[all-commits] [llvm/llvm-project] 168fc0: [NFC][SROA] Add test w/ select and non-capturing call

[Roman Lebedev via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 168fc01997249400fcbf9d672d95bc98ea58fee3
      https://github.com/llvm/llvm-project/commit/168fc01997249400fcbf9d672d95bc98ea58fee3
  Author: Roman Lebedev <lebedev.ri at gmail.com>
  Date:   2022-03-05 (Sat, 05 Mar 2022)

  Changed paths:
    M llvm/test/Transforms/SROA/non-capturing-call-readonly.ll

  Log Message:
  -----------
  [NFC][SROA] Add test w/ select and non-capturing call


  Commit: adc0984d81f570ecc38ea23e7f556b95c7831e4c
      https://github.com/llvm/llvm-project/commit/adc0984d81f570ecc38ea23e7f556b95c7831e4c
  Author: Roman Lebedev <lebedev.ri at gmail.com>
  Date:   2022-03-05 (Sat, 05 Mar 2022)

  Changed paths:
    M llvm/lib/Transforms/Scalar/SROA.cpp
    M llvm/test/Transforms/SROA/non-capturing-call-readonly.ll

  Log Message:
  -----------
  Reland [SROA] Maintain shadow/backing alloca when some slices are noncapturnig read-only calls to allow alloca partitioning/promotion

This is inspired by the original variant of D109749 by Graham Hunter,
but is a more general version.

Roughly, instead of promoting the alloca, we call it
a shadow/backing alloca, go through all it's slices,
clone(!) instructions that operated on it,
but make them operate on the cloned alloca,
and promote cloned alloca instead.

This keeps the shadow/backing alloca, and all the original instructions
around, which results in said shadow/backing alloca being
a perfect mirror/representation of the promoted alloca's content,
so calls that take the alloca as arguments (non-capturingly!)
can be supported.

For now, we require that the calls also don't modify the alloca's content,
but that is only to simplify the initial implementation,
and that will be supported in a follow-up.

Overall, this leads to *smaller* codesize:
https://llvm-compile-time-tracker.com/compare.php?from=a8b4f5bbab62091835205f3d648902432a4a5b58&to=aeae054055b125b011c1122f82c86457e159436f&stat=size-total
and is roughly neutral compile-time wise:
https://llvm-compile-time-tracker.com/compare.php?from=a8b4f5bbab62091835205f3d648902432a4a5b58&to=aeae054055b125b011c1122f82c86457e159436f&stat=instructions

This relands commit 703240c71fd640af7490069e8149d32d78d14da1,
that was reverted by commit 7405581f7ca3ba54be8a04c394a96fe6d980f073,
because the assertion `isa<LoadInst>(OrigInstr)` didn't hold in practice,
as the newly added test `@select_of_ptrs` shows:
If the pointers into alloca are used by select's/PHI's, then even if
we manage to fracture the alloca, some sub-alloca's will likely remain.
And if there are any non-capturing calls, then we will also decide to
keep the original backing alloca around, and we suddenly ~doubled
the alloca size, and the amount of memory traffic.
I'm not sure if this is a problem or we could live with it,
but let's leave that for later...

Reviewed By: djtodoro

Differential Revision: https://reviews.llvm.org/D113520


Compare: https://github.com/llvm/llvm-project/compare/b31a1b4746c7...adc0984d81f5