Specifying root signatures from HLSL

[turanszkij]

Hello,

Currently we need to use strings in HLSL when defining a root signature within the shader. This seems to originate from the time when HLSL was preparing to support DX12 features a long time ago. This feels like a workaround solution made for the old shader compiler and not a real language feature. The problem is that it is very easy to make a mistake in the string define, like leaving out a comma in a big multiline root signature definition. The error reporting is also hard to understand when making a mistake in the root signature string, especially if this string is long and multiline.

It would be great if there was a proper HLSL syntax to specify the root signature.

For example, a current string-style root signature:

"RootFlags(ALLOW_INPUT_ASSEMBLER_INPUT_LAYOUT), " \
"RootConstants(num32BitConstants=12, b999), " \
"CBV(b0), " \
"DescriptorTable( " \
	"UAV(u0, numDescriptors = 16, flags = DESCRIPTORS_VOLATILE | DATA_STATIC_WHILE_SET_AT_EXECUTE)" \
")," \
"DescriptorTable( " \
	"Sampler(s0, space = 1, offset = 0, numDescriptors = unbounded, flags = DESCRIPTORS_VOLATILE)" \
")," \
"DescriptorTable( " \
	"SRV(t0, space = 2, offset = 0, numDescriptors = unbounded, flags = DESCRIPTORS_VOLATILE | DATA_VOLATILE)" \
"), " \
"StaticSampler(s100, addressU = TEXTURE_ADDRESS_CLAMP, addressV = TEXTURE_ADDRESS_CLAMP, addressW = TEXTURE_ADDRESS_CLAMP, filter = FILTER_MIN_MAG_MIP_LINEAR)" 

Could be rewritten with a new syntax that is not just a big string, but more structured:

struct MyRootSignature
{
  RootFlags(ALLOW_INPUT_ASSEMBLER_INPUT_LAYOUT);
  RootConstants(num32BitConstants=12, b999);
  CBV(b0);
  DescriptorTable(
	UAV(u0, numDescriptors = 16, flags = DESCRIPTORS_VOLATILE | DATA_STATIC_WHILE_SET_AT_EXECUTE)
  );
  DescriptorTable(
	Sampler(s0, space = 1, offset = 0, numDescriptors = unbounded, flags = DESCRIPTORS_VOLATILE)
  );
  DescriptorTable(
	SRV(t0, space = 2, offset = 0, numDescriptors = unbounded, flags = DESCRIPTORS_VOLATILE | DATA_VOLATILE)
  );
  StaticSampler(s100, addressU = TEXTURE_ADDRESS_CLAMP, addressV = TEXTURE_ADDRESS_CLAMP, addressW = TEXTURE_ADDRESS_CLAMP, filter = FILTER_MIN_MAG_MIP_LINEAR);
};

And then this struct can easily be added to the rootsig declaration for a shader:

[RootSignature(MyRootSignature)]

The shader compiler also allows to specify a root signature define as a compile argument (-rootsig-define <value>), this is very useful too. Currently it needs a define that expands to a string, but it should accept the new root signature syntax as well.

The proposed syntax is just a really first attempt, I don't think it's very important that it would look like that.

Best regards,
Janos

[krupitskas]

Agree with Janos.
I think almost everyone came up with custom solution to simplify shader manufacturing. Would be great to standardize RootSignature object and deprecate string definition.