SPIR-V wishlist

[Jasper-Bekkers]

Tracking issue for features we think are missing or useful in to have in SPIR-V

• OpTrap: trigger a breakpoint

[Jasper-Bekkers]

Noticed a few comments other things in the codebase yesterday:

• Checked math as native SPIR-V ops could help slightly more optimized checked math routines

  rust-gpu/rustc_codegen_spirv/src/builder/builder_methods.rs

  Lines 218 to 226 in f79ca6c

  	fn checked_binop(
  	&mut self,
  	_oop: OverflowOp,
  	_ty: Ty<'_>,
  	_lhs: Self::Value,
  	_rhs: Self::Value,
  	) -> (Self::Value, Self::Value) {
  	panic!("TODO: Checked binary operations are not supported yet");
  	}

• Saturating math ops (already supported by some GPU ISAs - so should be easier to drive adoption for):

  rust-gpu/rustc_codegen_spirv/src/builder/mod.rs

  Lines 350 to 360 in 17814d6

  	sym::wrapping_add => math_intrinsic! {self, arg_tys, args, add, add, fadd},
  	sym::wrapping_sub => math_intrinsic! {self, arg_tys, args, sub, sub, fsub},
  	sym::wrapping_mul => math_intrinsic! {self, arg_tys, args, mul, mul, fmul},
  	sym::saturating_add => math_intrinsic! {self, arg_tys, args, add, add, fadd},
  	sym::saturating_sub => math_intrinsic! {self, arg_tys, args, sub, sub, fsub},
  	sym::unchecked_add => math_intrinsic! {self, arg_tys, args, add, add, fadd},
  	sym::unchecked_sub => math_intrinsic! {self, arg_tys, args, sub, sub, fsub},
  	sym::unchecked_mul => math_intrinsic! {self, arg_tys, args, mul, mul, fmul},
  	sym::unchecked_div => math_intrinsic! {self, arg_tys, args, sdiv, udiv, fdiv},
  	sym::unchecked_rem => math_intrinsic! {self, arg_tys, args, srem, urem, frem},
  	sym::unchecked_shl => math_intrinsic_int! {self, arg_tys, args, shl, shl},

• Equivalent of llvm.expect:

  rust-gpu/rustc_codegen_spirv/src/builder/mod.rs

  Lines 386 to 390 in 17814d6

  	fn expect(&mut self, cond: Self::Value, _expected: bool) -> Self::Value {
  	// TODO: llvm.expect
  	cond
  	}

• Equivalent of llvm.sideeffects:

  rust-gpu/rustc_codegen_spirv/src/builder/mod.rs

  Lines 391 to 395 in 17814d6

  	fn sideeffect(&mut self) {
  	// TODO: This is currently ignored.
  	// It corresponds to the llvm.sideeffect intrinsic - does spir-v have an equivalent?
  	}

• Equivalent of llvm.assume:

  rust-gpu/rustc_codegen_spirv/src/builder/mod.rs

  Lines 382 to 385 in 17814d6

  	fn assume(&mut self, _val: Self::Value) {
  	// TODO: llvm.assume
  	}

[khyperia]

For the "equivalent of llvm.*", there are a lot of these, those are just the ones that have been hit so far - I've just been JITting them into the codebase. For a reasonably full list, look at this file - for example, llvm.expect.i1 (sym::likely/sym::unlikely), llvm.debugtrap (sym::breakpoint), and so forth. (including llvm.trap (sym::abort), which was already mentioned at the start of this thread)

https://github.com/rust-lang/rust/blob/e88e908e66cd1e6e30d789b37bcd774951d01856/compiler/rustc_codegen_llvm/src/intrinsic.rs

[Jasper-Bekkers]

• Potentially request OpControlBarrier for non-uniform controlflow (useful for #29)

[nhaehnle]

* Potentially request OpControlBarrier  for non-uniform controlflow (useful for #29)

To clarify, for #29 the thinking is that OpControlBarrier without undefined behavior may be useful. This would be done by making the behavior a guaranteed hang when it's not executed in uniform control flow...

[Jasper-Bekkers]

@khyperia
not only is this legal spir-v, it is unspecified what the return value is (like, it's not specified as undefined, it's unspecified)

         %f_ = OpFunction %int None %7
          %9 = OpLabel
          %x = OpVariable %_ptr_Function_int Function
         %12 = OpLoad %int %x
               OpReturnValue %12
               OpFunctionEnd

can we get this fixed in the spec, please?
this is equivalent to the glsl

int f() {
    int x; // explicitly uninitialized
    return x;
}