As discussed on https://github.com/llvm/llvm-project/issues/54682,
MemorySSA currently has a bug when computing the clobber of calls
that access loop-varying locations. I think a "proper" fix for this
on the MemorySSA side might be non-trivial, but we can easily work
around this in MemCpyOpt:
Currently, MemCpyOpt uses a location-less getClobberingMemoryAccess()
call to find a clobber on either the src or dest location, and then
refines it for the src and dest clobber. This was intended as an
optimization, as the location-less API is cached, while the
location-affected APIs are not.
However, I don't think this really makes a difference in practice,
because I don't think anything will use the cached clobbers on
those calls later anyway. On CTMark, this patch seems to be very
mildly positive actually.
So I think this is a reasonable way to avoid the problem for now,
though MemorySSA should also get a fix.
Differential Revision: https://reviews.llvm.org/D122911
The search for the clobbering call is fairly expensive if uses are not optimized at construction. Defer the clobber walk to the point in the implementation we need it; there are a bunch of bailouts before that point. (e.g. If the source pointer is not an alloca, we can't do callslotopt.)
On a test case which involves a bunch of copies from argument pointers, this switches memcpyopt from > 1/2 second to < 10ms.
We have the same code repeated in both callers, sink it into callee.
The motivation here isn't just code style, we can also defer the relatively expensive aliasing checks until the cheap structural preconditions have been validated. (e.g. Don't bother aliasing if src is not an alloca.) This helps compile time significantly.
Currently writtenBetween can miss clobbers of Loc between End and Start,
if End is a MemoryUse.
To guarantee we see all write clobbers of Loc between Start and End
for MemoryUses, restrict to Start and End being in the same block
and check all accesses between them.
This fixes 2 mis-compiles illustrated in
llvm/test/Transforms/MemCpyOpt/memcpy-byval-forwarding-clobbers.ll
Reviewed By: nikic
Differential Revision: https://reviews.llvm.org/D119929
This is a recommit of the patch without changes. The reason for
the revert has been addressed in D117679.
-----
The user scanning loop above looks through pointer casts, so we
also need to strip pointer casts in the capture check. Previously
the source was incorrectly considered not captured if a bitcast
was passed to the call.
This is a recommit of the patch without changes. The reason for
the revert has been addressed in D117679.
-----
Call slot optimization is currently supposed to be prevented if
the call can capture the source pointer. Due to an implementation
bug, this check currently doesn't trigger if a bitcast of the source
pointer is passed instead. I'm somewhat afraid of the fallout of
fixing this bug (due to heavy reliance on call slot optimization
in rust), so I'd like to strengthen the capture reasoning a bit first.
In particular, I believe that the capture is fine as long as a)
the call itself cannot depend on the pointer identity, because
neither dest has been captured before/at nor src before the
call and b) there is no potential use of the captured pointer
before the lifetime of the source alloca ends, either due to
lifetime.end or a return from a function. At that point the
potentially captured pointer becomes dangling.
Differential Revision: https://reviews.llvm.org/D115615
Call slot optimization currently merges the metadata between the
call and the load. However, we also need to merge in the metadata
of the store.
Part of the reason why we might have gotten away with this
previously is that usually the load and the store are the same
instruction (a memcpy), this can only happen if call slot
optimization occurs on an actual load/store pair.
This addresses the issue reported in
https://reviews.llvm.org/D115615#3251386.
Differential Revision: https://reviews.llvm.org/D117679
This casued a miscompile due to call slot optimization replacing a call
argument without considering the call's !noalias metadata, see discussion on
the code review.
> Call slot optimization is currently supposed to be prevented if
> the call can capture the source pointer. Due to an implementation
> bug, this check currently doesn't trigger if a bitcast of the source
> pointer is passed instead. I'm somewhat afraid of the fallout of
> fixing this bug (due to heavy reliance on call slot optimization
> in rust), so I'd like to strengthen the capture reasoning a bit first.
>
> In particular, I believe that the capture is fine as long as a)
> the call itself cannot depend on the pointer identity, because
> neither dest has been captured before/at nor src before the
> call and b) there is no potential use of the captured pointer
> before the lifetime of the source alloca ends, either due to
> lifetime.end or a return from a function. At that point the
> potentially captured pointer becomes dangling.
>
> Differential Revision: https://reviews.llvm.org/D115615
Also reverting the dependent commit:
> [MemCpyOpt] Look through pointer casts when checking capture
>
> The user scanning loop above looks through pointer casts, so we
> also need to strip pointer casts in the capture check. Previously
> the source was incorrectly considered not captured if a bitcast
> was passed to the call.
This reverts commit 487a34ed9d
and 00e6869463.
The user scanning loop above looks through pointer casts, so we
also need to strip pointer casts in the capture check. Previously
the source was incorrectly considered not captured if a bitcast
was passed to the call.
Call slot optimization is currently supposed to be prevented if
the call can capture the source pointer. Due to an implementation
bug, this check currently doesn't trigger if a bitcast of the source
pointer is passed instead. I'm somewhat afraid of the fallout of
fixing this bug (due to heavy reliance on call slot optimization
in rust), so I'd like to strengthen the capture reasoning a bit first.
In particular, I believe that the capture is fine as long as a)
the call itself cannot depend on the pointer identity, because
neither dest has been captured before/at nor src before the
call and b) there is no potential use of the captured pointer
before the lifetime of the source alloca ends, either due to
lifetime.end or a return from a function. At that point the
potentially captured pointer becomes dangling.
Differential Revision: https://reviews.llvm.org/D115615
This patch fixes a variety of crashes resulting from the `MemCpyOptPass`
casting `TypeSize` to a constant integer, whether implicitly or
explicitly.
Since the `MemsetRanges` requires a constant size to work, all but one
of the fixes in this patch simply involve skipping the various
optimizations for scalable types as cleanly as possible.
The optimization of `byval` parameters, however, has been updated to
work on scalable types in theory. In practice, this optimization is only
valid when the length of the `memcpy` is known to be larger than the
scalable type size, which is currently never the case. This could
perhaps be done in the future using the `vscale_range` attribute.
Some implicit casts have been left as they were, under the knowledge
they are only called on aggregate types. These should never be
scalably-sized.
Reviewed By: nikic, tra
Differential Revision: https://reviews.llvm.org/D109329
When converting a store into a memset, we currently insert the new
MemoryDef after the store MemoryDef, which requires all uses to be
renamed to the new def using a whole block scan. Instead, we can
insert the new MemoryDef before the store and not rename uses,
because we know that the location is immediately overwritten, so
all uses should still refer to the old MemoryDef. Those uses will
get renamed when the old MemoryDef is actually dropped, which is
efficient.
I expect something similar can be done for some of the other MSSA
updates in MemCpyOpt. This is an alternative to D107513, at least
for this particular case.
Differential Revision: https://reviews.llvm.org/D107702
The MemorySSA-based implementation has been enabled for a few months
(since D94376). This patch drops the old MDA-based implementation
entirely.
I've kept this to only the basic cleanup of dropping various
conditions -- the code could be further cleaned up now that there
is only one implementation.
Differential Revision: https://reviews.llvm.org/D102113
Attempt to enable MemCpyOpt unconditionally in D104801 uncovered the fact that
there are users that do not expect LLVM to materialize `memset` intrinsic.
While other passes can do that, too, MemCpyOpt triggers it more frequently and
breaks sanitizers and some downstream users.
For now introduce a flag to force-enable the flag and opt-in only CUDA
compilation with NVPTX back-end.
Differential Revision: https://reviews.llvm.org/D106401
- Loads from the constant memory (either explicit one or as the source
of memory transfer intrinsics) won't alias any stores.
Reviewed By: asbirlea, efriedma
Differential Revision: https://reviews.llvm.org/D107605
Rather than blocking the whole MemCpyOpt pass if the libcalls are
not available, only disable creation of new memset/memcpy intrinsics
where only load/stores were used previously. This only affects the
store merging and load-store conversion optimization. Other
optimizations are derived from existing intrinsics, which are
well-defined in the absence of libcalls -- not having the libcalls
just means that call simplification won't convert them to intrinsics.
This is a weaker variation of D104801, which dropped these checks
entirely. Ideally we would not couple emission of intrinsics to
libcall availability at all, but as the intrinsics may be legalized
to libcalls we need to be a bit careful right now.
Differential Revision: https://reviews.llvm.org/D106769
The patch does not depend on the availability of the library functions for
memcpy/memset as it operates on LLVM intrinsics. The optimizations are useful
on the targets that have these functions disabled (e.g. NVPTX & AMDGPU).
Differential Revision: https://reviews.llvm.org/D104801
Apparently, it is legal to use memcpy/memset with pointer types
other than i8*. Prior to 81fcdae68c
this case was silently miscompiled, as the i8 offset calculation
was performed on some other type. Now it would crash due to a
type mismatch. Fix this by inserting an explicit bitcast to i8*.
Currently all AA analyses marked as preserved are stateless, not taking
into account their dependent analyses. So there's no need to mark them
as preserved, they won't be invalidated unless their analyses are.
SCEVAAResults was the one exception to this, it was treated like a
typical analysis result. Make it like the others and don't invalidate
unless SCEV is invalidated.
Reviewed By: asbirlea
Differential Revision: https://reviews.llvm.org/D102032
As a follow-up to D95982, this patch continues unblocking optimizations that are blocked by pseudu probe instrumention.
The optimizations unblocked are:
- In-block load propagation.
- In-block dead store elimination
- Memory copy optimization that turns stores to consecutive memories into a memset.
These optimizations are local to a block, so they shouldn't affect the profile quality.
Reviewed By: wmi
Differential Revision: https://reviews.llvm.org/D100075
This makes the memcpy-memcpy and memcpy-memset optimizations work for
variable sizes as long as they are equal, relaxing the old restriction
that they are constant integers. If they're not equal, the old
requirement that they are constant integers with certain size
restrictions is used.
The implementation works by pushing the length tests further down in the
code, which reveals some places where it's enough that the lengths are
equal (but not necessarily constant).
Differential Revision: https://reviews.llvm.org/D100870
Add MemorySSAWrapperPass as a dependency to MemCpyOptLegacyPass,
since MemCpyOpt now uses MemorySSA by default.
Differential Revision: https://reviews.llvm.org/D98484
This fixes a regression from the MemDep-based implementation:
MemDep completely ignores lifetime.start intrinsics that aren't
MustAlias -- this is probably unsound, but it does mean that the
MemDep based implementation successfully eliminated memcpy's from
lifetime.start if the memcpy happens at an offset, rather than
the base address of the alloca.
Add a special case for the case where the lifetime.start spans the
whole alloca (which is pretty much the only kind of lifetime.start
that frontends ever emit), as we don't need to figure out our exact
aliasing relationship in that case, the whole alloca is dead prior
to the call.
If this doesn't cover all practically relevant cases, then it
would be possible to make use of the recently added PartialAlias
clobber offsets to make this more precise.
If a memset destination is overwritten by a memcpy and the sizes
are exactly the same, then the memset is simply dead. We can
directly drop it, instead of replacing it with a memset of zero
size, which is particularly ugly for the case of a dynamic size.
If the call is readnone, then there may not be any MemoryAccess
associated with the call. Bail out in that case.
This fixes the issue reported at
https://reviews.llvm.org/D94376#2578312.
This enables use of MemorySSA instead of MemDep in MemCpyOpt. To
allow this without significant compile-time impact, the MemCpyOpt
pass is moved directly before DSE (in the cases where this was not
already the case), which allows us to reuse the existing MemorySSA
analysis.
Unlike the MemDep-based implementation, the MemorySSA-based MemCpyOpt
can also perform simple optimizations across basic blocks.
Differential Revision: https://reviews.llvm.org/D94376
This is a straightforward port of MemCpyOpt to MemorySSA following
the approach of D26739. MemDep queries are replaced with MSSA queries
without changing the overall structure of the pass. Some care has
to be taken to account for differences between these APIs
(MemDep also returns reads, MSSA doesn't).
Differential Revision: https://reviews.llvm.org/D89207
When performing a call slot optimization to a GEP destination, it
will currently usually fail, because the GEP is directly before the
memcpy and as such does not dominate the call. We should move it
above the call if that satisfies the domination requirement.
I think that a constant-index GEP is the only useful thing to move
here, as otherwise isDereferenceablePointer couldn't look through
it anyway. As such I'm not trying to generalize this further.
Differential Revision: https://reviews.llvm.org/D89623
This adds an -enable-memcpyopt-memoryssa option that currently does
nothing apart from requiring MSSA as a dependency. The tests are
split to run both with the option disabled and enabled. I went with
this rather than the separate directory DSE uses, as I found it
convenient to have a direct side-by-side comparison of differences.
Differential Revision: https://reviews.llvm.org/D89206
moveUp() moves instructions, so we should move the corresponding
memory accesses as well. We should also move the store instruction
itself: Even though we'll end up removing it later, this gives us
a correct MemoryDef to replace.
The implementation is somewhat more complicated than it should be,
because we also handle the case where P does not have a memory
access due to a degnerate AA pipeline. Hopefully, the need for this
will go away in the future, when the rest of the pass is based on
MSSA.
Differential Revision: https://reviews.llvm.org/D88778
If the memcpy operands are the same (which is allowed since D86815)
then the memcpy is effectively a no-op and the partially overlapping
memset is not dead.
Differential Revision: https://reviews.llvm.org/D89192
MemCpyOpt can shorten a memset if it is later partially overwritten
by a memcpy. It checks that the destination is not read in between,
but we also need to make sure that the destination cannot be observed
via unwinding.
Differential Revision: https://reviews.llvm.org/D89190
MemCpyOpt can hoist stores while load+store pairs into memcpy.
This hoisting can currently result in stores being executed that
weren't guaranteed to execute in the original problem.
Differential Revision: https://reviews.llvm.org/D89154
Fangrui Song