[ET-VK] Fake u16vecn for devserver

## Context

Copy-pasted from the newly added `maybe_fake_u16vec3` function in the codegen script:

> There is a latency benefit to using u16vecn variables to store texture position variables instead of ivecn, likely due to reduced register pressure. However, SwiftShader does not support 16 bit integer types in shaders, so this is a crude way to fallback to using ivecn to store texture positions so that testing with SwiftShader is still possible.

Differential Revision: [D65501674](https://our.internmc.facebook.com/intern/diff/D65501674/)

[ghstack-poisoned]

Author: SS-JIA

backends/vulkan/runtime/gen_vulkan_spv.py

@@ -540,6 +540,7 @@ def __init__(
         env: Dict[Any, Any],
         glslc_path: Optional[str],
         glslc_flags: str = "",
+        fake_u16vecn: bool = False,
     ) -> None:
         if isinstance(src_dir_paths, str):
             self.src_dir_paths = [src_dir_paths]
@@ -549,6 +550,7 @@ def __init__(
         self.env = env
         self.glslc_path = glslc_path
         self.glslc_flags = glslc_flags
+        self.fake_u16vecn = fake_u16vecn
 
         self.glsl_src_files: Dict[str, str] = {}
         self.template_yaml_files: List[str] = []
@@ -705,6 +707,22 @@ def constructOutputMap(self) -> None:
                     self.create_shader_params(),
                 )
 
+    def maybe_fake_u16vecn(self, input_text: str) -> str:
+        """
+        There is a latency benefit to using u16vecn variables to store texture position
+        variables instead of ivecn, likely due to reduced register pressure. However,
+        SwiftShader does not support 16 bit integer types in shaders, so this is a crude
+        way to fallback to using ivecn to store texture positions so that testing with
+        SwiftShader is still possible.
+        """
+        if not self.fake_u16vecn:
+            return input_text
+        if "codegen-nosub" in input_text:
+            return input_text
+
+        input_text = input_text.replace("u16vec", "ivec")
+        return input_text
+
     def generateSPV(self, output_dir: str) -> Dict[str, str]:
         output_file_map = {}
 
@@ -716,6 +734,7 @@ def process_shader(shader_paths_pair):
 
             with codecs.open(source_glsl, "r", encoding="utf-8") as input_file:
                 input_text = input_file.read()
+                input_text = self.maybe_fake_u16vecn(input_text)
                 output_text = preprocess(input_text, shader_params)
 
             glsl_out_path = os.path.join(output_dir, f"{shader_name}.glsl")
@@ -1029,6 +1048,7 @@ def main(argv: List[str]) -> int:
     parser.add_argument("-c", "--glslc-path", required=True, help="")
     parser.add_argument("-t", "--tmp-dir-path", required=True, help="/tmp")
     parser.add_argument("-o", "--output-path", required=True, help="")
+    parser.add_argument("--fake-u16vecn", action="store_true", default=False)
     parser.add_argument("--optimize_size", action="store_true", help="")
     parser.add_argument("--optimize", action="store_true", help="")
     parser.add_argument(
@@ -1056,7 +1076,11 @@ def main(argv: List[str]) -> int:
         glslc_flags += "-O"
 
     shader_generator = SPVGenerator(
-        options.glsl_paths, env, options.glslc_path, glslc_flags
+        options.glsl_paths,
+        env,
+        options.glslc_path,
+        glslc_flags,
+        fake_u16vecn=options.fake_u16vecn,
     )
     output_spv_files = shader_generator.generateSPV(options.tmp_dir_path)
 

backends/vulkan/runtime/graph/ops/glsl/q_8w_linear.glsl

@@ -6,6 +6,8 @@
  * LICENSE file in the root directory of this source tree.
  */
 
+// codegen-nosub
+
 #version 450 core
 
 #define PRECISION ${PRECISION}

backends/vulkan/targets.bzl

@@ -27,6 +27,7 @@ def vulkan_spv_shader_lib(name, spv_filegroups, is_fbcode = False):
         select({
             "DEFAULT": "",
             "ovr_config//os:android": "--optimize",
+            "ovr_config//os:linux": "--fake-u16vecn",
         })
     )