Add compile-time option for architecture mode

This allows to decode x86-32 machine code on a 64-bit platform (but
not vice versa). As a side-effect, we also get rid of pointer-size
detection for architecture selection.

decode.h

@@ -5,6 +5,10 @@
 #include <stddef.h>
 #include <stdint.h>
 
+#if defined(ARCH_X86_64) && __SIZEOF_POINTER__ < 8
+#error "Decoding x86-64 requires a 64-bit architecture"
+#endif
+
 #ifndef ssize_t
 #define ssize_t intptr_t
 #endif

meson.build

@@ -29,14 +29,13 @@ if get_option('warning_level').to_int() >= 3
                         language: 'c')
 endif
 
-c_compiler = meson.get_compiler('c')
+archmode = get_option('archmode')
-pointer_size = c_compiler.sizeof('void*')
+if archmode == '32'
-if pointer_size == 4
   add_project_arguments(['-DARCH_386'], language: 'c')
-elif pointer_size == 8
+elif archmode == '64'
   add_project_arguments(['-DARCH_X86_64'], language: 'c')
 else
-  error('Invalid pointer size')
+  error('Invalid architecture mode')
 endif
 
 instr_data = custom_target('tables',

meson_options.txt

@@ -0,0 +1 @@
+option('archmode', type: 'combo', choices: ['32', '64'])

tests/meson.build

@@ -15,7 +15,7 @@ cases = [
 test_driver = executable('test_driver', 'driver.c',
                          dependencies: libx86decode,
                          c_args: ['-D_GNU_SOURCE'])
-test_args = files('common.sh') + [test_driver.full_path(), '@0@'.format(pointer_size * 8)]
+test_args = files('common.sh') + [test_driver.full_path(), get_option('archmode')]
 
 foreach case : cases
   test(case[0], sh, args: test_args + files(case[1]))