[llvm-dev] Proposed new min and max intrinsics

[Thomas Lively via llvm-dev]

Hi everyone,

I have implemented a pair of new target-independent intrinsics and it would
be great if I could get some feedback about whether we want to merge them
in. The intrinsics are @llvm.minimum and @llvm.maximum, and are IEEE
754-2018 analogues to @llvm.minnum and @llvm.minnum. The differences are
that the new intrinsics propagate NaNs and consider -0.0 to be strictly
less than 0.0. The names are taken from the corresponding operations in the
IEEE 754-2018 draft specification. You can find the patches implementing
the intrinsics in the stack starting at https://reviews.llvm.org/D52764.

The context for this proposed change is that the WebAssembly backend needs
these intrinsics to make its min and max instructions available at the IR
level, since they follow the draft IEEE 754-2018 semantics. Originally I
was thinking of implementing the intrinsics as @wasm.min and @wasm.max, but
since these operations are in the draft standard, I thought they might be
more appropriate as target-independent. If the community does not want to
merge these intrinsics now, I can always go back to making them
wasm-specific.

The way I have currently implemented the intrinsics, they lower directly to
the fminnan and fmaxnan ISel DAG nodes, which already existed. As far as I
can tell, these DAG nodes are only used in the ARM, AArch64, SystemZ, and
WebAssembly backends and lower to instructions that follow the draft IEEE
754-2018 semantics, so these targets do not need to take any action to
support the new intrinsics. Before this change, the only way an ISel DAG
could contain fminnan or fmaxnan nodes was if they were produced by a
select pattern. The select pattern matching code does not produce mins or
maxes if it would be possible that both arguments could be zeros, so
tightening the semantics of fminnan and fmaxnan to match IEEE 754-2018 will
not invalidate any existing code.

Best,

Thomas Lively
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