[LLVMdev] AVX broadcast Vs. vector constant pool load

[Cameron McInally]

Hey guys,

I'm currently investigating broadcasts from the constant pool on Sandy
Bridge. I see this comment in llvm/lib/Target/X86/X86ISelLowering.cpp:

   // Handle the broadcasting a single constant scalar from the constant
pool
   // into a vector. On Sandybridge it is still better to load a constant
vector
   // from the constant pool and not to broadcast it from a scalar.

Would anyone be able to explain why it is better to load a vector from the
constant pool rather than broadcast a scalar?

I checked out Agner Fog's tables, but it wasn't so obvious to me...

vmovaps y, m256:
  Uops: 1
  Lat: 4
  Throughput: 1

vbroadcastsd y, m64:
  Uops: 2
  Lat: [Not or cannot be measured]
  Throughput: 1


Thanks in advance,

Cameron
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