[PATCH] D77590: [ARM] MVE saturating truncates

Wed Apr 8 05:23:25 PDT 2020

dmgreen added inline comments.

================
Comment at: llvm/lib/Target/ARM/ARMISelLowering.cpp:14796
+
+    // Create a VQMOVNB, Signed extended in to the top half. That extend will
+    // hopefully be removed if only the bottom bits are demanded (though a
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SjoerdMeijer wrote:
> sorry, just double checking again, should this be: 
> 
>   Signed extended in to the bottom half. 
We are producing a:
v8i16 %X = VQMOVNB v8i16 undef, v4i32 %in
v4i32 %Y = bitcast %X to v4i32
v4i32 %res = SIGN_EXTEND_INREG %Y, v4i16

So the VQMOVNB will set the "bottom" lanes (of the v8i16), but leave the top lanes undef. The SIGN_EXTEND_INREG will extend the bottom half into the top half of the v4i32, maing sure it's the same value as before with the min/max pair. Hopefully the SIGN_EXTEND_INREG will be demand-bitted away in a lot of cases, leaving the VQMOVNB.

I've tried to write that into the comment.

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D77590/new/

https://reviews.llvm.org/D77590