Permit the MIR inliner to inline diverging functions #106428
Merged
Conversation
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
rustbot
added
S-waiting-on-review
|
@bors try @rust-timer queue |
This comment has been minimized.
This comment has been minimized.
rustbot
added
the
S-waiting-on-perf
|
⌛ Trying commit e4ddc8a0f6cac4ea10db08d70f4d15cfd3a5ecaf with merge 696235b229d3d1166bd68a49240f1d7880b2ff2d... |
|
☀️ Try build successful - checks-actions |
This comment has been minimized.
This comment has been minimized.
|
Finished benchmarking commit (696235b229d3d1166bd68a49240f1d7880b2ff2d): comparison URL. Overall result: ❌✅ regressions and improvements - no action neededBenchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR may lead to changes in compiler perf. @bors rollup=never Instruction countThis is a highly reliable metric that was used to determine the overall result at the top of this comment.
Max RSS (memory usage)ResultsThis is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.
CyclesResultsThis is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.
|
rustbot
removed
the
S-waiting-on-perf
Which codegen test is this? |
|
This function in particular: rust/src/test/codegen/unchecked_shifts.rs Line 19 in 5c18bc6 |
Do you happen to have the error log (or new output) for that one?
Can at least answer that one: This is a |
|
Ah, found the file: unchecked_shifts.txt |
|
@nikic I think that I don't have a strong opinion on how the LLVM-level UB-ness for that is preserved, just that it's still there. If it ending up producing a But I'm slightly concerned that this means that everthing using Would it perhaps be worth a mir-transform test that EDIT: Oh, wait, out-of-bounds shift is Aside: reason number 2947643 that I want |
saethlin
force-pushed
the
inline-diverging-functions
branch
2 times, most recently
from
9fd6e31
to
0dd6e02
Compare
|
I've rebased this up since #106294 is now merged. By default, the |
|
I could have sworn we ensure that we emit one Turning off MIR inlining also causes the |
|
Don't block this PR on that one assume. Do whatever you need for that codegen test; I can always update the implementation of Just so long as you're convinced that this additional inlining doesn't regress people using |
cjgillot
reviewed
Jan 18, 2023
saethlin
force-pushed
the
inline-diverging-functions
branch
from
0dd6e02
to
f8d0b47
Compare
I looked for examples of other codegen in the standard library which depends on I'm going to try hunting for examples in the ecosystem. |
This comment has been minimized.
This comment has been minimized.
cjgillot
reviewed
Jan 21, 2023
| _2 = move _1; // scope 0 at $DIR/unwrap_unchecked.rs:+1:5: +1:8 | ||
| StorageLive(_3); // scope 0 at $DIR/unwrap_unchecked.rs:+1:9: +1:27 | ||
| _4 = discriminant(_2); // scope 1 at $SRC_DIR/core/src/option.rs:LL:COL | ||
| switchInt(move _4) -> [0: bb1, 1: bb3, otherwise: bb2]; // scope 1 at $SRC_DIR/core/src/option.rs:LL:COL |
There was a problem hiding this comment.
As all other targets are unreachable, we could InstCombine this pattern into:
_5 = Eq(_4, const 1_size);
Assume(_5);
goto -> bb3
Would this help LLVM ?
There was a problem hiding this comment.
My instinct is that we should keep the "otherwise unreachable" part so LLVM knows that _4 is definitely 1, and can optimize backwards using that.
Thus perhaps something like
Suggested change
| switchInt(move _4) -> [0: bb1, 1: bb3, otherwise: bb2]; // scope 1 at $SRC_DIR/core/src/option.rs:LL:COL | |
| switchInt(move _4) -> [1: bb3, otherwise: bb2]; // scope 1 at $SRC_DIR/core/src/option.rs:LL:COL |
might be best?
There was a problem hiding this comment.
I believe that transformation is accomplished by DeduplicateBlocks, which I noted back here: #106428 (comment) though I'll admit I was a bit vague.
There was a problem hiding this comment.
DeduplicateBlocks can be a heavy hammer, since it hashes basic blocks. Detecting unreachable blocks is trivial, since we only need to test the terminator. That's why I suggested modifying InstCombine. SimplifyCfg could do the trick too.
About the final solution: either the assume or the reduced SwitchInt, I don't mind, as long as LLVM understands.
saethlin
force-pushed
the
inline-diverging-functions
branch
from
f8d0b47
to
4b1a179
Compare
|
@bors try @rust-timer queue |
|
☀️ Test successful - checks-actions |
This was referenced Mar 26, 2023
|
Finished benchmarking commit (2420bd3): comparison URL. Overall result: ❌✅ regressions and improvements - no action needed@rustbot label: -perf-regression Instruction countThis is a highly reliable metric that was used to determine the overall result at the top of this comment.
Max RSS (memory usage)ResultsThis is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.
CyclesThis benchmark run did not return any relevant results for this metric. |
|
The job Click to see the possible cause of the failure (guessed by this bot) |
This was referenced Mar 26, 2023
bors
added a commit
to rust-lang-ci/rust
that referenced
this pull request
Apr 21, 2023
…illot Deduplicate unreachable blocks, for real this time In rust-lang#106428 (in particular rust-lang@41eda69) we noticed that inlining `unreachable_unchecked` can produce duplicate unreachable blocks. So we improved two MIR optimizations: `SimplifyCfg` was given a simplify to deduplicate unreachable blocks, then `InstCombine` was given a combiner to deduplicate switch targets that point at the same block. The problem is that change doesn't actually work. Our current pass order is ``` SimplifyCfg (does nothing relevant to this situation) Inline (produces multiple unreachable blocks) InstCombine (doesn't do anything here, oops) SimplifyCfg (produces the duplicate SwitchTargets that InstCombine is looking for) ``` So in here, I have factored out the specific function from `InstCombine` and placed it inside the simplify that produces the case it is looking for. This should ensure that it runs in the scenario it was designed for. Fixes rust-lang#110551 r? `@cjgillot`
RalfJung
pushed a commit
to RalfJung/rust-analyzer
that referenced
this pull request
Apr 20, 2024
Deduplicate unreachable blocks, for real this time In rust-lang/rust#106428 (in particular rust-lang/rust@41eda69) we noticed that inlining `unreachable_unchecked` can produce duplicate unreachable blocks. So we improved two MIR optimizations: `SimplifyCfg` was given a simplify to deduplicate unreachable blocks, then `InstCombine` was given a combiner to deduplicate switch targets that point at the same block. The problem is that change doesn't actually work. Our current pass order is ``` SimplifyCfg (does nothing relevant to this situation) Inline (produces multiple unreachable blocks) InstCombine (doesn't do anything here, oops) SimplifyCfg (produces the duplicate SwitchTargets that InstCombine is looking for) ``` So in here, I have factored out the specific function from `InstCombine` and placed it inside the simplify that produces the case it is looking for. This should ensure that it runs in the scenario it was designed for. Fixes rust-lang/rust#110551 r? `@cjgillot`
RalfJung
pushed a commit
to RalfJung/rust-analyzer
that referenced
this pull request
Apr 27, 2024
Deduplicate unreachable blocks, for real this time In rust-lang/rust#106428 (in particular rust-lang/rust@41eda69) we noticed that inlining `unreachable_unchecked` can produce duplicate unreachable blocks. So we improved two MIR optimizations: `SimplifyCfg` was given a simplify to deduplicate unreachable blocks, then `InstCombine` was given a combiner to deduplicate switch targets that point at the same block. The problem is that change doesn't actually work. Our current pass order is ``` SimplifyCfg (does nothing relevant to this situation) Inline (produces multiple unreachable blocks) InstCombine (doesn't do anything here, oops) SimplifyCfg (produces the duplicate SwitchTargets that InstCombine is looking for) ``` So in here, I have factored out the specific function from `InstCombine` and placed it inside the simplify that produces the case it is looking for. This should ensure that it runs in the scenario it was designed for. Fixes rust-lang/rust#110551 r? `@cjgillot`
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
Add this suggestion to a batch that can be applied as a single commit.
This suggestion is invalid because no changes were made to the code.
Suggestions cannot be applied while the pull request is closed.
Suggestions cannot be applied while viewing a subset of changes.
Only one suggestion per line can be applied in a batch.
Add this suggestion to a batch that can be applied as a single commit.
Applying suggestions on deleted lines is not supported.
You must change the existing code in this line in order to create a valid suggestion.
Outdated suggestions cannot be applied.
This suggestion has been applied or marked resolved.
Suggestions cannot be applied from pending reviews.
Suggestions cannot be applied on multi-line comments.
Suggestions cannot be applied while the pull request is queued to merge.
Suggestion cannot be applied right now. Please check back later.
This heuristic prevents inlining of
hint::unreachable_unchecked, which in turn makesOption/Result::unwrap_uncheckeda bad inlining candidate. I looked through the changes tocore,alloc,std, andhashbrownby hand and they all seem reasonable. Let's see how this looks in perf...Based on rustc-perf it looks like this regresses ctfe-stress, and the cachegrind diff indicates that this regression is in
InterpCx::statement. I don't know how to do any deeper analysis because that function is enormous in the try toolchain, which has no debuginfo in it. And a local build produces significantly different codegen for that function, even with LTO.