Merge pull request #1308 from Yawning/fix/amd64-syscalls

src: Fix the syscall intrinsic code generation for Linux and Windows

src/llvm_backend_proc.cpp

@@ -2002,7 +2002,22 @@ lbValue lb_build_builtin_proc(lbProcedure *p, Ast *expr, TypeAndValue const &tv,
 						constraints = gb_string_appendc(constraints, regs[i]);
 						constraints = gb_string_appendc(constraints, "}");
 					}
-					
+
+					// The SYSCALL instruction stores the address of the
+					// following instruction into RCX, and RFLAGS in R11.
+					//
+					// RSP is not saved, but at least on Linux it appears
+					// that the kernel system-call handler does the right
+					// thing.
+					//
+					// Some but not all system calls will additionally
+					// clobber memory.
+					//
+					// TODO: FreeBSD is different and will also clobber
+					// R8, R9, and R10.  Additionally CF is used to
+					// indicate an error instead of -errno.
+					constraints = gb_string_appendc(constraints, ",~{rcx},~{r11},~{memory}");
+
 					inline_asm = llvm_get_inline_asm(func_type, make_string_c(asm_string), make_string_c(constraints));
 				}
 				break;