Add support for OpenBSD

It just rejects the generic System V ABI and has other syscall numbers.

Makefile

@@ -1,11 +1,13 @@
 # All we need is C99 and POSIX, which this should make available
 CFLAGS = -std=gnu99
-NAMES = bfc-amd64-linux
+NAMES = bfc-amd64-linux bfc-amd64-openbsd
 
 all: $(NAMES)
 
-%: %.c
-	$(CC) $(CPPFLAGS) $(CFLAGS) $< -o $@
+%-linux: %.c
+	$(CC) $(CPPFLAGS) $(CFLAGS) $< -o $@ -DTARGET_LINUX
+%-openbsd: %.c
+	$(CC) $(CPPFLAGS) $(CFLAGS) $< -o $@ -DTARGET_OPENBSD
 clean:
 	rm -f $(NAMES)
 

README.adoc

@@ -1,8 +1,8 @@
 bfc
 ===
 
-'bfc' is a small, fast, self-contained, optimizing Brainfuck compiler for Linux
-on Intel x86-64.
+'bfc' is a small, fast, self-contained, optimizing Brainfuck compiler for *nix
+on Intel x86-64.  Currently supported targets are Linux and OpenBSD.
 
 Also included are several interpreters in various states of sophistication that
 document my progress as I was writing this, from the simplest approach to an
@@ -14,7 +14,7 @@ The compiler itself is platform agnostic.
 Building
 --------
 Build dependencies: a C99 compiler +
-Runtime dependencies: Linux
+Runtime dependencies: Linux or OpenBSD
 
  $ git clone https://github.com/pjanouch/bfc.git
  $ cd bfc

bfc-amd64.c

@@ -580,7 +580,13 @@ main (int argc, char *argv[])
 	//   args -> rdi, rsi, rdx, r10, r8, r9
 	//   trashed <- rcx, r11
 
+#ifdef TARGET_OPENBSD
+	enum { SYS_READ = 3, SYS_WRITE = 4, SYS_EXIT = 1 };
+#elif defined TARGET_LINUX
 	enum { SYS_READ = 0, SYS_WRITE = 1, SYS_EXIT = 60 };
+#else
+#error Target not supported
+#endif
 
 	CODE ("\xB8") DD (SYS_EXIT)  // mov eax, 0x3c
 	CODE ("\x48\x31\xFF")        // xor rdi, rdi
@@ -605,7 +611,7 @@ main (int argc, char *argv[])
 	size_t read_offset = buffer.len;
 	CODE ("\x50")                // push rax -- save tape position
 	CODE ("\xB8") DD (SYS_READ)  // mov eax, "SYS_READ"
-	CODE ("\x48\x89\xC7")        // mov rdi, rax -- STDIN_FILENO
+	CODE ("\xBF") DD (0)         // mov edi, "STDIN_FILENO"
 	CODE ("\x66\x6A\x00")        // push word 0 -- the default value for EOF
 	CODE ("\x48\x89\xE6")        // mov rsi, rsp -- the char starts at rsp
 	CODE ("\xBA") DD (1)         // mov edx, 1 -- count
@@ -623,7 +629,7 @@ main (int argc, char *argv[])
 	size_t write_offset = buffer.len;
 	CODE ("\x50")                // push rax -- save tape position
 	CODE ("\xB8") DD (SYS_WRITE) // mov eax, "SYS_WRITE"
-	CODE ("\x48\x89\xC7")        // mov rdi, rax -- STDOUT_FILENO
+	CODE ("\xBF") DD (1)         // mov edi, "STDOUT_FILENO"
 	CODE ("\x66\x53")            // push bx
 	CODE ("\x48\x89\xE6")        // mov rsi, rsp -- the char starts at rsp
 	CODE ("\xBA") DD (1)         // mov edx, 1 -- count
@@ -683,7 +689,12 @@ main (int argc, char *argv[])
 
 	// ELF header
 	CODE ("\x7F" "ELF\x02\x01\x01")     // ELF, 64-bit, little endian, v1
+#ifdef TARGET_OPENBSD
+	// OpenBSD either requires its ABI or a PT_NOTE with "OpenBSD" in it
+	CODE ("\x0C\x00" "\0\0\0\0\0\0\0")  // OpenBSD ABI, v0, padding
+#else
 	CODE ("\x00\x00" "\0\0\0\0\0\0\0")  // Unix System V ABI, v0, padding
+#endif
 	DW (2) DW (62) DD (1)               // executable, x86-64, v1
 	DQ (ELF_LOAD_CODE + ELF_META_SIZE)  // entry point address
 	DQ (ELF_HEADER_SIZE) DQ (0)         // program, section header offset