Invalid floating type error when compiling shader

[mbouchard2]

Description
Currently getting following error when compiling shader:
error: Invalid floating type
UNREACHABLE executed at C:__w\1\s\DXC\include\llvm/IR/Type.h:174!

Steps to Reproduce
https://godbolt.org/z/GMafYrfj9

If we just compile with ps_6_8 instead:
https://godbolt.org/z/eK91GGPe8

Actual Behavior
Since this actual shader do compile with shader model 6.8, I would expect to compile on 6.9 as well otherwise, the error should point to proper line number.

Environment

• DXC version 1.8.2505.32
• Host Operating System Windows 11 Version 23H2 (OS Build 22631.6060)

[V-FEXrt]

The assert is being raised here:

DirectXShaderCompiler/include/llvm/IR/Type.h

Line 174 in 6679358

default: llvm_unreachable("Invalid floating type");

getTypeID is expectedly a vector in the SM6.9 case which isn't hitting any of the branches. We could potentially update that function directly however the call site may also be the right place to update.

DirectXShaderCompiler/lib/Transforms/Scalar/Float2Int.cpp

Line 416 in 6679358

unsigned MaxRepresentableBits

Even in the SM6.8 case, this transform isn't applied because a float can't losslessly fit into the uint and it just bottoms out in doing nothing. The SM6.9 case should also just bottom out in doing nothing but exactly how to do nothing is unclear to me.

One option is to subtly rewrite the code to

    if (MinBW > 64) {
      DEBUG(dbgs() << "F2I: Value requires more than 64 bits to represent!\n");
      continue;
    }
    unsigned MaxRepresentableBits
      = APFloat::semanticsPrecision(ConvertedToTy->getFltSemantics()) - 1;
    if (MinBW > MaxRepresentableBits) {
      DEBUG(dbgs() << "F2I: Value not guaranteed to be representable!\n");
      continue;
    }

which checks MinBW before calling getFltSemantics. MinBW for a float2 is 65 so it fails the check and early exits as expected. This seems like a complete solution since all bigger vectors would have larger MinBW and all smaller "vectors" are just the scalar case which already works correctly. I'm having a hard time convincing myself this is a "correct" solution vs a solution that just happens to work though.

Willing to hear anyone's opinions