Generate padding fields in VolkDeviceTable to preserve ABI

To avoid issues with ABI compatibility of volk built with different sets
of defines, we now generate padding fields for each extension block with
the same number of function pointers. This should make sure that the
offset of every pointer for any given volk version remains stable,
simplifying binary builds.

Author: zeux

generate.py

@@ -157,8 +157,11 @@ def cdepends(key):
 	for key in block_keys:
 		blocks[key] = ''
 
+	devi = 0
+
 	for (group, cmdnames) in command_groups.items():
 		ifdef = '#if ' + group + '\n'
+		devt = 0
 
 		for key in block_keys:
 			blocks[key] += ifdef
@@ -176,6 +179,7 @@ def cdepends(key):
 				blocks['LOAD_DEVICE'] += '\t' + name + ' = (PFN_' + name + ')load(context, "' + name + '");\n'
 				blocks['DEVICE_TABLE'] += '\tPFN_' + name + ' ' + name + ';\n'
 				blocks['LOAD_DEVICE_TABLE'] += '\ttable->' + name + ' = (PFN_' + name + ')load(context, "' + name + '");\n'
+				devt += 1
 			elif is_descendant_type(types, type, 'VkInstance'):
 				blocks['LOAD_INSTANCE'] += '\t' + name + ' = (PFN_' + name + ')load(context, "' + name + '");\n'
 			elif type != '':
@@ -187,6 +191,11 @@ def cdepends(key):
 		for key in block_keys:
 			if blocks[key].endswith(ifdef):
 				blocks[key] = blocks[key][:-len(ifdef)]
+			elif key == 'DEVICE_TABLE':
+				devi += 1
+				blocks[key] += '#else\n'
+				blocks[key] += '\tPFN_vkVoidFunction padding' + str(devi) + '[' + str(devt) + '];\n'
+				blocks[key] += '#endif /* ' + group + ' */\n'
 			else:
 				blocks[key] += '#endif /* ' + group + ' */\n'
 

volk.c

@@ -21,6 +21,8 @@
 #	include <stdlib.h>
 #endif
 
+#include <string.h>
+
 #ifdef __cplusplus
 extern "C" {
 #endif
@@ -188,6 +190,7 @@ VkDevice volkGetLoadedDevice(void)
 
 void volkLoadDeviceTable(struct VolkDeviceTable* table, VkDevice device)
 {
+	memset(table, 0, sizeof(*table));
 	volkGenLoadDeviceTable(table, device, vkGetDeviceProcAddrStub);
 }