[LLVMdev] Help!!!!Help!!!! " LLVM ERROR: Cannot select: 0x9fc9680: i32 = fp32_to_fp16 0x9fc0750 [ID=16] " problem!!!!!!!!!!!!!!!!!!

[Andrea Di Biagio]

Not sure if this can help, but
if you really really want to have minimal half float support on Mips,
then one thing you could try to do is to hack MipsISelLowering.cpp
adding rules to expand float-half conversion SDNodes into library
calls.

+  setOperationAction(ISD::FP16_TO_FP32, MVT::f32, Expand);
+  setOperationAction(ISD::FP32_TO_FP16, MVT::i32, Expand);

(The MVT::i32 on the second rule is required because type i16 is
promoted to i32).

If you then convert every occurrence of 'fptrunc' from float to half
with calls to @llvm.convert.to.fp16, then you should be able to
compile (hopefully) with no errors.
That means, in your original example you would convert the following
IR statement:
  %Vt3_1 = fptrunc float %Vt_2 to half
into
  %Vt3_1 = call i16 @llvm.convert.to.fp16(float %Vt_2)

The downside is that you will have to add definitions for
'__gnu_f2h_ieee' and '__gnu_h2f_ieee' in the compiler runtime. That is
because the backend will expand all the float-half conversions into
library calls...
This workaround should work assuming that a) you can hack the backend,
and b) it is acceptable (i.e. a reasonable solution in your case) to
have a library call for every float-half conversion in your code.

On Wed, Jul 9, 2014 at 8:51 PM, Matt Arsenault
<Matthew.Arsenault at amd.com> wrote:
> I think that support for the half type is only implemented for ARM. Last I
> tried to use it, I found that none of it works even on x86, and the current
> handling of the half conversion SDNodes seem to rely on ARM specific
> assumptions

Just for the record,
since revision 212293 (committed only five days ago), the x86 backend
supports float half conversions.
On x86, if the target has F16C, there are ISel patterns to map
float-half conversions to specific instructions. If there is no F16C
support, then the backend expands float-half conversions into runtime
library calls.

Cheers,
Andrea