[SPIR-V] All attempts at decorating an instruction with Inline-SPIRV fail.

[devshgraphicsprogramming]

Description

I'm trying to make a proof-of-concept interval arithmetic library in HLSL.

This requires me to leverage the float_controls2 extension and set rounding modes per instruction.

I can't figure out the syntax to OpDecorate an isntruction.

Steps to Reproduce

Attempt 1, take an intermediate of the OpFAdd and attempt to `OpDecorate that.

template<typename T>
[[vk::ext_instruction(spv::OpFAdd)]]
T fAdd(T lhs, T rhs);

template<typename T>
[[vk::ext_instruction(spv::OpDecorate)]]
void decorateInstruction(T res, [[vk::ext_literal]] int32_t decoration, [[vk::ext_literal]] int32_t op0);

...

decorateInstruction(fAdd(lo,rhs.lo),spv::DecorationFPRoundingMode,spv::FPRoundingModeRTN);

Attempt 2, try and use your vk::ext_decorate on an intrinsic

template<typename T>
[[vk::ext_decorate(spv::DecorationFPRoundingMode,spv::FPRoundingModeRTN)]]
[[vk::ext_instruction(spv::OpFAdd)]]
T fAddRTN(T lhs, T rhs);

...

result.lo = fAddRTN(lo,rhs.lo);

Actual Behavior

Attempt 1 kind-of-works but only with -O0 and kind-of because OpDecorate can't return anything so while I can annotate an immediate, I can't get it out and use it meaningfully
https://godbolt.org/z/9E346eazG

         %54 = OpAccessChain %_ptr_Function_float %param_this %int_0
         %55 = OpLoad %float %54
               OpLine %2 43 37
         %56 = OpAccessChain %_ptr_Function_float %rhs %int_0
               OpLine %2 43 41
         %57 = OpLoad %float %56
               OpLine %2 43 29
         %58 = OpFAdd %float %55 %57
               OpDecorate %58 FPRoundingMode RTN

However if I progress onto -O1 the optimizer breaks the code altogether (goes from valid but dead to invalid), moving the decoration on the immediate result to a constant it optimized to
https://godbolt.org/z/5TYr85Kvf

   %float_12 = OpConstant %float 12
...
               OpDecorate %float_12 FPRoundingMode RTN

Attempt 2 doesn't emit any OpDecorate at all, even in -O0, its just ignored
https://godbolt.org/z/xnK6azeqM

Environment

• DXC version
• Host Operating System <!--- Host operating system and version --->

[eddyb]

Attempt 1 couldn't possibly work, you just disabled validation so you didn't see the fundamental issue with it: https://godbolt.org/z/7nqeMr17K

fatal error: generated SPIR-V is invalid: Decorate cannot appear in a function declaration
  OpDecorate %58 FPRoundingMode RTN

(all OpDecorate must go in a certain position in the file, before any instruction that can define any of the decorated IDs, i.e. before even all types/constants/global-variables etc.)

[eddyb]

The ext_decorate test has the example you want:

DirectXShaderCompiler/tools/clang/test/CodeGenSPIRV/spv.intrinsicDecorate.hlsl

Lines 46 to 47 in 905add5

	// spv::Decoration::FPRoundingMode = 39, RTZ=0
	[[vk::ext_decorate(39, 0), vk::ext_decorate_string(5635, "return variable")]] float4 ret = 1.0;

I wish there was a better documentation, but at least that seems to work: https://godbolt.org/z/xqTMnroqn

Well, except it decorates the variable, so maybe that test is doing something useless all along (and the decoration disappears with optimizations.

The exact needed thing is [[vk::ext_decorate(...)]] on an expression but I don't know if that's syntactically allowed?

---

EDIT: C++ attributes on expressions have been proposed but AFAICT not accepted yet into any future C++ standard:
https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2024/p3093r0.html

With that proposal, your code would look like this:

result.lo = (
  [[vk::ext_decorate(
      spv::DecorationFPRoundingMode,
      spv::FPRoundingModeRTN,
  )]]
  lo + rhs.lo
);

(the extra parens are necessary for syntactic disambiguation)

You could then define fAddRTN(a, b) as a regular function whose body is just return ([[...]] a + b);, and there might be different ways to make this more ergonomic etc.

[devshgraphicsprogramming]

Well, except it decorates the variable, so maybe that test is doing something useless all along (and the decoration disappears with optimizations.

        [[vk::ext_decorate(39, 0)]] fAdd(lo,rhs.lo);

fails with

<source>:43:11: error: 'ext_decorate' attribute cannot be applied to a statement
        [[vk::ext_decorate(39, 0)]] fAdd(lo,rhs.lo);
          ^                         ~~~~

btw if just my last example

template<typename T>
[[vk::ext_decorate(spv::DecorationFPRoundingMode,spv::FPRoundingModeRTN)]]
[[vk::ext_instruction(spv::OpFAdd)]]
T fAddRTN(T lhs, T rhs);

worked no "yet unapproved C++ standard addition" would have to be implemented.