AArch64 fails with "Don't know how to custom expand this" with illegal f16

[arsenm]

; RUN: llc -mtriple=aarch64-none-linux-gnu -mattr=-fp-armv8 < %s

define half @f16_return(float %arg) {
  %fptrunc = fptrunc float %arg to half
  ret half %fptrunc
}

Don't know how to custom expand this
UNREACHABLE executed at /Users/matt/src/llvm-project/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp:24529!

[llvmbot]

@llvm/issue-subscribers-backend-aarch64

Author: Matt Arsenault (arsenm)

``` ; RUN: llc -mtriple=aarch64-none-linux-gnu -mattr=-fp-armv8 < %s

define half @f16_return(float %arg) {
%fptrunc = fptrunc float %arg to half
ret half %fptrunc
}

Don't know how to custom expand this
UNREACHABLE executed at /Users/matt/src/llvm-project/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp:24529!
</details>

[sjoerdmeijer]

I don't mind taking a look at this. But I was wondering whether you managed to generate this IR from source code, or what your use case is?

The problem is in -mattr=-fp-armv8, where you specify not to have FP support. I will need to remind myself and take a look, but I don't think that is a valid architecture combination. Thus, I also don't know what the solution should be in terms of instructions to do this convert, but I will have a look to remind myself.

[ostannard]

AArch64 without FP is a valid combination for v8-R, see #74460.

LLVM currently emits calls to the GCC soft-float functions documented at https://gcc.gnu.org/onlinedocs/gccint/Soft-float-library-routines.html, but there isn't any mention of half-precision on that page. It looks like we do have the right function for this in compiler-rt, under the names __extendhfsf2 and __gnu_h2f_ieee: https://github.com/llvm/llvm-project/blob/main/compiler-rt/lib/builtins/extendhfsf2.c. I don't know if these names are documented anywhere though.

[arsenm]

I don't mind taking a look at this. But I was wondering whether you managed to generate this IR from source code, or what your use case is?

I didn't have a real use case, I was just looking for tests for half promotion legalization for a target with a more stable ABI