MSSA DSE starts at a killing store, finds an earlier store and
then checks that the earlier store is not read along any paths
(without being killed first). However, it uses the memory location
of the killing store for that, not the earlier store that we're
attempting to eliminate.
This has a number of problems:
* Mismatches between what BasicAA considers aliasing and what DSE
considers an overwrite (even though both are correct in isolation)
can result in miscompiles. This is PR48279, which D92045 tries to
fix in a different way. The problem is that we're using a location
from a store that is potentially not executed and thus may be UB,
in which case analysis results can be arbitrary.
* Metadata on the killing store may be used to determine aliasing,
but there is no guarantee that the metadata is valid, as the specific
killing store may not be executed. Using the metadata on the earlier
store is valid (it is the store we're removing, so on any execution
where its removal may be observed, it must be executed).
* The location is imprecise. For full overwrites the killing store
will always have a location that is larger or equal than the earlier
access location, so it's beneficial to use the earlier access
location. This is not the case for partial overwrites, in which
case either location might be smaller. There is some room for
improvement here.
Using the earlier access location means that we can no longer cache
which accesses are read for a given killing store, as we may be
querying different locations. However, it turns out that simply
dropping the cache has no notable impact on compile-time.
Differential Revision: https://reviews.llvm.org/D93523
This patch updates DSE + MemorySSA to use the same check as the legacy
implementation to determine if a location is killed by a free call.
This changes the existing behavior so that a free does not kill
locations before the start of the freed pointer.
This should fix PR48036.
Currently isOverwrite returns OW_MaybePartial even for accesss known not to overlap. This is not a big problem for legacy implementation (since isPartialOverwrite follows isOverwrite and clarifies the result). Contrary SSA based version does a lot of work to later find out that accesses don't overlap. Besides negative impact on compile time we quickly reach MemorySSAPartialStoreLimit and miss optimization opportunities.
Note: In fact, I think it would be cleaner implementation if isOverwrite returned fully clarified result in the first place whithout need to call isPartialOverwrite. This can be done as a follow up. What do you think?
Reviewed By: fhahn, asbirlea
Differential Revision: https://reviews.llvm.org/D90371
Currently we fail to eliminate some noop stores if there is a kill-able
store between the starting def and the load. This is because we
eliminate noop stores first.
In practice it seems like eliminating noop stores after the main
elimination for a def covers slightly more cases.
This patch improves the number of stores slightly in 2 cases for X86 -O3
-flto
Same hash: 235 (filtered out)
Remaining: 2
Metric: dse.NumRedundantStores
Program base patch diff
test-suite...ce/Benchmarks/PAQ8p/paq8p.test 2.00 3.00 50.0%
test-suite...006/453.povray/453.povray.test 18.00 21.00 16.7%
There might be other phase ordering issues, but it appears that they do
not show up in the test-suite/SPEC2000/SPEC2006. We can always tune the
ordering later.
Partly fixes PR47887.
Reviewed By: asbirlea, zoecarver
Differential Revision: https://reviews.llvm.org/D89650
Instead of getting the defining access we should be able to use
getClobberingMemoryAccess to skip non-aliasing MemoryDefs. No additional
checks should be needed, because we only remove the starting def if it
matches the defining access of the load. All we need to worry about is
that there are no (may)alias stores between the starting def and the
load and getClobberingMemoryAccess should guarantee that.
Partly fixes PR47887.
This improves the number of redundant stores removed in some cases
(numbers below for MultiSource, SPEC2000, SPEC2006 on X86 with -flto
-O3).
Same hash: 226 (filtered out)
Remaining: 11
Metric: dse.NumRedundantStores
Program base patch1 diff
test-suite...:: External/Povray/povray.test 1.00 5.00 400.0%
test-suite...chmarks/MallocBench/gs/gs.test 1.00 3.00 200.0%
test-suite...0/253.perlbmk/253.perlbmk.test 21.00 37.00 76.2%
test-suite...0.perlbench/400.perlbench.test 24.00 37.00 54.2%
test-suite.../Applications/SPASS/SPASS.test 3.00 4.00 33.3%
test-suite...006/453.povray/453.povray.test 15.00 18.00 20.0%
test-suite...T2006/445.gobmk/445.gobmk.test 27.00 29.00 7.4%
test-suite.../CINT2006/403.gcc/403.gcc.test 136.00 137.00 0.7%
test-suite.../CINT2000/176.gcc/176.gcc.test 6.00 6.00 0.0%
test-suite.../Benchmarks/Bullet/bullet.test NaN 3.00 nan%
test-suite.../Benchmarks/Ptrdist/bc/bc.test NaN 1.00 nan%
Reviewed By: asbirlea
Differential Revision: https://reviews.llvm.org/D89647
isMemTerminator checks if the current def is a memory terminator that
terminates the memory pointed to by DefLoc. We do not have to add any of
their users to the worklist, because the follow-on users cannot read the
memory in question.
This leads to more stores eliminated in the presence of lifetime calls.
Previously we added the users of those intrinsics to the worklist,
limiting elimination.
In terms of removed stores, this gives a nice boost on some benchmarks
(MultiSource/SPEC2000/SPEC2006 on X86 with -flto -O3):
Same hash: 205 (filtered out)
Remaining: 32
Metric: dse.NumFastStores
Program base patch diff
test-suite...000/197.parser/197.parser.test 4.00 8.00 100.0%
test-suite...rolangs-C++/family/family.test 4.00 7.00 75.0%
test-suite...marks/7zip/7zip-benchmark.test 1722.00 2189.00 27.1%
test-suite...CFP2000/177.mesa/177.mesa.test 30.00 38.00 26.7%
test-suite :: External/Nurbs/nurbs.test 44.00 49.00 11.4%
test-suite...lications/sqlite3/sqlite3.test 115.00 128.00 11.3%
test-suite...006/447.dealII/447.dealII.test 2715.00 3013.00 11.0%
test-suite...ProxyApps-C++/CLAMR/CLAMR.test 237.00 261.00 10.1%
test-suite...tions/lambda-0.1.3/lambda.test 40.00 44.00 10.0%
test-suite...3.xalancbmk/483.xalancbmk.test 1366.00 1475.00 8.0%
test-suite...abench/jpeg/jpeg-6a/cjpeg.test 13.00 14.00 7.7%
test-suite...oxyApps-C++/miniFE/miniFE.test 43.00 46.00 7.0%
test-suite...lications/ClamAV/clamscan.test 230.00 246.00 7.0%
test-suite...006/450.soplex/450.soplex.test 284.00 299.00 5.3%
test-suite...nsumer-jpeg/consumer-jpeg.test 21.00 22.00 4.8%
D70365 allows us to make attributes default. This is a follow up to
actually make nosync, nofree and willreturn default. The approach we
chose, for now, is to opt-in to default attributes to avoid introducing
problems to target specific intrinsics. Intrinsics with default
attributes can be created using `DefaultAttrsIntrinsic` class.
D18714 introduced writeonly attribute:
"Also start using the attribute for memset, memcpy, and memmove intrinsics,
and remove their special-casing in BasicAliasAnalysis."
But actually, writeonly was not attached to memmove - oversight, it seems.
So let's add it. As we can see, this helps DSE to eliminate redundant stores.
Reviewed By: jdoerfert
Differential Revision: https://reviews.llvm.org/D89724
isNoopIntrinsic returns true for some intrinsics that are modeled in
MemorySSA but do not actually read or write any memory and do not block
DSE. Such intrinsics should not be considered as read-clobbers.
After investigation by @asbirlea, the issue that caused the
revert appears to be an issue in the original source, rather
than a problem with the compiler.
This patch enables MemorySSA DSE again.
This reverts commit 915310bf14.
Summary:
Adds support for "following" memory through MSSA PHI arguments. This will help catch more noop stores that exist between blocks.
Originally part of D79391.
Reviewers: fhahn, jfb, asbirlea
Differential Revision: https://reviews.llvm.org/D82588
There appears to be a mis-compile with MemorySSA-backed DSE in
combination with llvm.lifetime.end. It currently appears like
DSE is doing the right thing and the llvm.lifetime.end markers
are incorrect. The reverted patch uncovers the mis-compile.
This patch temporarily switches back to the legacy DSE
implementation, while we investigate.
This reverts commit 9d172c8e9c.
When looking for memory defs killed by memory terminators the code
currently incorrectly ignores the size argument of llvm.lifetime.end.
This patch updates the code to use isMemTerminator and updates
isMemTerminator to use isOverwrite() to make sure locations that are
outside the range marked as dead by llvm.lifetime.end are not
considered. Note that isOverwrite is only used for llvm.lifetime.end,
because free-like functions make the whole underlying object dead.
llvm.lifetime.end accepts a size parameters to limit the size of the
location marked as dead. Add a few tests with stores to locations after
the part that has been marked as dead.
This switches to using DSE + MemorySSA by default again, after
fixing the issues reported after the first commit.
Notable fixes fc82006331, a0017c2bc2.
This reverts commit 3a59628f3c.
AliasAnalysis/MemoryLocation does not account for loops. Two
MemoryLocation can be must-overwrite, even if the first one writes
multiple locations in a loop.
This patch prevents removing such stores, by only considering candidates
that are known to be loop invariant, or executed in the same BB.
Currently the invariant check is quite conservative and only considers
Alloca and Alloca-like instructions and arguments as invariant base pointers.
It also considers GEPs with all constant indices and invariant bases as
invariant.
This can be improved in the future, but the current implementation has
only minor impact on the total number of stores eliminated (25903 vs
26047 for the baseline). There are some 2-10% swings for some individual
benchmarks. In roughly half of the cases, the number of stores removed
increases actually, because we skip candidates that are unlikely to be
valid candidates early.
When deleting stores at the end of a function, we have to do PHI
translation, otherwise we might miss reads in different iterations of a
loop. See multiblock-loop-carried-dependence.ll for details.
This fixes a mis-compile and surprisingly also increases the number of
eliminated stores from 26047 to 26572 for MultiSource/SPEC2000/SPEC2006
on X86 with -O3 -flto. This is most likely because we save budget by not
exploring through MemoryPhis, which are less likely to result in valid
candidates for elimination.
The issue was reported post-commit for fb109c42d9.
The tests have been updated and I plan to move them from the MSSA
directory up.
Some end-to-end tests needed small adjustments. One difference to the
legacy DSE is that legacy DSE also deletes trivially dead instructions
that are unrelated to memory operations. Because MemorySSA-backed DSE
just walks the MemorySSA, we only visit/check memory instructions. But
removing unrelated dead instructions is not really DSE's job and other
passes will clean up.
One noteworthy change is in llvm/test/Transforms/Coroutines/ArgAddr.ll,
but I think this comes down to legacy DSE not handling instructions that
may throw correctly in that case. To cover this with MemorySSA-backed
DSE, we need an update to llvm.coro.begin to treat it's return value to
belong to the same underlying object as the passed pointer.
There are some minor cases MemorySSA-backed DSE currently misses, e.g. related
to atomic operations, but I think those can be implemented after the switch.
This has been discussed on llvm-dev:
http://lists.llvm.org/pipermail/llvm-dev/2020-August/144417.html
For the MultiSource/SPEC2000/SPEC2006 the number of eliminated stores
goes from ~17500 (legayc DSE) to ~26300 (MemorySSA-backed). More numbers
and details in the thread on llvm-dev.
Impact on CTMark:
```
Legacy Pass Manager
exec instrs size-text
O3 + 0.60% - 0.27%
ReleaseThinLTO + 1.00% - 0.42%
ReleaseLTO-g. + 0.77% - 0.33%
RelThinLTO (link only) + 0.87% - 0.42%
RelLO-g (link only) + 0.78% - 0.33%
```
http://llvm-compile-time-tracker.com/compare.php?from=3f22e96d95c71ded906c67067d75278efb0a2525&to=ae8be4642533ff03803967ee9d7017c0d73b0ee0&stat=instructions
```
New Pass Manager
exec instrs. size-text
O3 + 0.95% - 0.25%
ReleaseThinLTO + 1.34% - 0.41%
ReleaseLTO-g. + 1.71% - 0.35%
RelThinLTO (link only) + 0.96% - 0.41%
RelLO-g (link only) + 2.21% - 0.35%
```
http://195.201.131.214:8000/compare.php?from=3f22e96d95c71ded906c67067d75278efb0a2525&to=ae8be4642533ff03803967ee9d7017c0d73b0ee0&stat=instructions
Reviewed By: asbirlea, xbolva00, nikic
Differential Revision: https://reviews.llvm.org/D87163
MemoryLocation has been taught about memcpy.inline, which means we can
get the memory locations read and written by it. This means DSE can
handle memcpy.inline
Atomic stores are modeled as MemoryDef to model the fact that they may
not be reordered, depending on the ordering constraints.
Atomic stores that are monotonic or weaker do not limit re-ordering, so
we do not have to treat them as potential read clobbers.
Note that llvm/test/Transforms/DeadStoreElimination/MSSA/atomic.ll
already contains a set of negative test cases.
Reviewed By: asbirlea
Differential Revision: https://reviews.llvm.org/D87386
This adjusts the description of `llvm.memcpy` to also allow operands
to be equal. This is in line with what Clang currently expects.
This change is intended to be temporary and followed by re-introduce
a variant with the non-overlapping guarantee for cases where we can
actually ensure that property in the front-end.
See the links below for more details:
http://lists.llvm.org/pipermail/cfe-dev/2020-August/066614.html
and PR11763.
Reviewed By: nikic
Differential Revision: https://reviews.llvm.org/D86815
Some tests from multibuild-malloc-free.ll do not actually use malloc or
free and where split out to multiblock-throwing.ll, but not removed from
the original file. This patch cleans that up. It also moves @test22 to
simple.ll, because it does not involve multiple blocks.
This patch moves the tests for the old MemDepAnalysis based DSE
implementation to the MemDepAnalysis subdirectory and updates them to
pass -enable-dse-memoryssa=false.
This is in preparation for the switch to MemorySSA-backed DSE.
This changes getDomMemoryDef to check if a Current is a valid
candidate for elimination before checking for reads. Before the change,
we were spending a lot of compile-time in checking for read accesses for
Current that might not even be removable.
This patch flips the logic, so we skip Current if they cannot be
removed before checking all their uses. This is much more efficient in
practice.
It also adds a more aggressive limit for checking partially overlapping
stores. The main problem with overlapping stores is that we do not know
if they will lead to elimination until seeing all of them. This patch
limits adds a new limit for overlapping store candidates, which keeps
the number of modified overlapping stores roughly the same.
This is another substantial compile-time improvement (while also
increasing the number of stores eliminated). Geomean -O3 -0.67%,
ReleaseThinLTO -0.97%.
http://llvm-compile-time-tracker.com/compare.php?from=0a929b6978a068af8ddb02d0d4714a2843dd8ba9&to=2e630629b43f64b60b282e90f0d96082fde2dacc&stat=instructions
Reviewed By: asbirlea
Differential Revision: https://reviews.llvm.org/D86487
Nikita Popov