[PATCH] D126953: Promote bf16 to f32 when the target doesn't support it

[Phoebe Wang via Phabricator via llvm-commits]

pengfei added inline comments.


================
Comment at: llvm/lib/CodeGen/SelectionDAG/LegalizeDAG.cpp:2912-2915
+    Op = DAG.getNode(
+        ISD::SHL, dl, MVT::i32, Op,
+        DAG.getConstant(16, dl,
+                        TLI.getShiftAmountTy(MVT::i32, DAG.getDataLayout())));
----------------
arsenm wrote:
> Why can this just shift into the high bits? Why don't the mantissa bits need to be adjusted down to the low bits?
Expand a normal value doesn't need to adjust the mantissa bits. We do have concerns like DAZ or signaling NaN are not respected. But BF16 is not a IEEE standard type. There's no such strict rule for it AFAIK. And GCC does it in the same way.


Repository:
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CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D126953/new/

https://reviews.llvm.org/D126953