LLVM Project Blog

LLVM Project News and Details from the Trenches

Wednesday, September 4, 2019

Announcing the program for the 2019 LLVM Developers' Meeting - Bay Area

Announcing the program for the 2019 LLVM Developers' Meeting in San Jose, CA! This program is the largest we have ever had and has over 11 tutorials, 29 technical talks, 24 lightning talks, 2 panels, 3 birds of a feather, 14 posters, and 4 SRC talks. Be sure to register to attend this event and hear some of these great talks.

Technical Talks
Student Research Competition
Birds of a Feather
Lightning Talks

Thursday, August 1, 2019

The LLVM Project is Moving to GitHub

The LLVM Project is Moving to GitHub

After several years of discussion and planning, the LLVM project is getting ready to complete the migration of its source code from SVN to GitHub!  At last year’s developer meeting, many interested community members convened at a series of round tables to lay out a plan to completely migrate LLVM source code from SVN to GitHub by the 2019 U.S. Developer’s Meeting.  We have made great progress over the last nine months and are on track to complete the migration on October 21, 2019.

As part of the migration to GitHub we are maintaining the ‘monorepo’ layout which currently exists in SVN.  This means that there will be a single git repository with one top-level directory for each LLVM sub-project.  This will be a change for those of you who are already using git and accessing the code via the official sub-project git mirrors (e.g. https://git.llvm.org/git/llvm.git) where each sub-project has its own repository.

One of the first questions people ask when they hear about the GitHub plans is: Will the project start using GitHub pull requests and issues?  And the answer to that for now is: no. The current transition plan focuses on migrating only the source code. We will continue to use Phabricator for code reviews, and bugzilla for issue tracking after the migration is complete.  We have not ruled out using pull requests and issues at some point in the future, but these are discussions we still need to have as a community.

The most important takeaway from this post, though, is that if you consume the LLVM source code in any way, you need to take action now to migrate your workflows.  If you manage any continuous integration or other systems that need read-only access to the LLVM source code, you should begin pulling from the official GitHub repository instead of SVN or the current sub-project mirrors.  If you are a developer that needs to commit code, please use the git-llvm script for committing changes.

We have created a status page, if you want to track the current progress of the migration.  We will be posting updates to this page as we get closer to the completion date.  If you run into issues of any kind with GitHub you can file a bug in bugzilla and mark it as a blocker of the github tracking bug.

This entire process has been a large community effort.  Many many people have put in time discussing, planning, and implementing all the steps required to make this happen.  Thank you to everyone who has been involved and let’s keep working to make this migration a success.

Blog post by Tom Stellard.

Friday, May 24, 2019

LLVM and Google Season of Docs

The LLVM Project is pleased to announce that we have been selected to participate in Google’s Season of Docs!

Our project idea list may be found here:

From now until May 29th, technical writers are encouraged to review the proposed project ideas and to ask any questions you have on our gsdocs@llvm.org mailing list. Other documentation ideas are allowed, but we can not guarantee that a mentor will be found for the project. You are encouraged to discuss new ideas on the mailing list prior to submitting your technical writer application, in order to start the process of finding a mentor.

When submitting your application for an LLVM documentation project, please consider the following:

  • Include Prior Experience: Do you have prior technical writing experience? We want to see this! Considering including links to prior documentation or attachments of documentation you have written. If you can’t include a link to the actual documentation, please describe in detail what you wrote, who the audience was, and any other important information that can help us gauge your prior experience. Please also include any experience with Sphinx or other documentation generation tools.
  • Take your time writing the proposal: We will be looking closely at your application to see how well it is written. Take the time to proofread and know who your audience is.
  • Propose your plan for our documentation project: We have given a rough idea of what changes or topics we envision for the documentation, but this is just a start. We expect you to take the idea and expand or modify it as you see fit. Review our existing documentation and see how it would compliment or replace other pieces. Optionally include an overview or document design or layout plan in your application.
  • Become familiar with our project: We don’t expect you to become a compiler expert, but we do expect you read up on our project to learn a bit about LLVM.

We look forward to working with some fabulous technical writers and improving our documentation. Again, please email gsdocs@llvm.org with your questions.

Friday, March 15, 2019

LLVM Numerics Blog

Keywords: Numerics, Clang, LLVM-IR, : 2019 LLVM Developers' Meeting, LLVMDevMtg.

The goal of this blog post is to start a discussion about numerics in LLVM – where we are, recent work and things that remain to be done.  There will be an informal discussion on numerics at the 2019 EuroLLVM conference next month. One purpose of this blog post is to refresh everyone's memory on where we are on the topic of numerics to restart the discussion.

In the last year or two there has been a push to allow fine-grained decisions on which optimizations are legitimate for any given piece of IR.  In earlier days there were two main modes of operation: fast-math and precise-math.  When operating under the rules of precise-math, defined by IEEE-754, a significant number of potential optimizations on sequences of arithmetic instructions are not allowed because they could lead to violations of the standard.  

For example: 

The Reassociation optimization pass is generally not allowed under precise code generation as it can change the order of operations altering the creation of NaN and Inf values propagated at the expression level as well as altering precision.  

Precise code generation is often overly restrictive, so an alternative fast-math mode is commonly used where all possible optimizations are allowed, acknowledging that this impacts the precision of results and possibly IEEE compliant behavior as well.  In LLVM, this can be enabled by setting the unsafe-math flag at the module level, or passing the -funsafe-math-optimizations to clang which then sets flags on the IR it generates.  Within this context the compiler often generates shorter sequences of instructions to compute results, and depending on the context this may be acceptable.  Fast-math is often used in computations where loss of precision is acceptable.  For example when computing the color of a pixel, even relatively low precision is likely to far exceed the perception abilities of the eye, making shorter instruction sequences an attractive trade-off.  In long-running simulations of physical events however loss of precision can mean that the simulation drifts from reality making the trade-off unacceptable.

Several years ago LLVM IR instructions gained the ability of being annotated with flags that can drive optimizations with more granularity than an all-or-nothing decision at the module level.  The IR flags in question are: 

nnan, ninf, nsz, arcp, contract, afn, reassoc, nsw, nuw, exact.  

Their exact meaning is described in the LLVM Language Reference Manual.   When all the flags are are enabled, we get the current fast-math behavior.  When these flags are disabled, we get precise math behavior.  There are also several options available between these two models that may be attractive to some applications.  In the past year several members of the LLVM community worked on making IR optimizations passes aware of these flags.  When the unsafe-math module flag is not set these optimization passes will work by examining individual flags, allowing fine-grained selection of the optimizations that can be enabled on specific instruction sequences.  This allows vendors/implementors to mix fast and precise computations in the same module, aggressively optimizing some instruction sequences but not others.

We now have good coverage of IR passes in the LLVM codebase, in particular in the following areas:
* Intrinsic and libcall management
* Instruction Combining and Simplification
* Instruction definition
* SDNode definition
* GlobalIsel Combining and code generation
* Selection DAG code generation
* DAG Combining
* Machine Instruction definition
* IR Builders (SDNode, Instruction, MachineInstr)
* CSE tracking
* Reassociation
* Bitcode

There are still some areas that need to be reworked for modularity, including vendor specific back-end passes.  

The following are some of the contributions mentioned above from the last 2 years of open source development:

https://reviews.llvm.org/D45781 : MachineInst support mapping SDNode fast math flags for support in Back End code generation 
https://reviews.llvm.org/D46322 : [SelectionDAG] propagate 'afn' and 'reassoc' from IR fast-math-flags
https://reviews.llvm.org/D45710 : Fast Math Flag mapping into SDNode
https://reviews.llvm.org/D46854 : [DAG] propagate FMF for all FPMathOperators
https://reviews.llvm.org/D48180 : updating isNegatibleForFree and GetNegatedExpression with fmf for fadd
https://reviews.llvm.org/D48057: easing the constraint for isNegatibleForFree and GetNegatedExpression
https://reviews.llvm.org/D47954 : Utilize new SDNode flag functionality to expand current support for fdiv
https://reviews.llvm.org/D47918 : Utilize new SDNode flag functionality to expand current support for fma
https://reviews.llvm.org/D47909 : Utilize new SDNode flag functionality to expand current support for fadd
https://reviews.llvm.org/D47910 : Utilize new SDNode flag functionality to expand current support for fsub
https://reviews.llvm.org/D47911 : Utilize new SDNode flag functionality to expand current support for fmul
https://reviews.llvm.org/D48289 : refactor of visitFADD for AllowNewConst cases
https://reviews.llvm.org/D47388 : propagate fast math flags via IR on fma and sub expressions
https://reviews.llvm.org/D47389 : guard fneg with fmf sub flags
https://reviews.llvm.org/D47026 : fold FP binops with undef operands to NaN
https://reviews.llvm.org/D47749 : guard fsqrt with fmf sub flags
https://reviews.llvm.org/D46447 : Mapping SDNode flags to MachineInstr flags
https://reviews.llvm.org/D50195 : extend folding fsub/fadd to fneg for FMF
https://reviews.llvm.org/rL339197 : [NFC] adding tests for Y - (X + Y) --> -X
https://reviews.llvm.org/D50417 : [InstCombine] fold fneg into constant operand of fmul/fdiv
https://reviews.llvm.org/rL339357 : extend folding fsub/fadd to fneg for FMF
https://reviews.llvm.org/D50996 : extend binop folds for selects to include true and false binops flag intersection
https://reviews.llvm.org/rL339938 : add a missed case for binary op FMF propagation under select folds
https://reviews.llvm.org/D51145 : Guard FMF context by excluding some FP operators from FPMathOperator
https://reviews.llvm.org/rL341138 : adding initial intersect test for Node to Instruction association
https://reviews.llvm.org/rL341565 : in preparation for adding nsw, nuw and exact as flags to MI
https://reviews.llvm.org/D51738 : add IR flags to MI
https://reviews.llvm.org/D52006 : Copy utilities updated and added for MI flags
https://reviews.llvm.org/rL342598 : add new flags to a DebugInfo lit test
https://reviews.llvm.org/D53874 : [InstSimplify] fold 'fcmp nnan oge X, 0.0' when X is not negative
https://reviews.llvm.org/D55668 : Add FMF management to common fp intrinsics in GlobalIsel
https://reviews.llvm.org/rL352396 : [NFC] TLI query with default(on) behavior wrt DAG combines for fmin/fmax target…
https://reviews.llvm.org/rL316753 (Fold fma (fneg x), K, y -> fma x, -K, y)
https://reviews.llvm.org/D57630 : Move IR flag handling directly into builder calls for cases translated from Instructions in GlobalIsel
https://reviews.llvm.org/rL332756 : adding baseline fp fold tests for unsafe on and off
https://reviews.llvm.org/rL334035 : NFC: adding baseline fneg case for fmf
https://reviews.llvm.org/rL325832 : [InstrTypes] add frem and fneg with FMF creators
https://reviews.llvm.org/D41342 : [InstCombine] Missed optimization in math expression: simplify calls exp functions
https://reviews.llvm.org/D52087 : [IRBuilder] Fixup CreateIntrinsic to allow specifying Types to Mangle.
https://reviews.llvm.org/D52075 : [InstCombine] Support (sub (sext x), (sext y)) --> (sext (sub x, y)) and (sub (zext x), (zext y)) --> (zext (sub x, y))
https://reviews.llvm.org/rL338059 : [InstCombine] fold udiv with common factor from muls with nuw
Commit: e0ab896a84be9e7beb59874b30f3ac51ba14d025 : [InstCombine] allow more fmul folds with ‘reassoc'
Commit: 3e5c120fbac7bdd4b0ff0a3252344ce66d5633f9 : [InstCombine] distribute fmul over fadd/fsub
https://reviews.llvm.org/D37427 : [InstCombine] canonicalize fcmp ord/uno with constants to null constant
https://reviews.llvm.org/D40130 : [InstSimplify] fold and/or of fcmp ord/uno when operand is known nnan
https://reviews.llvm.org/D40150 : [LibCallSimplifier] fix pow(x, 0.5) -> sqrt() transforms
https://reviews.llvm.org/D39642 : [ValueTracking] readnone is a requirement for converting sqrt to llvm.sqrt; nnan is not
https://reviews.llvm.org/D39304 : [IR] redefine 'reassoc' fast-math-flag and add 'trans' fast-math-flag
https://reviews.llvm.org/D41333 : [ValueTracking] ignore FP signed-zero when detecting a casted-to-integer fmin/fmax pattern
https://reviews.llvm.org/D5584 : Optimize square root squared (PR21126)
https://reviews.llvm.org/D42385 : [InstSimplify] (X * Y) / Y --> X for relaxed floating-point ops
https://reviews.llvm.org/D43160 : [InstSimplify] allow exp/log simplifications with only 'reassoc’ FMF
https://reviews.llvm.org/D43398 : [InstCombine] allow fdiv folds with less than fully 'fast’ ops
https://reviews.llvm.org/D44308 : [ConstantFold] fp_binop AnyConstant, undef --> NaN
https://reviews.llvm.org/D43765 : [InstSimplify] loosen FMF for sqrt(X) * sqrt(X) --> X
https://reviews.llvm.org/D44521 : [InstSimplify] fp_binop X, NaN --> NaN
https://reviews.llvm.org/D47202 : [CodeGen] use nsw negation for abs
https://reviews.llvm.org/D48085 : [DAGCombiner] restrict (float)((int) f) --> ftrunc with no-signed-zeros
https://reviews.llvm.org/D48401 : [InstCombine] fold vector select of binops with constant ops to 1 binop (PR37806)
https://reviews.llvm.org/D39669 : DAG: Preserve nuw when reassociating adds
https://reviews.llvm.org/D39417 : InstCombine: Preserve nuw when reassociating nuw ops
https://reviews.llvm.org/D51753 : [DAGCombiner] try to convert pow(x, 1/3) to cbrt(x)
https://reviews.llvm.org/D51630 : [DAGCombiner] try to convert pow(x, 0.25) to sqrt(sqrt(x))
https://reviews.llvm.org/D53650 : [FPEnv] Last BinaryOperator::isFNeg(...) to m_FNeg(...) changes
https://reviews.llvm.org/D54001 : [ValueTracking] determine sign of 0.0 from select when matching min/max FP
https://reviews.llvm.org/D51942 : [InstCombine] Fold (C/x)>0 into x>0 if possible
https://llvm.org/svn/llvm-project/llvm/trunk@348016 : [SelectionDAG] fold FP binops with 2 undef operands to undef
http://llvm.org/viewvc/llvm-project?view=revision&revision=346242 : propagate fast-math-flags when folding fcmp+fpext, part 2
http://llvm.org/viewvc/llvm-project?view=revision&revision=346240 : propagate fast-math-flags when folding fcmp+fpext
http://llvm.org/viewvc/llvm-project?view=revision&revision=346238 : [InstCombine] propagate fast-math-flags when folding fcmp+fneg, part 2
http://llvm.org/viewvc/llvm-project?view=revision&revision=346169 : [InstSimplify] fold select (fcmp X, Y), X, Y
http://llvm.org/viewvc/llvm-project?view=revision&revision=346234 : propagate fast-math-flags when folding fcmp+fneg
http://llvm.org/viewvc/llvm-project?view=revision&revision=346147 : [InstCombine] canonicalize -0.0 to +0.0 in fcmp
http://llvm.org/viewvc/llvm-project?view=revision&revision=346143 : [InstCombine] loosen FP 0.0 constraint for fcmp+select substitution
http://llvm.org/viewvc/llvm-project?view=revision&revision=345734 : [InstCombine] refactor fabs+fcmp fold; NFC
http://llvm.org/viewvc/llvm-project?view=revision&revision=345728 : [InstSimplify] fold 'fcmp nnan ult X, 0.0' when X is not negative
http://llvm.org/viewvc/llvm-project?view=revision&revision=345727 : [InstCombine] add assertion that InstSimplify has folded a fabs+fcmp; NFC

While multiple people have been working on finer-grained control over fast-math optimizations and other relaxed numerics modes, there has also been some initial progress on adding support for more constrained numerics models. There has been considerable progress towards adding and enabling constrained floating-point intrinsics to capture FENV_ACCESS ON and similar semantic models.

These experimental constrained intrinsics prohibit certain transforms that are not safe if the default floating-point environment is not in effect. Historically, LLVM has in practice basically “split the difference” with regard to such transforms; they haven’t been explicitly disallowed, as LLVM doesn’t model the floating-point environment, but they have been disabled when they caused trouble for tests or software projects. The absence of a formal model for licensing these transforms constrains our ability to enable them. Bringing language and backend support for constrained intrinsics across the finish line will allow us to include transforms that we disable as a matter of practicality today, and allow us to give developers an easy escape valve (in the form of FENV_ACCESS ON and similar language controls) when they need more precise control, rather than an ad-hoc set of flags to pass to the driver.

We should discuss these new intrinsics to make sure that they can capture the right models for all the languages that LLVM supports.

Here are some possible discussion items:

  • Should specialization be applied at the call level for edges in a call graph where the caller has special context to extend into the callee wrt to flags?
  • Should the inliner apply something similar to calls that meet inlining criteria?
  • What other part(s) of the compiler could make use of IR flags that are currently not covered?
  • What work needs to be done regarding code debt wrt current areas of implementation.