[PATCH] D23797: [X86][SSE] Improve awareness of (v)cvtpd2ps implicit zeroing of upper 64-bits of xmm result

[Simon Pilgrim via llvm-commits]

RKSimon added inline comments.

================
Comment at: lib/Target/X86/X86InstrSSE.td:2285
@@ -2284,1 +2284,3 @@
   // Match fpround and fpextend for 128/256-bit conversions
+  def : Pat<(v4f32 (bitconvert (X86vzmovl (v2f64 (bitconvert
+                               (v4f32 (X86vfpround (v2f64 VR128:$src)))))))),
----------------
RKSimon wrote:
> mkuper wrote:
> > I've only now realized we represent zeroing the two high lanes of a v4f32 with (v4f32 (bitconvert (X86vzmovl (v2f64 (bitconvert (v4f32 ...)))))) :-\
> > But there's nothing we can really do about this, right?
> > 
> > 
> Not much - we use VZEXT_MOVL to zero all but the first vector element.
> 
> An alternative would be to have VZEXT32_MOVL and VZEXT64_MOVL (or something similar) - it would affect a lot of existing lowering patterns and I'm it sure its worth it. We have a number of similar bitcasting pattern situation.
> An alternative would be to have VZEXT32_MOVL and VZEXT64_MOVL (or something similar) - it would affect a lot of existing lowering patterns and I'm it sure its worth it. We have a number of similar bitcasting pattern situation.

Sorry that should say "and I'm not sure its worth it."



Repository:
  rL LLVM

https://reviews.llvm.org/D23797