spirv: enhance printf debugging with temporary mock formatting

Author: ivanstepanovftw

lib/std/debug.zig

@@ -208,25 +208,37 @@ pub fn unlockStdErr() void {
 /// for use in "printf debugging." Use `std.log` functions for proper logging.
 pub fn print(comptime fmt: []const u8, args: anytype) void {
     if (builtin.zig_backend == .stage2_spirv) {
-        const ret = asm volatile (
-            \\      %set = OpExtInstImport "OpenCL.std"
+        const size = 20;
+        var fmt_fmt: [size]u8 = [1]u8{undefined} ** size;
+
+        // TODO: All formatting is done by std.
+        // std.fmt.bufPrint(&fmt_fmt, fmt, args) catch unreachable;
+        // Temporarely mock the fmt_fmt buffer to test printing 's\n'.
+        fmt_fmt[0] = 115;
+        fmt_fmt[1] = 115;
+        fmt_fmt[2] = 115;
+        fmt_fmt[3] = 115;
+        fmt_fmt[4] = 10;
+        fmt_fmt[5] = 0;
+
+        // Imports std and calls OpenCL.std.printf("%s\0", fmt_fmt)
+        _ = asm volatile (
+            \\      %std = OpExtInstImport "OpenCL.std"
             \\       %u8 = OpTypeInt 8 0
             \\      %u32 = OpTypeInt 32 0
             \\      %u64 = OpTypeInt 64 0
-            \\    %u64_4 = OpConstant %u64 4
-            \\%_arr_u8_4 = OpTypeArray %u8 %u64_4
-            \\%_ptr_UniformConstant__arr_u8_4 = OpTypePointer UniformConstant %_arr_u8_4
-            \\   %char_H = OpConstant %u8 72
-            \\   %char_i = OpConstant %u8 105
-            \\  %char__n = OpConstant %u8 10
-            \\   %char_0 = OpConstant %u8 0
-            \\%const_str = OpConstantComposite %_arr_u8_4 %char_H %char_i %char__n %char_0
-            \\      %fmt = OpVariable %_ptr_UniformConstant__arr_u8_4 UniformConstant %const_str
-            \\
-            \\      %ret = OpExtInst %u32 %set 184 %fmt
+            \\    %u64_3 = OpConstant %u64 3
+            \\%_arr_u8_3 = OpTypeArray %u8 %u64_3
+            \\%_ptr_UniformConstant__arr_u8_3 = OpTypePointer UniformConstant %_arr_u8_3
+            \\       %c1 = OpConstant %u8 37
+            \\       %c2 = OpConstant %u8 115
+            \\       %c3 = OpConstant %u8 0
+            \\%const_str = OpConstantComposite %_arr_u8_3 %c1 %c2 %c3
+            \\      %fmt = OpVariable %_ptr_UniformConstant__arr_u8_3 UniformConstant %const_str
+            \\      %ret = OpExtInst %u32 %std 184 %fmt %fmt_fmt
             : [ret] "" (-> u32),
+            : [fmt_fmt] "" (&fmt_fmt),
         );
-        _ = ret;
     } else {
         lockStdErr();
         defer unlockStdErr();