[libc] Add arm-32 syscall implementation.

[sivachandra]

Summary

The implementation currently supports only non-thumb mode. As a test for
the implementation, mmap and munmap functions have been enabled.

[michaelrj]

LGTM with one nit

libc/src/__support/OSUtil/linux/arm/syscall.h

25

nit: should this be formatted like the rest?

libc/config/linux/arm/entrypoints.txt

set(TARGET_LIBC_ENTRYPOINTS
     libc.src.string.strncpy
     libc.src.string.strnlen
     libc.src.string.strpbrk
     libc.src.string.strrchr
     libc.src.string.strspn
     libc.src.string.strstr
     libc.src.string.strtok
     libc.src.string.strtok_r
+
     # inttypes.h entrypoints
     libc.src.inttypes.imaxabs
     libc.src.inttypes.imaxdiv
     libc.src.inttypes.strtoimax
     libc.src.inttypes.strtoumax
  
     # stdlib.h entrypoints
     libc.src.stdlib.abs
set(TARGET_LIBC_ENTRYPOINTS
     libc.src.stdlib.qsort
     libc.src.stdlib.strtod
     libc.src.stdlib.strtof
     libc.src.stdlib.strtol
     libc.src.stdlib.strtold
     libc.src.stdlib.strtoll
     libc.src.stdlib.strtoul
     libc.src.stdlib.strtoull
+
+    # sys/mman.h entrypoints
+    libc.src.sys.mman.mmap
+    libc.src.sys.mman.munmap
 )
 
 set(TARGET_LIBM_ENTRYPOINTS
     # math.h entrypoints
     libc.src.math.fabs
     libc.src.math.fabsf
     libc.src.math.fabsl
     libc.src.math.fdim

libc/src/__support/OSUtil/linux/arm/CMakeLists.txt

+add_header_library(
+  linux_arm_util
+  HDRS
+    syscall.h
+  DEPENDS
+    libc.src.__support.common
+)

libc/src/__support/OSUtil/linux/arm/syscall.h

+//===--------- inline implementation of arm syscalls --------------* 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_SUPPORT_OSUTIL_LINUX_ARM_SYSCALL_H
+#define LLVM_LIBC_SRC_SUPPORT_OSUTIL_LINUX_ARM_SYSCALL_H
+
+#include "src/__support/common.h"
+
+#ifdef __thumb__
+#error "The arm syscall implementation does not yet support thumb flavor."
+#endif // __thumb__
+
+#define REGISTER_DECL_0                                                        \
+  register long r7 __asm__("r7") = number;                                     \
+  register long r0 __asm__("r0");
+#define REGISTER_DECL_1                                                        \
+  register long r7 __asm__("r7") = number;                                     \
+  register long r0 __asm__("r0") = arg1;
+#define REGISTER_DECL_2                                                        \
+  REGISTER_DECL_1                                                              \
+  register long r1 __asm__("r1") = arg2;
+#define REGISTER_DECL_3                                                        \
+  REGISTER_DECL_2                                                              \
+  register long r2 __asm__("r2") = arg3;
+#define REGISTER_DECL_4                                                        \
+  REGISTER_DECL_3                                                              \
+  register long r3 __asm__("r3") = arg4;
+#define REGISTER_DECL_5                                                        \
+  REGISTER_DECL_4                                                              \
+  register long r4 __asm__("r4") = arg5;
+#define REGISTER_DECL_6                                                        \
+  REGISTER_DECL_5                                                              \
+  register long r5 __asm__("r5") = arg6;
+
+#define REGISTER_CONSTRAINT_0 "r"(r7)
+#define REGISTER_CONSTRAINT_1 REGISTER_CONSTRAINT_0, "r"(r0)
+#define REGISTER_CONSTRAINT_2 REGISTER_CONSTRAINT_1, "r"(r1)
+#define REGISTER_CONSTRAINT_3 REGISTER_CONSTRAINT_2, "r"(r2)
+#define REGISTER_CONSTRAINT_4 REGISTER_CONSTRAINT_3, "r"(r3)
+#define REGISTER_CONSTRAINT_5 REGISTER_CONSTRAINT_4, "r"(r4)
+#define REGISTER_CONSTRAINT_6 REGISTER_CONSTRAINT_5, "r"(r5)
+
+#define SYSCALL_INSTR(input_constraint)                                        \
+  LIBC_INLINE_ASM("svc 0" : "=r"(r0) : input_constraint : "memory", "cc")
+
+namespace __llvm_libc {
+
+__attribute__((always_inline)) inline long syscall(long number) {
+  REGISTER_DECL_0;
+  SYSCALL_INSTR(REGISTER_CONSTRAINT_0);
+  return r0;
+}
+
+__attribute__((always_inline)) inline long syscall(long number, long arg1) {
+  REGISTER_DECL_1;
+  SYSCALL_INSTR(REGISTER_CONSTRAINT_1);
+  return r0;
+}
+
+__attribute__((always_inline)) inline long syscall(long number, long arg1,
+                                                   long arg2) {
+  REGISTER_DECL_2;
+  SYSCALL_INSTR(REGISTER_CONSTRAINT_2);
+  return r0;
+}
+
+__attribute__((always_inline)) inline long syscall(long number, long arg1,
+                                                   long arg2, long arg3) {
+  REGISTER_DECL_3;
+  SYSCALL_INSTR(REGISTER_CONSTRAINT_3);
+  return r0;
+}
+
+__attribute__((always_inline)) inline long
+syscall(long number, long arg1, long arg2, long arg3, long arg4) {
+  REGISTER_DECL_4;
+  SYSCALL_INSTR(REGISTER_CONSTRAINT_4);
+  return r0;
+}
+
+__attribute__((always_inline)) inline long
+syscall(long number, long arg1, long arg2, long arg3, long arg4, long arg5) {
+  REGISTER_DECL_5;
+  SYSCALL_INSTR(REGISTER_CONSTRAINT_5);
+  return r0;
+}
+
+__attribute__((always_inline)) inline long syscall(long number, long arg1,
+                                                   long arg2, long arg3,
+                                                   long arg4, long arg5,
+                                                   long arg6) {
+  REGISTER_DECL_6;
+  SYSCALL_INSTR(REGISTER_CONSTRAINT_6);
+  return r0;
+}
+
+} // namespace __llvm_libc
+
+#undef REGISTER_DECL_0
+#undef REGISTER_DECL_1
+#undef REGISTER_DECL_2
+#undef REGISTER_DECL_3
+#undef REGISTER_DECL_4
+#undef REGISTER_DECL_5
+#undef REGISTER_DECL_6
+
+#undef REGISTER_CONSTRAINT_0
+#undef REGISTER_CONSTRAINT_1
+#undef REGISTER_CONSTRAINT_2
+#undef REGISTER_CONSTRAINT_3
+#undef REGISTER_CONSTRAINT_4
+#undef REGISTER_CONSTRAINT_5
+#undef REGISTER_CONSTRAINT_6
+
+#endif // LLVM_LIBC_SRC_SUPPORT_OSUTIL_LINUX_ARM_SYSCALL_H

libc/src/__support/OSUtil/linux/syscall.h

 #define LLVM_LIBC_SRC_SUPPORT_OSUTIL_LINUX_SYSCALL_H
 
 #include "src/__support/architectures.h"
 
 #ifdef LLVM_LIBC_ARCH_X86_64
 #include "x86_64/syscall.h"
 #elif defined(LLVM_LIBC_ARCH_AARCH64)
 #include "aarch64/syscall.h"
+#elif defined(LLVM_LIBC_ARCH_ARM)
+#include "arm/syscall.h"
 #endif
 
 namespace __llvm_libc {
 
 template <typename... Ts>
 __attribute__((always_inline)) inline long syscall(long __number, Ts... ts) {
   static_assert(sizeof...(Ts) <= 6, "Too many arguments for syscall");
   return syscall(__number, (long)ts...);
 }
 
 } // namespace __llvm_libc
 
 #endif // LLVM_LIBC_SRC_SUPPORT_OSUTIL_LINUX_SYSCALL_H