The unrolling code was previously inserting new cloned blocks at the end of the function. The result of this with typical loop structures is that the new iterations are placed far from the initial iteration.
With unrolling, the general assumption is that the a) the loop is reasonable hot, and b) the first Count-1 copies of the loop are rarely (if ever) loop exiting. As such, placing Count-1 copies out of line is a fairly poor code placement choice. We'd much rather fall through into the hot (non-exiting) path. For code with branch profiles, later layout would fix this, but this may have a positive impact on non-PGO compiled code.
However, the real motivation for this change isn't performance. Its readability and human understanding. Having to jump around long distances in an IR file to trace an unrolled loop structure is error prone and tedious.
This is to show that we currently only convert the terminator to
unreachable, but don't clean up instructions before it (unless
trivial DCE removes them).
Also clean up excessive whitespace in this test.
Turns out simplifyLoopIVs sometimes returns a non-dead instruction in it's DeadInsts out param. I had done a bit of NFC cleanup which was only NFC if simplifyLoopIVs obeyed it's documentation. I'm simplfy dropping that part of the change.
Commit message from try 3:
Recommitting after fixing a bug found post commit. Amusingly, try 1 had been correct, and by reverting to incorporate last minute review feedback, I introduce the bug. Oops. :)
Original commit message:
The problem was that recursively deleting an instruction can delete instructions beyond the current iterator (via a dead phi), thus invalidating iteration. Test case added in LoopUnroll/dce.ll to cover this case.
LoopUnroll does a limited DCE pass after unrolling, but if you have a chain of dead instructions, it only deletes the last one. Improve the code to recursively delete all trivially dead instructions.
Differential Revision: https://reviews.llvm.org/D102511
Recommitting after fixing a bug found post commit. Amusingly, try 1 had been correct, and by reverting to incorporate last minute review feedback, I introduce the bug. Oops. :)
The problem was that recursively deleting an instruction can delete instructions beyond the current iterator (via a dead phi), thus invalidating iteration. Test case added in LoopUnroll/dce.ll to cover this case.
LoopUnroll does a limited DCE pass after unrolling, but if you have a chain of dead instructions, it only deletes the last one. Improve the code to recursively delete all trivially dead instructions.
Differential Revision: https://reviews.llvm.org/D102511
Recommitting after addressing a missed review comment, and updating an aarch64 test I'd missed.
LoopUnroll does a limited DCE pass after unrolling, but if you have a chain of dead instructions, it only deletes the last one. Improve the code to recursively delete all trivially dead instructions.
Differential Revision: https://reviews.llvm.org/D102511
LoopUnroll does a limited DCE pass after unrolling, but if you have a chain of dead instructions, it only deletes the last one. Improve the code to recursively delete all trivially dead instructions.
Differential Revision: https://reviews.llvm.org/D102511
This patch generalizes the UnrollLoop utility to support loops that exit
from the header instead of the latch. Usually, LoopRotate would take care
of must of those cases, but in some cases (e.g. -Oz), LoopRotate does
not kick in.
Codesize impact looks relatively neutral on ARM64 with -Oz + LTO.
Program master patch diff
External/S.../CFP2006/447.dealII/447.dealII 629060.00 627676.00 -0.2%
External/SPEC/CINT2000/176.gcc/176.gcc 1245916.00 1244932.00 -0.1%
MultiSourc...Prolangs-C/simulator/simulator 86100.00 86156.00 0.1%
MultiSourc...arks/Rodinia/backprop/backprop 66212.00 66252.00 0.1%
MultiSourc...chmarks/Prolangs-C++/life/life 67276.00 67312.00 0.1%
MultiSourc...s/Prolangs-C/compiler/compiler 69824.00 69788.00 -0.1%
MultiSourc...Prolangs-C/assembler/assembler 86672.00 86696.00 0.0%
Reviewers: efriedma, vsk, paquette
Reviewed By: paquette
Differential Revision: https://reviews.llvm.org/D61962
llvm-svn: 364398
Philip Reames