value() has undesired exception checking semantics and calls
__throw_bad_optional_access in libc++. Moreover, the API is unavailable without
_LIBCPP_NO_EXCEPTIONS on older Mach-O platforms (see
_LIBCPP_AVAILABILITY_BAD_OPTIONAL_ACCESS).
This enabled the select optimize patch for ARM Out of order AArch64
cores. It is trying to solve a problem that is difficult for the
compiler to fix. The criteria for when a csel is better or worse than a
branch depends heavily on whether the branch is well predicted and the
amount of ILP in the loop (as well as other criteria like the core in
question and the relative performance of the branch predictor). The
pass seems to do a decent job though, with the inner loop heuristics
being well implemented and doing a better job than I had expected in
general, even without PGO information.
I've been doing quite a bit of benchmarking. The headline numbers are
these for SPEC2017 on a Neoverse N1:
500.perlbench_r -0.12%
502.gcc_r 0.02%
505.mcf_r 6.02%
520.omnetpp_r 0.32%
523.xalancbmk_r 0.20%
525.x264_r 0.02%
531.deepsjeng_r 0.00%
541.leela_r -0.09%
548.exchange2_r 0.00%
557.xz_r -0.20%
Running benchmarks with a combination of the llvm-test-suite plus
several versions of SPEC gave between a 0.2% and 0.4% geomean
improvement depending on the core/run. The instruction count went down
by 0.1% too, which is a good sign, but the results can be a little
noisy. Some issues from other benchmarks I had ran were improved in
rGca78b5601466f8515f5f958ef8e63d787d9d812e. In summary well predicted
branches will see in improvement, badly predicted branches may get
worse, and on average performance seems to be a little better overall.
This patch enables the pass for AArch64 under -O3 for cores that will
benefit for it. i.e. not in-order cores that do not fit into the "Assume
infinite resources that allow to fully exploit the available
instruction-level parallelism" cost model. It uses a subtarget feature
for specifying when the pass will be enabled, which I have enabled under
cpu=generic as the performance increases for out of order cores seems
larger than any decreases for inorder, which were minor.
Differential Revision: https://reviews.llvm.org/D138990
This patch mechanically replaces None with std::nullopt where the
compiler would warn if None were deprecated. The intent is to reduce
the amount of manual work required in migrating from Optional to
std::optional.
This is part of an effort to migrate from llvm::Optional to
std::optional:
https://discourse.llvm.org/t/deprecating-llvm-optional-x-hasvalue-getvalue-getvalueor/63716
A `select i1 %c, i1 true, i1 %d` is just an or and a `select i1 %c, i1 %d, i1 false`
is just an and. There are better treated as such in the logic of SelectOpt, allowing
the backend to optimize them to and/or directly.
Differential Revision: https://reviews.llvm.org/D138490
This is some quick debug messages for the SelectOptimize pass, adding
some information for the costs that are measured from getInstructionCost
calls, and re-using the existing optimization remarks to print some
information about if transforms were performed or not.
Differential Revision: https://reviews.llvm.org/D138108
This patch replaces:
return Optional<T>();
with:
return None;
to make the migration from llvm::Optional to std::optional easier.
Specifically, I can deprecate None (in my source tree, that is) to
identify all the instances of None that should be replaced with
std::nullopt.
Note that "return None" far outnumbers "return Optional<T>();". There
are more than 2000 instances of "return None" in our source tree.
All of the instances in this patch come from functions that return
Optional<T> except Archive::findSym and ASTNodeImporter::import, where
we return Expected<Optional<T>>. Note that we can construct
Expected<Optional<T>> from any parameter convertible to Optional<T>,
which None certainly is.
This is part of an effort to migrate from llvm::Optional to
std::optional:
https://discourse.llvm.org/t/deprecating-llvm-optional-x-hasvalue-getvalue-getvalueor/63716
Differential Revision: https://reviews.llvm.org/D138464
This is a follow-up to D133777, which resolved a use-after-free case but
did not cover all possible memory bugs due to misplacement of loads.
In short, the overall problem was that sinked loads could be moved after
state-modifying instructions leading to memory bugs.
The solution is to restrict load sinking unless it is found to be sound.
i) Within a basic block (to-be-sinked load and select-user are in the same BB),
loads can be sinked only if there is no intervening state-modifying instruction.
This is a conservative approach to avoid resorting to alias analysis to detect
potential memory overlap.
ii) Across basic blocks, sinking of loads is avoided. This is because going over
multiple basic blocks looking for memory conflicts could be computationally
expensive and also unlikely to allow loads to sink. Further, experiments showed
that not sinking these loads has a slight positive performance effect.
Maybe for some of these loads, having some separation allows enough time
for the load to be executed in time for its user. This is not the case for
floating point operations that benefit more from sinking.
The solution in D133777 was essentially undone in this patch,
since the latter is a complete solution to the observed problem.
Overall, the performance impact of this patch is minimal.
Tested on two internal Google workloads with instrPGO.
Search application showed <0.05% perf difference,
while the database one showed a slight improvement,
but not statistically significant.
Reviewed By: davidxl
Differential Revision: https://reviews.llvm.org/D133999
When a select is converted to a branch and load instructions are sinked to the true/false blocks,
lifetime intrinsics (if present) could be made unsound if not moved.
This conservatively moves all lifetime intrinsics in a transformed BB to the end block to ensure
preserved lifetime semantics.
Reviewed By: davidxl
Differential Revision: https://reviews.llvm.org/D133777
In this patch we replace common code patterns with the use of utility
functions for dealing with profiling metadata. There should be no change
in functionality, as the existing checks should be preserved in all
cases.
Reviewed By: bogner, davidxl
Differential Revision: https://reviews.llvm.org/D128860
In this patch we replace common code patterns with the use of utility
functions for dealing with profiling metadata. There should be no change
in functionality, as the existing checks should be preserved in all
cases.
Reviewed By: bogner, davidxl
Differential Revision: https://reviews.llvm.org/D128860
Use container::size_type directly to avoid type mismatch causing build failures in Windows.
Original commit message:
This patch optimizes the transformation of selects to a branch when the heuristics deemed it profitable.
It aggressively sinks eligible instructions to the newly created true/false blocks to prevent their
execution on the common path and interleaves dependence slices to maximize ILP.
Depends on D120232
Reviewed By: davidxl
Differential Revision: https://reviews.llvm.org/D120233
This patch optimizes the transformation of selects to a branch when the heuristics deemed it profitable.
It aggressively sinks eligible instructions to the newly created true/false blocks to prevent their
execution on the common path and interleaves dependence slices to maximize ILP.
Depends on D120232
Reviewed By: davidxl
Differential Revision: https://reviews.llvm.org/D120233
This patch adds the loop-level heuristics for determining whether branches are more profitable than conditional moves.
These heuristics apply to only inner-most loops.
Depends on D120231
Reviewed By: davidxl
Differential Revision: https://reviews.llvm.org/D120232
This patch adds the base heuristics for determining whether branches are more profitable than conditional moves.
Base heuristics apply to all code apart from inner-most loops.
Depends on D122259
Reviewed By: davidxl
Differential Revision: https://reviews.llvm.org/D120231
This patch implements the actual transformation of selects to branches.
It includes only the base transformation without any sinking.
Depends on D120230
Reviewed By: davidxl
Differential Revision: https://reviews.llvm.org/D122259
This is the first commit for the cmov-vs-branch optimization pass.
The goal is to develop a new profile-guided and target-independent cost/benefit analysis
for selecting conditional moves over branches when optimizing for performance.
Initially, this new pass is expected to be enabled only for instrumentation-based PGO.
RFC: https://discourse.llvm.org/t/rfc-cmov-vs-branch-optimization/6040
Reviewed By: tejohnson
Differential Revision: https://reviews.llvm.org/D120230
