For some transformations like hot-cold split or coro split, it can outline its part of function ranges. Since sample loader is the early stage of backend and no split happens at that time, compiler can't recognize those function, so in llvm-profgen we should attribute the sample to the original function. This is already done for the body range samples since we use the symbols from dwarf which is created before the split.
But for branch samples, the call from master function to its outlined function is actually not a call to the original function, we shouldn't add head/callsie samples for it. So instead of dwarf symbol, we use the symbols from symbol table and ignore those functions with special suffixes(like `.cold` ,`.resume`) for accumulating the callsite/head samples.
Reviewed By: hoy, wenlei
Differential Revision: https://reviews.llvm.org/D110864
This change adds duplication factor multiplier while accumulating body samples for line-number based profile. The body sample count will be `duplication-factor * count`. Base discriminator and duplication factor is decoded from the raw discriminator, this requires some refactor works.
Differential Revision: https://reviews.llvm.org/D109934
In order to be consistent with compiler that interprets zero count as unexecuted(cold), this change reports zero-value count for unexecuted part of function code. For the implementation, it leverages the range counter, initializes all the executed function range with the zero-value. After all ranges are merged and converted into disjoint ranges, the remaining zero count will indicates the unexecuted(cold) part of the function.
This change also extends the current `findDisjointRanges` method which now can support adding zero-value range.
Reviewed By: hoy, wenlei
Differential Revision: https://reviews.llvm.org/D109713
This patch introduces non-CS AutoFDO profile generation into LLVM. The profile is supposed to be well consumed by compiler using `-fprofile-sample-use=[profile]`.
After range and branch counters are extracted from the LBR sample, here we go through each addresses for symbolization, create FunctionSamples and populate its sub fields like TotalSamples, BodySamples and HeadSamples etc. For inlined code, as we need to map back to original code, so we always add body samples to the leaf frame's function sample.
Reviewed By: wenlei, hoy
Differential Revision: https://reviews.llvm.org/D109551
Similar to https://reviews.llvm.org/D109637, there is a whole invalid line of message in perfscript.
```
warning: Invalid address in LBR record at line 14118674: Processed 14138923 events and lost 1 chunks!
warning: Invalid address in LBR record at line 14118676: Check IO/CPU overload!
```
This only happened for LBR only perfscript, hybridperfscript have a check of " 0x" to make sure it's the LBR perf line.
Reviewed By: hoy, wenlei
Differential Revision: https://reviews.llvm.org/D110424
Without preinliner, we need to tune down the cold count cutoff to merge/trim more context to limit profile size for large components. However it doesn't make sense for cold threshold to be higher than hot threshold, so we now change to use hot threshold as merging/trimming cut off instead.
Differential Revision: https://reviews.llvm.org/D110212
For large app, dumping disasm of the whole program can be slow and result in gianant output. Adding a switch to dump specific symbols only.
Reviewed By: wlei
Differential Revision: https://reviews.llvm.org/D110079
Currenlty PseudoProbeInserter is a pass conditioned on a target switch. It works well with a single clang invocation. It doesn't work so well when the backend is called separately (i.e, through the linker or llc), where user has always to pass -pseudo-probe-for-profiling explictly. I'm making the pass a default pass that requires no command line arg to trigger, but will be actually run depending on whether the CU comes with `llvm.pseudo_probe_desc` metadata.
Reviewed By: wenlei
Differential Revision: https://reviews.llvm.org/D110209
Turn on `use-context-cost-for-preinliner` to use context-sensitive byte size cost for preinliner decisions by default.
This is a more accurate proxy of inline cost than profile size. We tested on our large workload that it delivers measureable CPU improvement.
Differential Revision: https://reviews.llvm.org/D109893
Invalid frame addresses exist in call stack samples due to bad unwinding. This could happen to frame-pointer-based unwinding and the callee functions that do not have the frame pointer chain set up. It isn't common when the program is built with the frame pointer omission disabled, but can still happen with third-party static libs built with frame pointer omitted.
Reviewed By: wenlei
Differential Revision: https://reviews.llvm.org/D109638
Adding the compiler support of MD5 CS profile based on pervious context split work D107299. A MD5 CS profile is about 40% smaller than the string-based extbinary profile. As a result, the compilation is 15% faster.
There are a few conversion from real names to md5 names that have been made on the sample loader and context tracker side to get it work.
Reviewed By: wenlei, wmi
Differential Revision: https://reviews.llvm.org/D108342
This change aims at supporting LBR only sample perf script which is used for regular(Non-CS) profile generation. A LBR perf script includes a batch of LBR sample which starts with a frame pointer and a group of 32 LBR entries is followed. The FROM/TO LBR pair and the range between two consecutive entries (the former entry's TO and the latter entry's FROM) will be used to infer function profile info.
An example of LBR perf script(created by `perf script -F ip,brstack -i perf.data`)
```
40062f 0x40062f/0x4005b0/P/-/-/9 0x400645/0x4005ff/P/-/-/1 0x400637/0x400645/P/-/-/1 ...
4005d7 0x4005d7/0x4005e5/P/-/-/8 0x40062f/0x4005b0/P/-/-/6 0x400645/0x4005ff/P/-/-/1 ...
...
```
For implementation:
- Extended a new child class `LBRPerfReader` for the sample parsing, reused all the functionalities in `extractLBRStack` except for an extension to parsing leading instruction pointer.
- `HybridSample` is reused(just leave the call stack empty) and the parsed samples is still aggregated in `AggregatedSamples`. After that, range samples, branch sample, address samples are computed and recorded.
- Reused `ContextSampleCounterMap` to store the raw profile, since it's no need to aggregation by context, here it just registered one sample counter with a fake context key.
- Unified to use `show-raw-profile` instead of `show-unwinder-output` to dump the intermediate raw profile, see the comments of the format of the raw profile. For CS profile, it remains to output the unwinder output.
Profile generation part will come soon.
Differential Revision: https://reviews.llvm.org/D108153
Currently context strings contain a lot of duplicated function names and that significantly increase the profile size. This change split the context into a series of {name, offset, discriminator} tuples so function names used in the context can be replaced by the index into the name table and that significantly reduce the size consumed by context.
A follow-up improvement made in the compiler and profiling tools is to avoid reconstructing full context strings which is time- and memory- consuming. Instead a context vector of `StringRef` is adopted to represent the full context in all scenarios. As a result, the previous prevalent profile map which was implemented as a `StringRef` is now engineered as an unordered map keyed by `SampleContext`. `SampleContext` is reshaped to using an `ArrayRef` to represent a full context for CS profile. For non-CS profile, it falls back to use `StringRef` to represent a contextless function name. Both the `ArrayRef` and `StringRef` objects are underpinned by real array and string objects that are stored in producer buffers. For compiler, they are maintained by the sample reader. For llvm-profgen, they are maintained in `ProfiledBinary` and `ProfileGenerator`. Full context strings can be generated only in those cases of debugging and printing.
When it comes to profile format, nothing has changed to the text format, though internally CS context is implemented as a vector. Extbinary format is only changed for CS profile, with an additional `SecCSNameTable` section which stores all full contexts logically in the form of `vector<int>`, which each element as an offset points to `SecNameTable`. All occurrences of contexts elsewhere are redirected to using the offset of `SecCSNameTable`.
Testing
This is no-diff change in terms of code quality and profile content (for text profile).
For our internal large service (aka ads), the profile generation is cut to half, with a 20x smaller string-based extbinary format generated.
The compile time of ads is dropped by 25%.
Differential Revision: https://reviews.llvm.org/D107299
This is a follow up diff for BinarySizeContextTracker to track zero size for fully optimized inlinee. When an inlinee is fully optimized away, we won't be able to get its size through symbolizing instructions, hence we will treat the corresponding context size as unknown. However by traversing the inlined probe forest, we know what're original inlinees regardless of optimization. If a context show up in inlined probes, but not during symbolization, we know that it's fully optimized away hence its size is zero instead of unknown. It should provide more accurate size cost estimation for pre-inliner to make better inline decisions in llvm-profgen.
Differential Revision: https://reviews.llvm.org/D108350
This change enables llvm-profgen to use accurate context-sensitive post-optimization function byte size as a cost proxy to drive global preinline decisions.
To do this, BinarySizeContextTracker is introduced to track function byte size under different inline context during disassembling. In preinliner, we can not query context byte size under switch `context-cost-for-preinliner`. The tracker uses a reverse trie to keep size of functions under different context (callee as parent, caller as child), and it can give best/longest possible matching context size for given input context.
The new size cost is off by default. There're a few TODOs that needs to addressed: 1) avoid dangling string from `Offset2LocStackMap`, which will be addressed in split context work; 2) using inlinee's entry probe to make sure we have correct zero size for inlinee that's completely optimized away after inlining. Some tuning is also needed.
Differential Revision: https://reviews.llvm.org/D108180
Currently we use a centralized string map(StringMap<FunctionSamples> ProfileMap) to store the profile while populating the sample, which might cause the memory usage bottleneck. I saw in an extreme case, there are thousands of samples whose context stack depth is >= 100. The memory consumption can be greater than 100GB.
As here the context is used for inlining, we can assume we won't have so many of inlinees keeping inlined at the same root function, so this change tried to cap the context stack and merge the samples for peak memory reduction and this is done after recursion compression.
The default value is -1 meaning no depth limit, in the future we can tune to a smaller one.
Reviewed By: hoy, wenlei
Differential Revision: https://reviews.llvm.org/D107800
This change tried to integrate a new count based aggregated type of perf script. The only difference of the format is that an aggregated count is added at the head of the original sample which means the same samples are repeated to the given count times. This is used to reduce the perf script size.
e.g.
```
2
4005dc
400634
400684
7f68c5788793
0x4005c8/0x4005dc/P/-/-/0 ....
```
Implemented by a dedicated PerfReader `AggregatedHybridPerfReader`.
Differential Revision: https://reviews.llvm.org/D107192
This change supports to run without parsing MMap binary loading events instead it always assumes binary is loaded at the preferred address. This is used when we have assured no binary load address changes or we have pre-processed the addresses resolution. Warn if there's interior mmap event but without leading mmap events.
Reviewed By: hoy
Differential Revision: https://reviews.llvm.org/D107097
The linker or post-link optimizer can create an ELF image with multiple executable segments each of which will be loaded separately at run time. This breaks the assumption of llvm-profgen that currently only supports one base load address. What it ends up with is that the subsequent mmap events will be treated as an overwrite of the first mmap event which will in turn screw up address mapping. While it is non-trivial to support multiple separate load addresses and given that on x64 those segments will always be loaded at consecutive addresses (though via separate mmap
sys calls), I'm adding an error checking logic to bail out if that's violated and keep using a single load address which is the address of the first executable segment.
Also changing the disassembly output from printing section offset to printing the virtual address instead, which matches the behavior of objdump.
Differential Revision: https://reviews.llvm.org/D103178
As a follow-up to https://reviews.llvm.org/D104129, I'm cleaning up the danling probe related code in both the compiler and llvm-profgen.
I'm seeing a 5% size win for the pseudo_probe section for SPEC2017 and 10% for Ciner. Certain benchmark such as 602.gcc has a 20% size win. No obvious difference seen on build time for SPEC2017 and Cinder.
Reviewed By: wenlei
Differential Revision: https://reviews.llvm.org/D104477
If we have seen an inwards transition from external code to internal code, but not a following outwards transition, the inwards transition is likely due to interrupt which is usually unpaired. Ignore current and subsequent entries since they are likely from an unrelated pre-interrupt context.
LBR records from different interrupt context are unrelated and they should not be mixed together. Currenlty the OS does this for task-scheduling interrupt but not for all interrupts.
Reviewed By: wenlei, wlei
Differential Revision: https://reviews.llvm.org/D104276
Previously dangling samples were represented by INT64_MAX in sample profile while probes never executed were not reported. This was based on an observation that dangling probes were only at a smaller portion than zero-count probes. However, with compiler optimizations, dangling probes end up becoming at large portion of all probes in general and reporting them does not make sense from profile size point of view. This change flips sample reporting by reporting zero-count probes instead. This enabled dangling probe to be represented by none (missing entry in profile). This has a couple benefits:
1. Reducing sample profile size in optimize mode, even when the number of non-executed probes outperform the number of dangling probes, since INT64_MAX takes more space over 0 to encode.
2. Binary size savings. No need to encode dangling probe anymore, since missing probes are treated as dangling in the profile reader.
3. Reducing compiler work to track dangling probes. However, for probes that are real dead and removed, we still need the compiler to identify them so that they can be reported as zero-count, instead of mistreated as dangling probes.
4. Improving counts quality by respecting the counts already collected on the non-dangling copy of a probe. A probe, when duplicated, gets two copies at runtime. If one of them is dangling while the other is not, merging the two probes at profile generation time will cause the real samples collected on the non-dangling one to be discarded. Not reporting the dangling counterpart will keep the real samples.
5. Better readability.
6. Be consistent with non-CS dwarf line number based profile. Zero counts are trusted by the compiler counts inferencer while missing counts will be inferred by the compiler.
Note that the current patch does include any work for #3. There will be follow-up changes.
For #1, I've seen for a large Facebook service, the text profile is reduced by 7%. For extbinary profile, the size of LBRProfileSection is reduced by 35%.
For #4, I have seen general counts quality for SPEC2017 is improved by 10%.
Reviewed By: wenlei, wlei, wmi
Differential Revision: https://reviews.llvm.org/D104129
This change provides the option to merge and aggregate cold context by the last k frames instead of context-less name. By default K = 1 means the context-less one.
This is for better perf tuning. The more selective merging and trimming will rely on llvm-profgen's preinliner.
Reviewed By: wenlei, hoy
Differential Revision: https://reviews.llvm.org/D104131
Make extended binary the default output format for CSSPGO. This avoids having to pass flag every time when generating profile. It also matches llvm-profdata where binary profile is the default (should we switch to extbinary as default for llvm-profdata?).
We plan to compress name table for context profile, which depends on the built-in compression of extbinary.
Differential Revision: https://reviews.llvm.org/D103650
llvm-profgen uses profile summary based cold threshold to merge and trim cold context profile. This is to strike a good balance between profile size and performance.
We've been using 99.9% as the cutoff to save profile size without affecting performance. This change switch to use 99.9% instead of 99.9999% as default cold threshold cutoff for llvm-profgen.
Redundant switch csprof-cold-thres is also removed and tests cleaned up.
Differential Revision: https://reviews.llvm.org/D103071
Fixing an issue where samples collected for an untrackable frame is not reported. An untrackable frame refers to a frame whose caller is untrackable due to missing debug info or pseudo probe. Though the frame is connected to its parent frame through the frame pointer chain at runtime, the compiler cannot build the connection without debug info or pseudo probe. In such case we just need to report the untrackable frame as the base frame and all of its child frames.
With more samples reported I'm seeing this improves the performance of an internal benchmark by 2.5%.
Reviewed By: wenlei, wlei
Differential Revision: https://reviews.llvm.org/D102961
The change adds support for triming and merging cold context when mergine CSSPGO profiles using llvm-profdata. This is similar to the context profile trimming in llvm-profgen, however the flexibility to trim cold context after profile is generated can be useful.
Differential Revision: https://reviews.llvm.org/D100528
Report dangling probes for frames that have real samples collected. Dangling probes are the probes associated to an empty block. When reported, sample count on a dangling probe will not be trusted by the compiler and we will rely on the counts inference algorithm to get the probe a reasonable count. This actually fixes a bug where previously only those dangling probes with samples collected were reported.
This patch also fixes two existing issues. Pseudo probes are stored in `Address2ProbesMap` and their pointers are used in `PseudoProbeInlineTree`. Previously `std::vector` was used to store probes and the pointers to probes may get obsolete as the vector grows. I'm changing `std::vector` to `std::list` instead.
The other issue is that all outlined functions shared the same inline frame previously due to the unchanged `Index` value as the dummy inlineSite identifier.
Good results seen for SPEC2017 in general regarding profile quality.
Reviewed By: wenlei, wlei
Differential Revision: https://reviews.llvm.org/D100235
This patch fixed the following issues along side with some refactoring:
1. Fix bugs where StringRef for context string out live the underlying std::string. We now keep string table in profile generator to hold std::strings. We also do the same for bracketed context strings in profile writer.
2. Make sure profile output strictly follow (total sample, name) order. Previously, there's inconsistency between ProfileMap's key and FunctionSamples's name, leading to inconsistent ordering. This is now fixed by introducing context profile canonicalization. Assertions are also added to make sure ProfileMap's key and FunctionSamples's name are always consistent.
3. Enhanced error handling for profile writing to make sure we bubble up errors properly for both llvm-profgen and llvm-profdata when string table is not populated correctly for extended binary profile.
4. Keep all internal context representation bracket free. This avoids creating new strings for context trimming, merging and preinline. getNameWithContext API is now simplied accordingly.
5. Factor out the code for context trimming and merging into SampleContextTrimmer in SampleProf.cpp. This enables llvm-profdata to use the trimmer when merging profiles. Changes in llvm-profgen will be in separate patch.
Differential Revision: https://reviews.llvm.org/D100090
This change sets up a framework in llvm-profgen to estimate inline decision and adjust context-sensitive profile based on that. We call it a global pre-inliner in llvm-profgen.
It will serve two purposes:
1) Since context profile for not inlined context will be merged into base profile, if we estimate a context will not be inlined, we can merge the context profile in the output to save profile size.
2) For thinLTO, when a context involving functions from different modules is not inined, we can't merge functions profiles across modules, leading to suboptimal post-inline count quality. By estimating some inline decisions, we would be able to adjust/merge context profiles beforehand as a mitigation.
Compiler inline heuristic uses inline cost which is not available in llvm-profgen. But since inline cost is closely related to size, we could get an estimate through function size from debug info. Because the size we have in llvm-profgen is the final size, it could also be more accurate than the inline cost estimation in the compiler.
This change only has the framework, with a few TODOs left for follow up patches for a complete implementation:
1) We need to retrieve size for funciton//inlinee from debug info for inlining estimation. Currently we use number of samples in a profile as place holder for size estimation.
2) Currently the thresholds are using the values used by sample loader inliner. But they need to be tuned since the size here is fully optimized machine code size, instead of inline cost based on not yet fully optimized IR.
Differential Revision: https://reviews.llvm.org/D99146
Switch to use cold threshold from profile summary for cold context merging and trimming, instead of relying on hard coded values. Minor refactoring included for switch names, etc.
Differential Revision: https://reviews.llvm.org/D98921
This changes adds attribute field for metadata of context profile. Currently we have an inline attribute that indicates whether the leaf frame corresponding to a context profile was inlined in previous build.
This will be used to help estimating inlining and be taken into account when trimming context. Changes for that in llvm-profgen will follow. It will also help tuning.
Differential Revision: https://reviews.llvm.org/D98823
Previously we didn't support to keep the unique linkage name(-funique-internal-linkage-name) in llvm-profgen. As discussed in https://reviews.llvm.org/D96932, we choose to do canonicalization for it.
Now since "selected" is set as the default parameter of getCanonicalFnName in `D96932`, we don't need to add any attribute here for the previous usage and only fix the missing usage in the pseudo probe decoding.
Differential Revision: https://reviews.llvm.org/D98226
It appears some instructions doesn't have the debug location info and the symbolizer will return an empty call stack for them which will cause some crash later in profile unwinding. Actually we do not record the sample info for them, so this change just filter out those instruction.
As those instruction would appears at the begin and end of the instruction list, without them we need to add the boundary check for IP `advance` and `backward`.
Also for pseudo probe based profile, we actually don't need the symbolized location info, so here just change to use an empty stack for it. This could save half of the binary loading time.
Differential Revision: https://reviews.llvm.org/D96434
This include some changes related with PerfReader's the input check and command line change:
1) It appears there might be thousands of leading MMAP-Event line in the perfscript for large workload. For this case, the 4k threshold is not eligible to determine it's a hybrid sample. This change renovated the `isHybridPerfScript` by going through the script without threshold limitation checking whether there is a non-empty call stack immediately followed by a LBR sample. It will stop once it find a valid one.
2) Added several input validations for the command line switches in PerfReader.
3) Changed the command line `show-disassembly` to `show-disassembly-only`, it will print to stdout and exit early which leave an empty output profile.
Reviewed By: hoy, wenlei
Differential Revision: https://reviews.llvm.org/D96387
To align with https://reviews.llvm.org/D95547, we need to add brackets for context id before initializing the `SampleContext`.
Also added test cases for extended binary format from llvm-profgen side.
Differential Revision: https://reviews.llvm.org/D95929
when we skip the call stack starting with an external address, we should also skip the bottom LBR entry, otherwise it will cause a truncated context issue.
Reviewed By: hoy, wenlei
Differential Revision: https://reviews.llvm.org/D95480
This change allows merging and trimming cold context profile in llvm-profgen to solve profile size bloat problem. Currently when the profile's total sample is below threshold(supported by a switch), it will be considered cold and merged into a base context-less profile, which will at least keep the profile quality as good as the baseline(non-cs).
For example, two input profiles:
[main @ foo @ bar]:60
[main @ bar]:50
Under threshold = 100, the two profiles will be merge into one with the base context, get result:
[bar]:110
Added two switches:
`--csprof-cold-thres=<value>`: Specified the total samples threshold for a context profile to be considered cold, with 100 being the default. Any cold context profiles will be merged into context-less base profile by default.
`--csprof-keep-cold`: Force profile generation to keep cold context profiles instead of dropping them. By default, any cold context will not be written to output profile.
Results:
Though not yet evaluating it with the latest CSSPGO, our internal branch shows neutral on performance but significantly reduce the profile size. Detailed evaluation on llvm-profgen with CSSPGO will come later.
Differential Revision: https://reviews.llvm.org/D94111
This change compresses the context string by removing cycles due to recursive function for CS profile generation. Removing recursion cycles is a way to normalize the calling context which will be better for the sample aggregation and also make the context promoting deterministic.
Specifically for implementation, we recognize adjacent repeated frames as cycles and deduplicated them through multiple round of iteration.
For example:
Considering a input context string stack:
[“a”, “a”, “b”, “c”, “a”, “b”, “c”, “b”, “c”, “d”]
For first iteration,, it removed all adjacent repeated frames of size 1:
[“a”, “b”, “c”, “a”, “b”, “c”, “b”, “c”, “d”]
For second iteration, it removed all adjacent repeated frames of size 2:
[“a”, “b”, “c”, “a”, “b”, “c”, “d”]
So in the end, we get compressed output:
[“a”, “b”, “c”, “d”]
Compression will be called in two place: one for sample's context key right after unwinding, one is for the eventual context string id in the ProfileGenerator.
Added a switch `compress-recursion` to control the size of duplicated frames, default -1 means no size limit.
Added unit tests and regression test for this.
Differential Revision: https://reviews.llvm.org/D93556
This change compresses the context string by removing cycles due to recursive function for CS profile generation. Removing recursion cycles is a way to normalize the calling context which will be better for the sample aggregation and also make the context promoting deterministic.
Specifically for implementation, we recognize adjacent repeated frames as cycles and deduplicated them through multiple round of iteration.
For example:
Considering a input context string stack:
[“a”, “a”, “b”, “c”, “a”, “b”, “c”, “b”, “c”, “d”]
For first iteration,, it removed all adjacent repeated frames of size 1:
[“a”, “b”, “c”, “a”, “b”, “c”, “b”, “c”, “d”]
For second iteration, it removed all adjacent repeated frames of size 2:
[“a”, “b”, “c”, “a”, “b”, “c”, “d”]
So in the end, we get compressed output:
[“a”, “b”, “c”, “d”]
Compression will be called in two place: one for sample's context key right after unwinding, one is for the eventual context string id in the ProfileGenerator.
Added a switch `compress-recursion` to control the size of duplicated frames, default -1 means no size limit.
Added unit tests and regression test for this.
Differential Revision: https://reviews.llvm.org/D93556
This change implements profile generation infra for pseudo probe in llvm-profgen. During virtual unwinding, the raw profile is extracted into range counter and branch counter and aggregated to sample counter map indexed by the call stack context. This change introduces the last step and produces the eventual profile. Specifically, the body of function sample is recorded by going through each probe among the range and callsite target sample is recorded by extracting the callsite probe from branch's source.
Please refer https://groups.google.com/g/llvm-dev/c/1p1rdYbL93s and https://reviews.llvm.org/D89707 for more context about CSSPGO and llvm-profgen.
**Implementation**
- Extended `PseudoProbeProfileGenerator` for pseudo probe based profile generation.
- `populateBodySamplesWithProbes` reading range counter is responsible for recording function body samples and inferring caller's body samples.
- `populateBoundarySamplesWithProbes` reading branch counter is responsible for recording call site target samples.
- Each sample is recorded with its calling context(named `ContextId`). Remind that the probe based context key doesn't include the leaf frame probe info, so the `ContextId` string is created from two part: one from the probe stack strings' concatenation and other one from the leaf frame probe.
- Added regression test
Test Plan:
ninja & ninja check-llvm
Differential Revision: https://reviews.llvm.org/D92998
wlei