adding read builtin

ffi/SpirvInterpreter.h

@@ -103,6 +103,7 @@ SPV_API SpvResult SpvGetEntryPointByName(SpvRuntime runtime, const char* name, S
 SPV_API SpvResult SpvCallEntryPoint(SpvRuntime runtime, SpvWord entry_point_index);
 
 SPV_API SpvResult SpvReadOutput(SpvRuntime runtime, SpvByte* output, SpvSize output_size, SpvWord result);
+SPV_API SpvResult SpvReadBuiltIn(SpvRuntime runtime, SpvByte* output, SpvSize output_size, SpvBuiltIn builtin);
 
 SPV_API SpvResult SpvWriteInput(SpvRuntime runtime, const SpvByte* input, SpvSize input_size, SpvWord result);
 SPV_API SpvResult SpvWriteBuiltIn(SpvRuntime runtime, const SpvByte* input, SpvSize input_size, SpvBuiltIn builtin);

ffi/runtime.zig

@@ -91,6 +91,11 @@ export fn SpvReadOutput(rt: *spv.Runtime, output: [*]u8, output_size: u32, resul
     return .Success;
 }
 
+export fn SpvReadBuiltIn(rt: *spv.Runtime, output: [*]u8, output_size: u32, builtin: spv.spv.SpvBuiltIn) callconv(.c) ffi.Result {
+    rt.readBuiltIn(output[0..output_size], builtin) catch |err| return toCResult(err);
+    return .Success;
+}
+
 export fn SpvWriteInput(rt: *spv.Runtime, input: [*]const u8, input_size: u32, result: spv.SpvWord) callconv(.c) ffi.Result {
     rt.writeInput(input[0..input_size], result) catch |err| return toCResult(err);
     return .Success;

src/Runtime.zig

@@ -250,6 +250,14 @@ pub fn readOutput(self: *const Self, output: []u8, result: SpvWord) RuntimeError
     }
 }
 
+pub fn readBuiltIn(self: *const Self, output: []u8, builtin: spv.SpvBuiltIn) RuntimeError!void {
+    if (self.mod.builtins.get(builtin)) |result| {
+        _ = try self.results[result].variant.?.Variable.value.read(output);
+    } else {
+        return RuntimeError.NotFound;
+    }
+}
+
 pub fn writeInput(self: *const Self, input: []const u8, result: SpvWord) RuntimeError!void {
     if (std.mem.indexOfScalar(SpvWord, &self.mod.input_locations, result)) |_| {
         _ = try self.results[result].variant.?.Variable.value.writeConst(input);