Argument macro expansion in token-pasting operator

[antiagainst]

fxc and dxc have different behaviors when handling token-pasting operator (##). fxc does macro expansion on its arguments before concatenating, but dxc does not.

For the following HLSL source code:

#define SET_INDEX0                10
#define BINDING_INDEX0            5

#define SET(INDEX)                SET_INDEX##INDEX
#define BINDING(INDEX)            BINDING_INDEX##INDEX
#define SET_BIND(NAME,SET,BIND)   resource_set_##SET##_bind_##BIND##_##NAME
#define RESOURCE(NAME,INDEX)      SET_BIND(NAME, SET(INDEX), BINDING(INDEX))

#ifdef C_PROGRAM

const int resource_set_10_bind_5_val = 5;

int main() {
    return RESOURCE(val, 0);
}

#else

Texture2D<float4> resource_set_10_bind_5_tex;

float4 main() : SV_Target {
    return RESOURCE(tex, 0)[uint2(1, 2)];
}

#endif

fxc accepts it but dxc rejects it.

I think dxc is just following clang's handling of token-pasting operator. In C, token-pasting operator arguments are not macro-expanded first:

http://en.cppreference.com/w/cpp/preprocessor/replace
https://msdn.microsoft.com/en-us/library/09dwwt6y.aspx

(cl.exe /TC /DC_PROGRAM rejects the above program.)

Just wanted to raise this issue to see how we should proceed with this behavior mismatch. Do we want to mimic the fxc way in dxc?

[antiagainst]

cc @ehsannas @chaoticbob

[tafuri]

Interesting, I just recently run into this exact issue. But decided work around it since I could not find any documentation on how a HLSL preprocessor is expected to behave.

[MRozaNixxes]

I ran into this issue as well. We have a multiplatform engine that supports multiple languages. For constant buffers we have the following define:

#define CONSTANT_BUFFER(name, index) cbuffer name : register(b##index)

which is used as follows:

#define CB_INDEX 1

CONSTANT_BUFFER(Buffer, CB_INDEX)

This fails because it will not replace CB_INDEX with 1. I have the following workaround that works:

#define CONCAT(a, b) a##b
#define bCB(index) CONCAT(b, index)
#define CONSTANT_BUFFER(name, index) cbuffer name : register(bCB(index))

[antiagainst]

We now have the -flegacy-macro-expansion option to let dxc to follow fxc's macro expansion behavior.