[PATCH v1 2/8] tools/nolibc: add support for powerpc64

[Zhangjin Wu <falcon@tinylab.org>]

This follows the 64-bit PowerPC ABI [1], refers to the slides: "A new
ABI for little-endian PowerPC64 Design & Implementation" [2] and the
musl code in arch/powerpc64/crt_arch.h.

Firstly, stdu and clrrdi are used instead of stwu and clrrwi for
powerpc64.

Second, the stack frame size is increased to 32 bytes for powerpc64, 32
bytes is the minimal stack frame size supported described in [2].

Besides, the TOC pointer (GOT pointer) must be saved to r2.

This works on both little endian and big endian 64-bit PowerPC.

[1]: https://refspecs.linuxfoundation.org/ELF/ppc64/PPC-elf64abi.pdf
[2]: https://www.llvm.org/devmtg/2014-04/PDFs/Talks/Euro-LLVM-2014-Weigand.pdf

Signed-off-by: Zhangjin Wu <falcon@tinylab.org>
---
 tools/include/nolibc/arch-powerpc.h | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/tools/include/nolibc/arch-powerpc.h b/tools/include/nolibc/arch-powerpc.h
index 100ec0f412dc..7b28ebcfcc23 100644
--- a/tools/include/nolibc/arch-powerpc.h
+++ b/tools/include/nolibc/arch-powerpc.h
@@ -143,6 +143,19 @@
 /* startup code */
 void __attribute__((weak, noreturn, optimize("Os", "omit-frame-pointer"))) __no_stack_protector _start(void)
 {
+#ifdef __powerpc64__
+	/* On 64-bit PowerPC, save TOC/GOT pointer to r2 */
+	extern char TOC __asm__ (".TOC.");
+	register volatile long r2 __asm__ ("r2") = (void *)&TOC - (void *)_start;
+
+	__asm__ volatile (
+		"mr     3, 1\n"         /* save stack pointer to r3, as arg1 of _start_c */
+		"clrrdi 1, 1, 4\n"      /* align the stack to 16 bytes                   */
+		"li     0, 0\n"         /* zero the frame pointer                        */
+		"stdu   1, -32(1)\n"    /* the initial stack frame                       */
+		"bl     _start_c\n"     /* transfer to c runtime                         */
+	);
+#else
 	__asm__ volatile (
 		"mr     3, 1\n"         /* save stack pointer to r3, as arg1 of _start_c */
 		"clrrwi 1, 1, 4\n"      /* align the stack to 16 bytes                   */
@@ -150,6 +163,7 @@ void __attribute__((weak, noreturn, optimize("Os", "omit-frame-pointer"))) __no_
 		"stwu   1, -16(1)\n"    /* the initial stack frame                       */
 		"bl     _start_c\n"     /* transfer to c runtime                         */
 	);
+#endif
 	__builtin_unreachable();
 }
 
-- 
2.25.1