-- Definition of the type time_t -------------------------------------===//
+//===-- Definition of the type time_t, for use during the libc build ------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
@@ -9,10 +9,10 @@
 #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;
+#ifdef LIBC_TYPES_TIME_T_IS_32_BIT
+#include "time_t_32.h"
 #else
-typedef __INT64_TYPE__ time_t;
+#include "time_t_64.h"
 #endif
 
 #endif // LLVM_LIBC_TYPES_TIME_T_H

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_32_H
+#define LLVM_LIBC_TYPES_TIME_T_32_H
+
+typedef __INT32_TYPE__ time_t;
+
+#endif // LLVM_LIBC_TYPES_TIME_T_32_H

libc/include/llvm-libc-types/time_t_64.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_64_H
+#define LLVM_LIBC_TYPES_TIME_T_64_H
+
+typedef __INT64_TYPE__ time_t;
+
+#endif // LLVM_LIBC_TYPES_TIME_T_64_H