Starting with Skylake, the LBR contains the precise number of cycles between the two
consecutive branches.
Making use of this will hopefully make the measurements more precise than the
existing methods of using RDTSC.
Differential Revision: https://reviews.llvm.org/D77422
New change: check for existence of field `cycles` in perf_branch_entry before enabling this mode.
This should prevent compilation errors when building for older kernel whose headers don't support it.
From @erichkeane:
```
This patch doesn't seem to build for me:
/iusers/ekeane1/workspaces/llvm-project/llvm/tools/llvm-exegesis/lib/X86/X86Counter.cpp: In function ‘llvm::Error llvm::exegesis::parseDataBuffer(const char*, size_t, const void*, const void*, llvm::SmallVector<long int, 4>*)’:
/iusers/ekeane1/workspaces/llvm-project/llvm/tools/llvm-exegesis/lib/X86/X86Counter.cpp:99:37: error: ‘struct perf_branch_entry’ has no member named ‘cycles’
CycleArray->push_back(Entry.cycles);
I'm on RHEL7, so I have kernel 3.10, so it doesn't have 'cycles'.
According ot this: https://elixir.bootlin.com/linux/v4.3/source/include/uapi/linux/perf_event.h#L963 kernel 4.3 is the first time that 'cycles' appeared in this structure.
```
Starting with Skylake, the LBR contains the precise number of cycles between the two
consecutive branches.
Making use of this will hopefully make the measurements more precise than the
existing methods of using RDTSC.
Differential Revision: https://reviews.llvm.org/D77422
Summary:
As noted in documentation, different repetition modes have different trade-offs:
> .. option:: -repetition-mode=[duplicate|loop]
>
> Specify the repetition mode. `duplicate` will create a large, straight line
> basic block with `num-repetitions` copies of the snippet. `loop` will wrap
> the snippet in a loop which will be run `num-repetitions` times. The `loop`
> mode tends to better hide the effects of the CPU frontend on architectures
> that cache decoded instructions, but consumes a register for counting
> iterations.
Indeed. Example:
>>! In D74156#1873657, @lebedev.ri wrote:
> At least for `CMOV`, i'm seeing wildly different results
> | | Latency | RThroughput |
> | duplicate | 1 | 0.8 |
> | loop | 2 | 0.6 |
> where latency=1 seems correct, and i'd expect the througput to be close to 1/2 (since there are two execution units).
This isn't great for analysis, at least for schedule model development.
As discussed in excruciating detail in
>>! In D74156#1924514, @gchatelet wrote:
>>>! In D74156#1920632, @lebedev.ri wrote:
>> ... did that explanation of the question i'm having made any sense?
>
> Thx for digging in the conversation !
> Ok it makes more sense now.
>
> I discussed it a bit with @courbet:
> - We want the analysis tool to stay simple so we'd rather not make it knowledgeable of the repetition mode.
> - We'd like to still be able to select either repetition mode to dig into special cases
>
> So we could add a third `min` repetition mode that would run both and take the minimum. It could be the default option.
> Would you have some time to look what it would take to add this third mode?
there appears to be an agreement that it is indeed sub-par,
and that we should provide an optional, measurement (not analysis!) -time
way to rectify the situation.
However, the solutions isn't entirely straight-forward.
We can just add an actual 'multiplexer' `MinSnippetRepetitor`, because
if we just concatenate snippets produced by `DuplicateSnippetRepetitor`
and `LoopSnippetRepetitor` and run+measure that, the measurement will
naturally be different from what we'd get by running+measuring
them separately and taking the min.
([[ https://www.wolframalpha.com/input/?i=%28x%2By%29%2F2+%21%3D+min%28x%2C+y%29 | `time(D+L)/2 != min(time(D), time(L))` ]])
Also, it seems best to me to have a single snippet instead of generating
a snippet per repetition mode, since the only difference here is that the
loop repetition mode reserves one register for loop counter.
As far as i can tell, we can either teach `BenchmarkRunner::runConfiguration()`
to produce a single report given multiple repetitors (as in the patch),
or do that one layer higher - don't modify `BenchmarkRunner::runConfiguration()`,
produce multiple reports, don't actually print each one, but aggregate them somehow
and only print the final one.
Initially i've gone ahead with the latter approach, but it didn't look like a natural fit;
the former (as in the diff) does seem like a better fit to me.
There's also a question of the test coverage. It sure currently does work here:
```
$ ./bin/llvm-exegesis --opcode-name=CMOV64rr --mode=inverse_throughput --repetition-mode=duplicate
Check generated assembly with: /usr/bin/objdump -d /tmp/snippet-8fb949.o
---
mode: inverse_throughput
key:
instructions:
- 'CMOV64rr RAX RAX R11 i_0x0'
- 'CMOV64rr RBP RBP R15 i_0x0'
- 'CMOV64rr RBX RBX RBX i_0x0'
- 'CMOV64rr RCX RCX RBX i_0x0'
- 'CMOV64rr RDI RDI R10 i_0x0'
- 'CMOV64rr RDX RDX RAX i_0x0'
- 'CMOV64rr RSI RSI RAX i_0x0'
- 'CMOV64rr R8 R8 R8 i_0x0'
- 'CMOV64rr R9 R9 RDX i_0x0'
- 'CMOV64rr R10 R10 RBX i_0x0'
- 'CMOV64rr R11 R11 R14 i_0x0'
- 'CMOV64rr R12 R12 R9 i_0x0'
- 'CMOV64rr R13 R13 R12 i_0x0'
- 'CMOV64rr R14 R14 R15 i_0x0'
- 'CMOV64rr R15 R15 R13 i_0x0'
config: ''
register_initial_values:
- 'RAX=0x0'
- 'R11=0x0'
- 'EFLAGS=0x0'
- 'RBP=0x0'
- 'R15=0x0'
- 'RBX=0x0'
- 'RCX=0x0'
- 'RDI=0x0'
- 'R10=0x0'
- 'RDX=0x0'
- 'RSI=0x0'
- 'R8=0x0'
- 'R9=0x0'
- 'R14=0x0'
- 'R12=0x0'
- 'R13=0x0'
cpu_name: bdver2
llvm_triple: x86_64-unknown-linux-gnu
num_repetitions: 10000
measurements:
- { key: inverse_throughput, value: 0.819, per_snippet_value: 12.285 }
error: ''
info: instruction has tied variables, using static renaming.
assembled_snippet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
...
$ ./bin/llvm-exegesis --opcode-name=CMOV64rr --mode=inverse_throughput --repetition-mode=loop
Check generated assembly with: /usr/bin/objdump -d /tmp/snippet-051eb3.o
---
mode: inverse_throughput
key:
instructions:
- 'CMOV64rr RAX RAX R11 i_0x0'
- 'CMOV64rr RBP RBP RSI i_0x0'
- 'CMOV64rr RBX RBX R9 i_0x0'
- 'CMOV64rr RCX RCX RSI i_0x0'
- 'CMOV64rr RDI RDI RBP i_0x0'
- 'CMOV64rr RDX RDX R9 i_0x0'
- 'CMOV64rr RSI RSI RDI i_0x0'
- 'CMOV64rr R9 R9 R12 i_0x0'
- 'CMOV64rr R10 R10 R11 i_0x0'
- 'CMOV64rr R11 R11 R9 i_0x0'
- 'CMOV64rr R12 R12 RBP i_0x0'
- 'CMOV64rr R13 R13 RSI i_0x0'
- 'CMOV64rr R14 R14 R14 i_0x0'
- 'CMOV64rr R15 R15 R10 i_0x0'
config: ''
register_initial_values:
- 'RAX=0x0'
- 'R11=0x0'
- 'EFLAGS=0x0'
- 'RBP=0x0'
- 'RSI=0x0'
- 'RBX=0x0'
- 'R9=0x0'
- 'RCX=0x0'
- 'RDI=0x0'
- 'RDX=0x0'
- 'R12=0x0'
- 'R10=0x0'
- 'R13=0x0'
- 'R14=0x0'
- 'R15=0x0'
cpu_name: bdver2
llvm_triple: x86_64-unknown-linux-gnu
num_repetitions: 10000
measurements:
- { key: inverse_throughput, value: 0.6083, per_snippet_value: 8.5162 }
error: ''
info: instruction has tied variables, using static renaming.
assembled_snippet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
...
$ ./bin/llvm-exegesis --opcode-name=CMOV64rr --mode=inverse_throughput --repetition-mode=min
Check generated assembly with: /usr/bin/objdump -d /tmp/snippet-c7a47d.o
Check generated assembly with: /usr/bin/objdump -d /tmp/snippet-2581f1.o
---
mode: inverse_throughput
key:
instructions:
- 'CMOV64rr RAX RAX R11 i_0x0'
- 'CMOV64rr RBP RBP R10 i_0x0'
- 'CMOV64rr RBX RBX R10 i_0x0'
- 'CMOV64rr RCX RCX RDX i_0x0'
- 'CMOV64rr RDI RDI RAX i_0x0'
- 'CMOV64rr RDX RDX R9 i_0x0'
- 'CMOV64rr RSI RSI RAX i_0x0'
- 'CMOV64rr R9 R9 RBX i_0x0'
- 'CMOV64rr R10 R10 R12 i_0x0'
- 'CMOV64rr R11 R11 RDI i_0x0'
- 'CMOV64rr R12 R12 RDI i_0x0'
- 'CMOV64rr R13 R13 RDI i_0x0'
- 'CMOV64rr R14 R14 R9 i_0x0'
- 'CMOV64rr R15 R15 RBP i_0x0'
config: ''
register_initial_values:
- 'RAX=0x0'
- 'R11=0x0'
- 'EFLAGS=0x0'
- 'RBP=0x0'
- 'R10=0x0'
- 'RBX=0x0'
- 'RCX=0x0'
- 'RDX=0x0'
- 'RDI=0x0'
- 'R9=0x0'
- 'RSI=0x0'
- 'R12=0x0'
- 'R13=0x0'
- 'R14=0x0'
- 'R15=0x0'
cpu_name: bdver2
llvm_triple: x86_64-unknown-linux-gnu
num_repetitions: 10000
measurements:
- { key: inverse_throughput, value: 0.6073, per_snippet_value: 8.5022 }
error: ''
info: instruction has tied variables, using static renaming.
assembled_snippet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
...
```
but i open to suggestions as to how test that.
I also have gone with the suggestion to default to this new mode.
This was irking me for some time, so i'm happy to finally see progress here.
Looking forward to feedback.
Reviewers: courbet, gchatelet
Reviewed By: courbet, gchatelet
Subscribers: mstojanovic, RKSimon, llvm-commits, courbet, gchatelet
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D76921
Summary:
This adds a `-max-configs-per-opcode` option to limit the number of
configs per opcode.
Reviewers: gchatelet
Subscribers: tschuett, llvm-commits
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D68642
llvm-svn: 374054
Summary: Removed excess new lines from documentations. As far as I can tell, it seems as though restructured text is agnostic to new lines, the use of new lines was inconsistent and had no effect on how the files were being displayed.
Reviewers: jhenderson, rupprecht, JDevlieghere
Reviewed By: jhenderson
Subscribers: llvm-commits
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D63971
llvm-svn: 365105
Sphinx allows for definitions of command-line options using
`.. option <name>` and references to those options via `:option:<name>`.
However, it looks like there is no scoping of these options by default,
meaning that links can end up pointing to incorrect documents. See for
example the llvm-mca document, which contains references to -o that,
prior to this patch, pointed to a different document. What's worse is
that these links appear to be non-deterministic in which one is picked
(on my machine, some references end up pointing to opt, whereas on the
live docs, they point to llvm-dwarfdump, for example).
The fix is to add the .. program <name> tag. This essentially namespaces
the options (definitions and references) to the named program, ensuring
that the links are kept correct.
Reviwed by: andreadb
Differential Revision: https://reviews.llvm.org/D63873
llvm-svn: 364538
Summary:
This is an alternative to D59539.
Let's suppose we have measured 4 different opcodes, and got: `0.5`, `1.0`, `1.5`, `2.0`.
Let's suppose we are using `-analysis-clustering-epsilon=0.5`.
By default now we will start processing the `0.5` point, find that `1.0` is it's neighbor, add them to a new cluster.
Then we will notice that `1.5` is a neighbor of `1.0` and add it to that same cluster.
Then we will notice that `2.0` is a neighbor of `1.5` and add it to that same cluster.
So all these points ended up in the same cluster.
This may or may not be a correct implementation of dbscan clustering algorithm.
But this is rather horribly broken for the reasons of comparing the clusters with the LLVM sched data.
Let's suppose all those opcodes are currently in the same sched cluster.
If i specify `-analysis-inconsistency-epsilon=0.5`, then no matter
the LLVM values this cluster will **never** match the LLVM values,
and thus this cluster will **always** be displayed as inconsistent.
The solution is obviously to split off some of these opcodes into different sched cluster.
But how do i do that? Out of 4 opcodes displayed in the inconsistency report,
which ones are the "bad ones"? Which ones are the most different from the checked-in data?
I'd need to go in to the `.yaml` and look it up manually.
The trivial solution is to, when creating clusters, don't use the full dbscan algorithm,
but instead "pick some unclustered point, pick all unclustered points that are it's neighbor,
put them all into a new cluster, repeat". And just so as it happens, we can arrive
at that algorithm by not performing the "add neighbors of a neighbor to the cluster" step.
But that won't work well once we teach analyze mode to operate in on-1D mode
(i.e. on more than a single measurement type at a time), because the clustering would
depend on the order of the measurements.
Instead, let's just create a single cluster per opcode, and put all the points of that opcode into said cluster.
And simultaneously check that every point in that cluster is a neighbor of every other point in the cluster,
and if they are not, the cluster (==opcode) is unstable.
This is //yet another// step to bring me closer to being able to continue cleanup of bdver2 sched model..
Fixes [[ https://bugs.llvm.org/show_bug.cgi?id=40880 | PR40880 ]].
Reviewers: courbet, gchatelet
Reviewed By: courbet
Subscribers: tschuett, jdoerfert, RKSimon, llvm-commits
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D59820
llvm-svn: 357152
Summary:
Up until the point i have looked in the source, i didn't even understood that
i can disable 'cluster' output. I have always silenced it via ` &> /dev/null`.
(And hoped it wasn't contributing much of the run time.)
While i expect that it has it's use-cases i never once needed it so far.
If i forget to silence it, console is completely flooded with that output.
How about not expecting users to opt-out of analyses,
but to explicitly specify the analyses that should be performed?
Reviewers: courbet, gchatelet
Reviewed By: courbet
Subscribers: tschuett, RKSimon, llvm-commits
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D57648
llvm-svn: 353021
Summary:
The pfm counters are now in the ExegesisTarget rather than the
MCSchedModel (PR39165).
This also compresses the pfm counter tables (PR37068).
Reviewers: RKSimon, gchatelet
Subscribers: mgrang, llvm-commits
Differential Revision: https://reviews.llvm.org/D52932
llvm-svn: 345243
Summary:
We try to recover gracefully on instructions that would crash the
program.
This includes some refactoring of runMeasurement() implementations.
Reviewers: gchatelet
Subscribers: tschuett, llvm-commits
Differential Revision: https://reviews.llvm.org/D53371
llvm-svn: 344695
Summary:
This is a step towards fixing PR38048.
Note that right now the measurements are given per instruction. We'll
need to give measurements a per code snippet and update the analysis (PR38731).
Reviewers: gchatelet
Subscribers: tschuett, llvm-commits
Differential Revision: https://reviews.llvm.org/D52041
llvm-svn: 342947
Summary:
[llvm-exegesis][RFC] Automatic Measurement of Instruction Latency/Uops
This is the code corresponding to the RFC "llvm-exegesis Automatic Measurement of Instruction Latency/Uops".
The RFC is available on the LLVM mailing lists as well as the following document
for easier reading:
https://docs.google.com/document/d/1QidaJMJUyQdRrFKD66vE1_N55whe0coQ3h1GpFzz27M/edit?usp=sharing
Subscribers: mgorny, gchatelet, orwant, llvm-commits
Differential Revision: https://reviews.llvm.org/D44519
llvm-svn: 329156
Clement Courbet