Use ASM unknown function trampolines on GN

The GN build doesn't currently support unknown function trampolines. To
fix that, we can use the assembly trampolines. To generate
the asm_offset assembly file, we can build it in a static library and
pass -S to it; that causes the assembly file to be copied into the .a
file. The python script can then extract it. This requires access to the
"ar" executable, so this mechanism is only used if a path to ar is
specified in //build_overrides/vulkan_loader.gni.

To fix incremental builds, "gen_defines.asm" must be included using
"#include", not ".include", since .include isn't a preprocessor
directive and isn't picked up by clang's depfile creation.

BUILD.gn

@@ -36,9 +36,7 @@ if (is_win) {
 }
 
 config("vulkan_internal_config") {
-  defines = [
-    "VK_ENABLE_BETA_EXTENSIONS",
-  ]
+  defines = [ "VK_ENABLE_BETA_EXTENSIONS" ]
   if (is_clang || !is_win) {
     cflags = [
       "-Wno-conversion",
@@ -99,6 +97,52 @@ if (!is_android) {
   } else {
     library_type = "static_library"
   }
+  support_unknown_function_handling = false
+  if (ar_path != "" && !is_win && (current_cpu == "arm64" || current_cpu == "x86_64")) {
+    support_unknown_function_handling = true
+    static_library("asm_offset") {
+      sources = [ "loader/asm_offset.c" ]
+      deps = [
+        "$vulkan_headers_dir:vulkan_headers",
+      ]
+
+      if (is_fuchsia) {
+        deps += [
+          ":dlopen_fuchsia",
+        ]
+      }
+
+      # Output raw assembly instead of compiled object file. The assembly will be included as a member of the output ar file.
+      cflags = [ "-S" ]
+      configs += [ ":vulkan_internal_config" ]
+      configs += [ ":vulkan_loader_config" ]
+    }
+
+    action("gen_defines") {
+      script = "scripts/parse_asm_values.py"
+      deps = [ ":asm_offset" ]
+
+      inputs = [
+        "$target_out_dir/libasm_offset.a",
+        ar_path,
+      ]
+      if (current_cpu == "arm64") {
+        cpu = "aarch64"
+      } else {
+        cpu = "x86_64"
+      }
+      args = [
+        rebase_path("$target_gen_dir/gen_defines.asm", root_build_dir),
+        rebase_path("$target_out_dir/libasm_offset.a", root_build_dir),
+        "GAS",
+        "Clang",
+        cpu,
+        rebase_path(ar_path, root_build_dir),
+        "libasm_offset.asm_offset.c.o",
+      ]
+      outputs = [ "$target_gen_dir/gen_defines.asm" ]
+    }
+  }
 
   target(library_type, "libvulkan") {
     sources = [
@@ -135,8 +179,6 @@ if (!is_android) {
       "loader/unknown_function_handling.h",
       "loader/unknown_function_handling.c",
       "loader/vk_loader_layer.h",
-
-      # TODO(jmadill): Use assembler where available.
       "loader/vk_loader_platform.h",
       "loader/wsi.c",
       "loader/wsi.h",
@@ -217,6 +259,19 @@ if (!is_android) {
 
       runtime_deps = [ "//sdk/lib/fdio:fdio_sdk" ]
     }
+    if (support_unknown_function_handling) {
+      if (current_cpu == "arm64") {
+        sources += [ "loader/unknown_ext_chain_gas_aarch.S" ]
+      } else if (current_cpu == "x86_64") {
+        sources += [ "loader/unknown_ext_chain_gas_x86.S" ]
+      } else {
+        assert(false, "Unexpected CPU $current_cpu")
+      }
+
+      defines += ["UNKNOWN_FUNCTIONS_SUPPORTED=1"]
+      deps += [ ":gen_defines" ]
+      include_dirs = [ "$target_gen_dir" ]
+    }
   }
 }
 

loader/unknown_ext_chain_gas_aarch.S

@@ -24,7 +24,7 @@
 // VkPhysicalDevice or a dispatchable object it can unwrap the object, possibly overwriting the wrapped physical device, and then
 // jump to the next function in the call chain
 
-.include "gen_defines.asm"
+#include "gen_defines.asm"
 
 /*
  * References:
@@ -56,6 +56,7 @@
 .macro PhysDevExtTramp num
 .global vkPhysDevExtTramp\num
 #if defined(__ELF__)
+ .type vkPhysDevExtTramp\num, @function
  .hidden vkPhysDevExtTramp\num
 #endif
 .balign 4
@@ -77,6 +78,7 @@ vkPhysDevExtTramp\num:
 .macro PhysDevExtTermin num
 .global vkPhysDevExtTermin\num
 #if defined(__ELF__)
+ .type vkPhysDevExtTermin\num, @function
  .hidden vkPhysDevExtTermin\num
 #endif
 .balign 4

loader/unknown_ext_chain_gas_x86.S

@@ -33,7 +33,7 @@
 #endif
 
 .intel_syntax noprefix
-.include "gen_defines.asm"
+#include "gen_defines.asm"
 
 .ifdef X86_64
 

scripts/gn/secondary/build_overrides/vulkan_loader.gni

@@ -21,3 +21,5 @@ vulkan_gen_subdir = ""
 # Vulkan loader build options
 vulkan_loader_shared = true
 
+# Path to ar or llvm-ar
+ar_path = ""

scripts/parse_asm_values.py

@@ -25,6 +25,8 @@
 import os.path
 from os.path import exists
 import re
+import subprocess
+import traceback
 
 
 # Where to write the "gen_defines.asm" file
@@ -57,12 +59,23 @@
             "DISPATCH_OFFSET_ICD_TERM",
             "EXT_OFFSET_DEVICE_DISPATCH" ]
 
-try:
-    with open(source_asm_file, 'r') as f:
-        asm_intermediate_file = f.read()
-except IOError:
-    print("Could not open assembler file:", source_asm_file)
-    sys.exit(1)
+if os.path.splitext(source_asm_file)[1] == ".a":
+    try:
+        ar_path = sys.argv[6]
+        asm_archive_member = sys.argv[7]
+        subprocess_result = subprocess.Popen([ar_path, "p", source_asm_file, asm_archive_member], stdout=subprocess.PIPE)
+        asm_intermediate_file = subprocess_result.stdout.read().decode("utf-8")
+    except IOError:
+        print("Could not open assembler archive file:", source_asm_file)
+        traceback.print_exc()
+        sys.exit(1)
+else:
+    try:
+        with open(source_asm_file, 'r') as f:
+            asm_intermediate_file = f.read()
+    except IOError:
+        print("Could not open assembler file:", source_asm_file)
+        sys.exit(1)
 
 with open(destination_file, "w", encoding="utf-8") as dest:
     if assembler_type == "MASM":