Fix CUDA lowering of standard TIR math intrinsics so they use precise
CUDA math functions by default instead of fast-math `__*f` functions.
This fixes the default behavior reported in #19546, where operators such
as `tirx.exp` could lower to `__expf` even though fast math was not
explicitly requested.
This change adds a CUDA target attribute, `enable_fast_math`, which
defaults to `false`. When the attribute is unset or false, standard math
intrinsics lower through the normal CUDA math rule, for example `expf`,
`logf`, `sinf`, `cosf`, `powf`, and `rsqrtf` for `float32`. When users
explicitly enable the attribute on the target, the lowering pass also
checks the `cuda.fastmath.FLowerIntrinsic` rules before the normal CUDA
lowering rules.
Users can opt in to fast math by constructing a CUDA target with the
attribute:
```py
tvm.target.Target({"kind": "cuda", "enable_fast_math": True})
target = tvm.target.Target({
"tag": "nvidia/nvidia-a100",
"enable_fast_math": True,
})
```
The fast-math lowering path currently covers the CUDA math operators
registered with `cuda.fastmath.FLowerIntrinsic`: `tirx.exp`,
`tirx.exp10`, `tirx.log`, `tirx.log2`, `tirx.log10`, `tirx.tan`,
`tirx.cos`, `tirx.sin`, `tirx.tanh`, and `tirx.pow`.
`tirx.rsqrt` is also registered for CUDA lowering so it maps to the CUDA
reciprocal-square-root intrinsic instead of being legalized as `1 /
sqrt(x)`.
Add CUDA codegen tests
`tests/python/codegen/test_target_codegen_cuda_fastmath.py` that check
the lowered IR, generated CUDA source, and runtime results for the
supported math intrinsics across floating point dtypes and both default
and fast-math targets.
ConvolutedDog
48f346bb07