ResNet18 takes minutes to compile on refback.

[stellaraccident]

This is in a Release with asserts (-UNDEBUG) build. Without asserts, it is maybe 3x faster but still quite slow. Testing against the refback so not sure how much we care.

[silvasean]

It's actually the running that takes multiple minutes, which makes sense since refback is refback.

Compilation is nearly instantaneous. The weights are the the main thing slowing it down.

• 1 second. npcomp-opt ResNet18Module.mlir >/dev/null (with full weights, no "opaque" constants) takes 1 second. (i.e. "cat")
• +0.1 sec. Adding in the -torchscript-to-npcomp-backend-pipeline to that command results in 1.1 seconds total.
  npcomp-opt -torchscript-to-npcomp-backend-pipeline ResNet18Module.mlir >/dev/null
• +0.1 sec. Adding refback lowering pipeline results in 1.2 seconds total: npcomp-opt -torchscript-to-npcomp-backend-pipeline -refback-lowering-pipeline /tmp/ResNet18Module.mlir >/dev/null

[silvasean]

Actually, looking at the end-to-end time, it looks like conversion from MLIR to LLVM IR is the unbearably slow part of compilation. Filed an upstream bug: https://bugs.llvm.org/show_bug.cgi?id=51650