[libc] Add x86_64 versions of setjmp() and longjmp()

None of the x86_64 builds currently have any way of invoking these
functions.  They are included only to avoid introducing unnecessary
architecture-specific dependencies into the self-test suite.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/x86_64/Makefile

@@ -40,6 +40,7 @@ endif
 
 # x86_64-specific directories containing source files
 #
+SRCDIRS		+= arch/x86_64/core
 SRCDIRS		+= arch/x86_64/prefix
 
 # Include common x86 Makefile

src/arch/x86_64/core/setjmp.S

@@ -0,0 +1,65 @@
+FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL )
+
+	.text
+	.code64
+
+	/* Must match jmp_buf structure layout */
+	.struct	0
+env_retaddr:	.quad	0
+env_stack:	.quad	0
+env_rbx:	.quad	0
+env_rbp:	.quad	0
+env_r12:	.quad	0
+env_r13:	.quad	0
+env_r14:	.quad	0
+env_r15:	.quad	0
+	.previous
+
+/*
+ * Save stack context for non-local goto
+ */
+	.globl	setjmp
+setjmp:
+	/* Save return address */
+	movq	0(%rsp), %rax
+	movq	%rax, env_retaddr(%rdi)
+	/* Save stack pointer */
+	movq	%rsp, env_stack(%rdi)
+	/* Save other registers */
+	movq	%rbx, env_rbx(%rdi)
+	movq	%rbp, env_rbp(%rdi)
+	movq	%r12, env_r12(%rdi)
+	movq	%r13, env_r13(%rdi)
+	movq	%r14, env_r14(%rdi)
+	movq	%r15, env_r15(%rdi)
+	/* Return 0 when returning as setjmp() */
+	xorq	%rax, %rax
+	ret
+	.size	setjmp, . - setjmp
+
+/*
+ * Non-local jump to a saved stack context
+ */
+	.globl	longjmp
+longjmp:
+	/* Get result in %rax */
+	movq	%rsi, %rax
+	/* Force result to non-zero */
+	testq	%rax, %rax
+	jnz	1f
+	incq	%rax
+1:	/* Restore stack pointer */
+	movq	env_stack(%rdi), %rsp
+	/* Restore other registers */
+	movq	env_rbx(%rdi), %rbx
+	movq	env_rbp(%rdi), %rbp
+	movq	env_r12(%rdi), %r12
+	movq	env_r13(%rdi), %r13
+	movq	env_r14(%rdi), %r14
+	movq	env_r15(%rdi), %r15
+	/* Replace return address on the new stack */
+	popq	%rcx	/* discard */
+	pushq	env_retaddr(%rdi)
+	/* Return to setjmp() caller */
+	ret
+	.size	longjmp, . - longjmp

src/arch/x86_64/include/setjmp.h

@@ -0,0 +1,34 @@
+#ifndef _SETJMP_H
+#define _SETJMP_H
+
+FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
+
+#include <stdint.h>
+
+/** A jump buffer */
+typedef struct {
+	/** Saved return address */
+	uint64_t retaddr;
+	/** Saved stack pointer */
+	uint64_t stack;
+	/** Saved %rbx */
+	uint64_t rbx;
+	/** Saved %rbp */
+	uint64_t rbp;
+	/** Saved %r12 */
+	uint64_t r12;
+	/** Saved %r13 */
+	uint64_t r13;
+	/** Saved %r14 */
+	uint64_t r14;
+	/** Saved %r15 */
+	uint64_t r15;
+} jmp_buf[1];
+
+extern int __asmcall __attribute__ (( returns_twice ))
+setjmp ( jmp_buf env );
+
+extern void __asmcall __attribute__ (( noreturn ))
+longjmp ( jmp_buf env, int val );
+
+#endif /* _SETJMP_H */