libcontext: Initial support for Apple Silicon

The assembly for apple_arm64 is adapted from linux_arm64 target with small modifications:

1. Re-enable FPU conditional save/restore as apple_x86_64 does
2. Workaround limitation in relocation in assembly for clang

thirdparty/libcontext/libcontext.cpp

@@ -531,6 +531,97 @@ __asm (
 
 #endif
 
+#if defined(LIBCONTEXT_PLATFORM_apple_arm64) && defined(LIBCONTEXT_COMPILER_gcc)
+__asm (
+".cpu    generic+fp+simd\n"
+".text\n"
+".align  2\n"
+".global _jump_fcontext\n"
+"_jump_fcontext:\n"
+"    # prepare stack for GP + FPU\n"
+"    sub  sp, sp, #0xb0\n"
+"    # test if fpu env should be preserved\n"
+"    cmp  w3, #0\n"
+"    b.eq  1f\n"
+"    # save d8 - d15\n"
+"    stp  d8,  d9,  [sp, #0x00]\n"
+"    stp  d10, d11, [sp, #0x10]\n"
+"    stp  d12, d13, [sp, #0x20]\n"
+"    stp  d14, d15, [sp, #0x30]\n"
+"1:\n"
+"    # save x19-x30\n"
+"    stp  x19, x20, [sp, #0x40]\n"
+"    stp  x21, x22, [sp, #0x50]\n"
+"    stp  x23, x24, [sp, #0x60]\n"
+"    stp  x25, x26, [sp, #0x70]\n"
+"    stp  x27, x28, [sp, #0x80]\n"
+"    stp  x29, x30, [sp, #0x90]\n"
+"    # save LR as PC\n"
+"    str  x30, [sp, #0xa0]\n"
+"    # store RSP (pointing to context-data) in first argument (x0).\n"
+"    # STR cannot have sp as a target register\n"
+"    mov  x4, sp\n"
+"    str  x4, [x0]\n"
+"    # restore RSP (pointing to context-data) from A2 (x1)\n"
+"    mov  sp, x1\n"
+"    # test if fpu env should be preserved\n"
+"    cmp  w3, #0\n"
+"    b.eq  2f\n"
+"    # load d8 - d15\n"
+"    ldp  d8,  d9,  [sp, #0x00]\n"
+"    ldp  d10, d11, [sp, #0x10]\n"
+"    ldp  d12, d13, [sp, #0x20]\n"
+"    ldp  d14, d15, [sp, #0x30]\n"
+"2:\n"
+"    # load x19-x30\n"
+"    ldp  x19, x20, [sp, #0x40]\n"
+"    ldp  x21, x22, [sp, #0x50]\n"
+"    ldp  x23, x24, [sp, #0x60]\n"
+"    ldp  x25, x26, [sp, #0x70]\n"
+"    ldp  x27, x28, [sp, #0x80]\n"
+"    ldp  x29, x30, [sp, #0x90]\n"
+"    # use third arg as return value after jump\n"
+"    # and as first arg in context function\n"
+"    mov  x0, x2\n"
+"    # load pc\n"
+"    ldr  x4, [sp, #0xa0]\n"
+"    # restore stack from GP + FPU\n"
+"    add  sp, sp, #0xb0\n"
+"    ret x4\n"
+);
+
+#endif
+
+#if defined(LIBCONTEXT_PLATFORM_apple_arm64) && defined(LIBCONTEXT_COMPILER_gcc)
+__asm (
+".cpu    generic+fp+simd\n"
+".text\n"
+".align  2\n"
+".global _make_fcontext\n"
+"_make_fcontext:\n"
+"    # shift address in x0 (allocated stack) to lower 16 byte boundary\n"
+"    and x0, x0, ~0xF\n"
+"    # reserve space for context-data on context-stack\n"
+"    sub  x0, x0, #0xb0\n"
+"    # third arg of make_fcontext() == address of context-function\n"
+"    # store address as a PC to jump in\n"
+"    str  x2, [x0, #0xa0]\n"
+"    # save address of finish as return-address for context-function\n"
+"    # will be entered after context-function returns (LR register)\n"
+"    # need to relocate manually because of Clang limitation\n"
+"    adrp x1, finish@PAGE\n"
+"    add  x1, x1, finish@PAGEOFF\n"
+"    str  x1, [x0, #0x98]\n"
+"    ret  x30 \n"
+"finish:\n"
+"    # exit code is zero\n"
+"    mov  x0, #0\n"
+"    # exit application\n"
+"    bl  _exit\n"
+);
+
+#endif
+
 #if defined(LIBCONTEXT_PLATFORM_linux_arm32) && defined(LIBCONTEXT_COMPILER_gcc)
 __asm (
 ".text\n"

thirdparty/libcontext/libcontext.h

@@ -73,6 +73,9 @@
     #elif defined(__x86_64__)
         #define LIBCONTEXT_PLATFORM_apple_x86_64
         #define LIBCONTEXT_CALL_CONVENTION
+    #elif __aarch64__
+        #define LIBCONTEXT_PLATFORM_apple_arm64
+        #define LIBCONTEXT_CALL_CONVENTION
     #endif
     #endif
 #elif defined (_MSC_VER)