Add some debug output to `outline` to assist in debugging + understanding the
code.
This will say
- How many things we found worth turning into outlined functions
- Whether or not candidates were pruned via the outlining algorithm
- The function created (if it was created)
- Where the calls were inserted
- What instruction was used to create the call
Sample output below:
```
NUMBER OF POTENTIAL FUNCTIONS: 5
WALKING FUNCTION LIST
PRUNED: 0/2 candidates
OUTLINE: Expected benefit (12 B) > threshold (1 B)
NEW FUNCTION: OUTLINED_FUNCTION_0
CREATE OUTLINED CALLS
CALL: OUTLINED_FUNCTION_0 in bar:<unknown>
.. BL @OUTLINED_FUNCTION_0, implicit-def $lr, implicit $sp
CALL: OUTLINED_FUNCTION_0 in bar:<unknown>
.. BL @OUTLINED_FUNCTION_0, implicit-def $lr, implicit $sp
PRUNED: 2/2 candidates
SKIP: Expected benefit (0 B) < threshold (1 B)
PRUNED: 0/2 candidates
OUTLINE: Expected benefit (8 B) > threshold (1 B)
NEW FUNCTION: OUTLINED_FUNCTION_1
CREATE OUTLINED CALLS
CALL: OUTLINED_FUNCTION_1 in bar:<unknown>
.. BL @OUTLINED_FUNCTION_1, implicit-def $lr, implicit $sp
CALL: OUTLINED_FUNCTION_1 in bar:<unknown>
.. BL @OUTLINED_FUNCTION_1, implicit-def $lr, implicit $sp
PRUNED: 2/2 candidates
SKIP: Expected benefit (0 B) < threshold (1 B)
PRUNED: 2/2 candidates
SKIP: Expected benefit (0 B) < threshold (1 B)
```
We add a field `IsOutlined` to indicate whether a MachineFunction
is outlined and set it true for outlined functions in MachineOutliner.
Reviewed By: paquette
Differential Revision: https://reviews.llvm.org/D146191
Outlining isn't always a win when the saved instruction count is >= 1.
The overhead of representing a new function in the binary depends on
exception metadata and alignment. So parameterize this for local tuning.
Reviewed By: paquette
Differential Revision: https://reviews.llvm.org/D136774
I think it's good practice to avoid having default ctors unless they're really
valid/useful. For OutlinedFunction the default ctor was used to represent a
bail-out value for getOutliningCandidateInfo(), so I changed the API to return
an optional<getOutliningCandidateInfo> instead which seems a tad cleaner.
Differential Revision: https://reviews.llvm.org/D146375
Add a test for statistics as well.
The mapper size stats were nested in a loop unnecessarily. Move them out.
Give existing stats better names, and add one which also tracks the number of
sentinels added.
This had no debug output. Since it was committed as NFC, it had no testcase.
The me of today was nerdsniped by the me of 6 years ago and decided that this
ought to have a testcase and some debug output.
The MachineOutliner + SuffixTree both used `std::vector` everywhere because I
didn't know any better at the time.
At least for small types, such as `unsigned` and iterators, I can't see any
particular reason to use std::vector over `SmallVector` here.
Recommit with bug fixes + added testcases to the outliner. Also adds some
debug output.
We found a case in the Swift benchmarks where the MachineOutliner introduces
about a 20% compile time overhead in comparison to building without the
MachineOutliner.
The origin of this slowdown is that the benchmark has long blocks which incur
lots of LRU checks for lots of candidates.
Imagine a case like this:
```
bb:
i1
i2
i3
...
i123456
```
Now imagine that all of the outlining candidates appear early in the block, and
that something like, say, NZCV is defined at the end of the block.
The outliner has to check liveness for certain registers across all candidates,
because outlining from areas where those registers are used is unsafe at call
boundaries.
This is fairly wasteful because in the previously-described case, the outlining
candidates will never appear in an area where those registers are live.
To avoid this, precalculate areas where we will consider outlining from.
Anything outside of these areas is mapped to illegal and not included in the
outlining search space. This allows us to reduce the size of the outliner's
suffix tree as well, giving us a potential memory win.
By precalculating areas, we can also optimize other checks too, like whether
or not LR is live across an outlining candidate.
Doing all of this is about a 16% compile time improvement on the case.
This is likely useful for other targets (e.g. ARM + RISCV) as well, but for now,
this only implements the AArch64 path. The original "is the MBB safe" method
still works as before.
Add `nooutline` + update LangRef to say it exists.
This makes it possible to say "don't outline from this function ever."
We want to be able to toggle whether or not a function should be in the search
set regardless of default behaviour.
Add testcases for the IR Outliner + Machine Outliner.
Also remove an unnecessary check for an empty function in the Machine Outliner.
Differential Revision: https://reviews.llvm.org/D140438
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
1. When checking if a candidate contains a CFI instruction, actually
iterate over all of the instructions, instead of stopping halfway
through.
2. Make sure copied CFI directives refer to the correct instruction.
Fixes https://github.com/llvm/llvm-project/issues/55842
Differential Revision: https://reviews.llvm.org/D126930
This reverts commit 7f230feeeac8a67b335f52bd2e900a05c6098f20.
Breaks CodeGenCUDA/link-device-bitcode.cu in check-clang,
and many LLVM tests, see comments on https://reviews.llvm.org/D121169
We found a case in the Swift benchmarks where the MachineOutliner introduces
about a 20% compile time overhead in comparison to building without the
MachineOutliner.
The origin of this slowdown is that the benchmark has long blocks which incur
lots of LRU checks for lots of candidates.
Imagine a case like this:
```
bb:
i1
i2
i3
...
i123456
```
Now imagine that all of the outlining candidates appear early in the block, and
that something like, say, NZCV is defined at the end of the block.
The outliner has to check liveness for certain registers across all candidates,
because outlining from areas where those registers are used is unsafe at call
boundaries.
This is fairly wasteful because in the previously-described case, the outlining
candidates will never appear in an area where those registers are live.
To avoid this, precalculate areas where we will consider outlining from.
Anything outside of these areas is mapped to illegal and not included in the
outlining search space. This allows us to reduce the size of the outliner's
suffix tree as well, giving us a potential memory win.
By precalculating areas, we can also optimize other checks too, like whether
or not LR is live across an outlining candidate.
Doing all of this is about a 16% compile time improvement on the case.
This is likely useful for other targets (e.g. ARM + RISCV) as well, but for now,
this only implements the AArch64 path. The original "is the MBB safe" method
still works as before.
Useful for debugging + evaluating improvements to the outliner.
Stats are the number of illegal, legal, and invisible instructions in the
unsigned vector, and it's total length.
We have the `clang -cc1` command-line option `-funwind-tables=1|2` and
the codegen option `VALUE_CODEGENOPT(UnwindTables, 2, 0) ///< Unwind
tables (1) or asynchronous unwind tables (2)`. However, this is
encoded in LLVM IR by the presence or the absence of the `uwtable`
attribute, i.e. we lose the information whether to generate want just
some unwind tables or asynchronous unwind tables.
Asynchronous unwind tables take more space in the runtime image, I'd
estimate something like 80-90% more, as the difference is adding
roughly the same number of CFI directives as for prologues, only a bit
simpler (e.g. `.cfi_offset reg, off` vs. `.cfi_restore reg`). Or even
more, if you consider tail duplication of epilogue blocks.
Asynchronous unwind tables could also restrict code generation to
having only a finite number of frame pointer adjustments (an example
of *not* having a finite number of `SP` adjustments is on AArch64 when
untagging the stack (MTE) in some cases the compiler can modify `SP`
in a loop).
Having the CFI precise up to an instruction generally also means one
cannot bundle together CFI instructions once the prologue is done,
they need to be interspersed with ordinary instructions, which means
extra `DW_CFA_advance_loc` commands, further increasing the unwind
tables size.
That is to say, async unwind tables impose a non-negligible overhead,
yet for the most common use cases (like C++ exceptions), they are not
even needed.
This patch extends the `uwtable` attribute with an optional
value:
- `uwtable` (default to `async`)
- `uwtable(sync)`, synchronous unwind tables
- `uwtable(async)`, asynchronous (instruction precise) unwind tables
Reviewed By: MaskRay
Differential Revision: https://reviews.llvm.org/D114543
This patch implements a new MachineFunction in the ARM backend for
placing BTI instructions. It is similar to the existing AArch64
aarch64-branch-targets pass.
BTI instructions are inserted into basic blocks that:
- Have their address taken
- Are the entry block of a function, if the function has external
linkage or has its address taken
- Are mentioned in jump tables
- Are exception/cleanup landing pads
Each BTI instructions is placed in the beginning of a BB after the
so-called meta instructions (e.g. exception handler labels).
Each outlining candidate and the outlined function need to be in agreement about
whether BTI placement is enabled or not. If branch target enforcement is
disabled for a function, the outliner should not covertly enable it by emitting
a call to an outlined function, which begins with BTI.
The cost mode of the outliner is adjusted to account for the extra BTI
instructions in the outlined function.
The ARM Constant Islands pass will maintain the count of the jump tables, which
reference a block. A `BTI` instruction is removed from a block only if the
reference count reaches zero.
PAC instructions in entry blocks are replaced with PACBTI instructions (tests
for this case will be added in a later patch because the compiler currently does
not generate PAC instructions).
The ARM Constant Island pass is adjusted to handle BTI
instructions correctly.
Functions with static linkage that don't have their address taken can
still be called indirectly by linker-generated veneers and thus their
entry points need be marked with BTI or PACBTI.
The changes are tested using "LLVM IR -> assembly" tests, jump tables
also have a MIR test. Unfortunately it is not possible add MIR tests
for exception handling and computed gotos because of MIR parser
limitations.
This patch is part of a series that adds support for the PACBTI-M extension of
the Armv8.1-M architecture, as detailed here:
https://community.arm.com/arm-community-blogs/b/architectures-and-processors-blog/posts/armv8-1-m-pointer-authentication-and-branch-target-identification-extension
The PACBTI-M specification can be found in the Armv8-M Architecture Reference
Manual:
https://developer.arm.com/documentation/ddi0553/latest
The following people contributed to this patch:
- Mikhail Maltsev
- Momchil Velikov
- Ties Stuij
Reviewed By: ostannard
Differential Revision: https://reviews.llvm.org/D112426
The compiler needs to mark register $x0 as live in for the following case.
$x1 = ADDXri $sp, 16, 0
BL @spam, csr_darwin_aarch64_aapcs, implicit-def dead $lr, implicit $sp, implicit $x0, implicit killed $x1, implicit-def $sp, implicit-def dead $x0
Reviewed By: paquette
Differential Revision: https://reviews.llvm.org/D95267
The debug location is removed from any outlined instruction. This
causes the MachineVerifier to crash on outlined DBG_VALUE
instructions.
Then, debug instructions are "invisible" to the outliner, that is, two
ranges of instructions from different functions are considered
identical if the only difference is debug instructions. Since a debug
instruction from one function is unlikely to provide sensible debug
information about all functions, sharing an outlined sequence, this
patch just removes debug instructions from the outlined functions.
Differential Revision: https://reviews.llvm.org/D89485
We only need to include MachineInstrBundle.h, but exposes an implicit dependency in MachineOutliner.h.
Also, remove duplicate includes from LiveRegUnits.cpp + MachineOutliner.cpp.
This prevents the outlined functions from pulling in a lot of unnecessary code
in our downstream libraries/linker. Which stops outlining making codesize
worse in c++ code with no-exceptions.
Differential Revision: https://reviews.llvm.org/D57254
This moves the SuffixTree test used in the Machine Outliner and moves it into Support for use in other outliners elsewhere in the compilation pipeline.
Differential Revision: https://reviews.llvm.org/D80586
Prior to D69446 I had done some NFC cleanup to make landing an iterative
outliner a cleaner more straight-forward patch. Since then, it seems that has
landed but I noticed some ways it could be cleaned up. Specifically:
1) doOutline was meant to be the re-runable function, but instead
runOnceOnModule was created that just calls doOutline.
2) In D69446 we discussed that the flag allowing the re-run of the
outliner should be a flag to tell how many additional times to run
the outliner again, not the total number of times. I don't think it
makes sense to introduce a flag, but print an error if the flag is
set to 0.
This is an NFCi, the i being that I get rid of the way that the
machine-outline-runs flag could be used to tell the outliner to not run
at all, and because I renamed the flag to '-machine-outliner-reruns'.
Differential Revision: https://reviews.llvm.org/D79070
Preserving liveness can be useful even late in the pipeline, if we're
doing substantial optimization work afterwards. (See, for example,
D76065.) Teach MachineOutliner how to correctly set live-ins on the
basic block in outlined functions.
Differential Revision: https://reviews.llvm.org/D78605
[MachineOutliner] fix test for excluding CFI and add test to include CFI in outlining
New test to check that we only outline CFI instruction if all CFI
Instructions in the function would be captured by the outlining
adding x86 tests analagous to AARCH64 cfi tests
Revision: https://reviews.llvm.org/D77852
Summary: The following change is to allow the machine outlining can be applied for Nth times, where N is specified by the compiler option. By default the value of N is 1. The motivation is that the repeated machine outlining can further reduce code size. Please refer to the presentation "Improving Swift Binary Size via Link Time Optimization" in LLVM Developers' Meeting in 2019.
Reviewers: aschwaighofer, tellenbach, paquette
Reviewed By: paquette
Subscribers: tellenbach, hiraditya, llvm-commits, jinlin
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D71027
Summary: The following change is to allow the machine outlining can be applied for Nth times, where N is specified by the compiler option. By default the value of N is 1. The motivation is that the repeated machine outlining can further reduce code size. Please refer to the presentation "Improving Swift Binary Size via Link Time Optimization" in LLVM Developers' Meeting in 2019.
Reviewers: aschwaighofer, tellenbach, paquette
Reviewed By: paquette
Subscribers: tellenbach, hiraditya, llvm-commits, jinlin
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D71027
Summary: The following change is to allow the machine outlining can be applied for Nth times, where N is specified by the compiler option. By default the value of N is 1. The motivation is that the repeated machine outlining can further reduce code size. Please refer to the presentation "Improving Swift Binary Size via Link Time Optimization" in LLVM Developers' Meeting in 2019.
Reviewers: aschwaighofer, tellenbach, paquette
Reviewed By: paquette
Subscribers: tellenbach, hiraditya, llvm-commits, jinlin
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D71027
This will address the issue: P8198 and P8199 (from D73534).
The methods was not handle bundles properly.
Differential Revision: https://reviews.llvm.org/D74904
Use the isCandidateForCallSiteEntry().
This should mostly be an NFC, but there are some parts ensuring
the moveCallSiteInfo() and copyCallSiteInfo() operate with call site
entry candidates (both Src and Dest should be the call site entry
candidates).
Differential Revision: https://reviews.llvm.org/D74122
This is how it should've been and brings it more in line with
std::string_view. There should be no functional change here.
This is mostly mechanical from a custom clang-tidy check, with a lot of
manual fixups. It uncovers a lot of minor inefficiencies.
This doesn't actually modify StringRef yet, I'll do that in a follow-up.
Conservatively always save + restore LR in noreturn functions.
These functions do not end in a RET, and so they aren't guaranteed to have an
instruction which uses LR in any way. So, as a result, you can end up in
unfortunate situations where you can't backtrace out of these functions in a
debugger.
Remove the old noreturn test, and add a new one which is more descriptive.
Remove the restriction that we can't outline from noreturn functions as well
since we now do the right thing.
Having this function be recursive could use up way too much stack space.
Rewrite it as an iterative traversal in the tree instead to prevent this.
Fixes PR44344.
