[cfe-dev] [analyzer][RFC] Get info from the LLVM IR for precision
Gábor Horváth via cfe-dev
cfe-dev at lists.llvm.org
Fri Aug 7 01:35:46 PDT 2020
Speaking of the pipeline, I think we should strive for a general
Basically, the proposal is using the analyses of LLVM IR as an oracle for
certain properties of conservatively evaluated functions. In the (possibly
far) future, Clang might get additional analyses based on the CFG or a new
middle level IR. With the optimal solution it should be possible to
replace, add, remove, or maybe even combine oracles easily. I do not insist
on large efforts for generalizing as we do not have multiple oracles to
verify the approach, but whenever we make a design decision I think this is
something that we want to keep in mind.
On Thu, 6 Aug 2020 at 21:42, Gábor Márton <martongabesz at gmail.com> wrote:
> Yes, this is a good point. And a reason to assemble our own CSA specific
> llvm pipeline to avoid such removal of the static functions. We may want to
> skip the inliner pass. Or ... I assume there is a module pass that removes
> the unused static function, so as a better alternative, we could skip that
> from the pipeline.
> On Thu, Aug 6, 2020 at 8:24 PM Gábor Horváth <xazax.hun at gmail.com> wrote:
>> I like the idea of piggybacking some analysis in the LLVM IR. However, I
>> have some concerns as well. I am not well versed in the LLVM optimizer, but
>> I do see potential side effects. E.g. what if a static function is inlined
>> to ALL call sites, thus the original function can be removed. We will no
>> longer be able to get all the useful info for that function? It would be
>> unfortunate if the analysis result would depend on inlining heuristics. It
>> would make the analyzer even harder to debug or understand.
>> On Thu, 6 Aug 2020 at 19:20, Artem Dergachev via cfe-dev <
>> cfe-dev at lists.llvm.org> wrote:
>>> Umm, ok!~
>>> Static analysis is commonly run in debug builds and those are typically
>>> unoptimized. It is not common for a project to have a release+asserts build
>>> but we are relying on asserts for analysis, so debug builds are commonly
>>> used for analysis. If your project completely ignores debug builds its
>>> usefulness drops a lot.
>>> Sounds like we want to disconnect this new fake codegen from compiler
>>> flags entirely. Like, the AST will depend on compiler flags, but we should
>>> not be taking -O flags into account at all, but pick some default -O2
>>> regardless of flags; and ideally all flags should be ignored by default, to
>>> ensure experience as consistent as possible.
>>> You'd also have to make sure that running CodeGen doesn't have unwanted
>>> side effects such as emitting a .o file.
>>> Would something like that actually work?
>>> And if it would, would this also address the usual concerns about making
>>> warnings depend on optimizations? Because, like, optimizations now remain
>>> consistent and no longer depend on optimization flags used for actual code
>>> generation or interact with code generation; they're now simply another
>>> analysis performed on the AST that depends solely on the AST.
>>> On 8/6/20 2:06 AM, Gábor Márton wrote:
>>> > you're "just" generating llvm::Function for a given AST FunctionDecl
>>> "real quick" and looking at the attributes. This is happening on-demand and
>>> cached, right?
>>> This works differently. We generate the llvm code for the whole
>>> translation unit during parsing. It is the Parser and the Sema that calls
>>> into the callbacks of the CodeGenerator via the ASTConsumer interface. This
>>> is the exact same mechanism that is used for the Backend (see the
>>> BackendConsumer). We register both the CodeGenerator ast consumer and the
>>> AnalysisAstConsumer with the AnalysisAction (we use a MultiplexConsumer).
>>> By the time we start the symbolic execution in
>>> AnalysisConsumer::HandleTranslationUnit, the CodeGen is already done (since
>>> CodeGen is added first to the MultiplexConsumer so
>>> its HandleTranslationUnit and other callbacks are called back earlier).
>>> About caching, the llvm code is cached, we generate that only once, then
>>> during the function call evaluation we search it in the llvm::Module using
>>> the mangled name as the key (we don't cache the mangled names now, but we
>>> It would be possible to directly call the callbacks of the CodeGenerator
>>> on-demand, without registering that to the FrontendAction. Actually, my
>>> first attempt was to call HandleTopLevelDecl for a given FunctionDecl on
>>> demand when we needed the llvm code. However, this is a false attempt for
>>> the following reasons: (1) Could not support ObjC/C++ because I could not
>>> get all the information that the Sema has when it calls to
>>> HandleTopLevelDeclInObjCContainer. In fact, I think it is not supported to
>>> call these callbacks directly, just indirectly through a registered
>>> ASTConsumer because we may not know how the Parser and the Sema calls to
>>> these. (2) It is not enough to get the llvm code for a function in
>>> isolation. E.g., for the "readonly" attribute we must enable alias analysis
>>> on global variables (see GlobalsAAResult), so we must emit llvm code for
>>> global variables.
>>> > 1.1. But it sounds like for the CTU users it may amplify the
>>> imperfections of ASTImporter.
>>> > 2.1. Again, it's worse with CTU because imported ASTs have so far
>>> never been tested for compatibility with CodeGen.
>>> We should not call the CodeGen on a merged AST. ASTImporter does not
>>> support the ASTConsumer interface. In the case of CTU, I think we should
>>> generate the IR for each TU in isolation. And we should probably want to
>>> extend the CrossTranslationUnit interface to give back the llvm::Function
>>> for a given FunctionDecl. Or we could make this more transparent and the
>>> IRContext in this prototype could be CTU aware.
>>> > Just to be clear, we should definitely avoid having our analysis
>>> results depend on optimization levels. It should be possible to avoid that,
>>> There is a dependency we will never be able to get rid of: CodeGen
>>> generates lifetime markers
>>> <https://llvm.org/docs/LangRef.html#memory-use-markers> only when the
>>> optimization level is greater or eq to 2 (-O2, -O3) .These lifetime markers
>>> are needed to get the precise pureness info out of GlobalsAA.
>>> > The way i imagined this, we're only interested in picking up LLVM
>>> analyses, which can be run over unoptimized IR just fine(?)
>>> Yes, but we need to set the optimization level so CodeGen generates
>>> lifetime markers. Indeed, there are many llvm analyses that simply do not
>>> change the IR and just populate their results. And we could simply use the
>>> results in CSA.
>>> > We should probably not be optimizing the IR at all in the process(?)
>>> Some llvm passes may invalidate the results of previous analyses and
>>> then we need to rerun those. I am not an expert, but I think if we run an
>>> analysis again after another analysis that optimizes the IR (i.e truncates
>>> it) then our results could be more precise. And that is the reason why we
>>> see multiple passes for the same analyses when we do optimizations. And
>>> perhaps this is the exact job of the PassManager to orchestrate this (?).
>>> There are passes that extend the IR (e.g InferFunctionAttrsPass), we may
>>> not need these strictly speaking, but I really don't know how the different
>>> analyses use the function attributes.
>>> Maybe we need the IR both in unoptimized form and in optimized form.
>>> Also, we may want to have our own CSA specific pipeline, but having the
>>> default O2 pipeline seems to simplify things.
>>> On Wed, Aug 5, 2020 at 11:22 PM Artem Dergachev <noqnoqneo at gmail.com>
>>>> Just to be clear, we should definitely avoid having our analysis
>>>> results depend on optimization levels. It should be possible to avoid that,
>>>> right? The way i imagined this, we're only interested in picking up LLVM
>>>> analyses, which can be run over unoptimized IR just fine(?) We should
>>>> probably not be optimizing the IR at all in the process(?)
>>>> On 05.08.2020 12:17, Artem Dergachev wrote:
>>>> I'm excited that this is actually moving somewhere!
>>>> Let's see what consequences do we have here. I have some thoughts but i
>>>> don't immediately see any architecturally catastrophic consequences; you're
>>>> "just" generating llvm::Function for a given AST FunctionDecl "real quick"
>>>> and looking at the attributes. This is happening on-demand and cached,
>>>> right??? I'd love to hear more opinions. Here's what i see:
>>>> 1. We can no longer mutate the AST for analysis purposes without the
>>>> risk of screwing up subsequent codegen. And the risk would be pretty high
>>>> because hand-crafting ASTs is extremely difficult. Good thing we aren't
>>>> actually doing this.
>>>> 1.1. But it sounds like for the CTU users it may amplify the
>>>> imperfections of ASTImporter.
>>>> 2. Ok, yeah, we now may have crashes in CodeGen during analysis.
>>>> Normally they shouldn't be that bad because this would mean that CodeGen
>>>> would crash during normal compilation as well. And that's rare; codegen
>>>> crashes are much more rare than analyzer crashes. Of course a difference
>>>> can be triggered by #ifndef __clang_analyzer__ but it still remains a proof
>>>> of valid crashing code, so that should be rare.
>>>> 2.1. Again, it's worse with CTU because imported ASTs have so far
>>>> never been tested for compatibility with CodeGen.
>>>> Let's also talk about the benefits. First of all, *we still need the
>>>> source code available during analysis*. This isn't about peeking into
>>>> binary dependencies and it doesn't immediately aid CTU in any way; this is
>>>> entirely about improving upon conservative evaluation on the currently
>>>> available AST, for functions that are already available for inlining but
>>>> are not being inlined for whatever reason. In fact, in some cases we may
>>>> later prefer such LLVM IR-based evaluation to inlining, which may improve
>>>> analysis performance (i.e., less path explosion) *and* correctness (eg.,
>>>> avoid unjustified state splits).
>>>> On 05.08.2020 08:29, Gábor Márton via cfe-dev wrote:
>>>> I have been working on a prototype that makes it possible to access the
>>>> IR from the components of the Clang Static Analyzer.
>>>> There are many important and useful analyses in the LLVM layer that we
>>>> can use during the path sensitive analysis. Most notably, the "readnone"
>>>> and "readonly" function attributes (https://llvm.org/docs/LangRef.html)
>>>> which can be used to identify "pure" functions (those without side
>>>> effects). In the prototype I am using the pureness info from the IR to
>>>> avoid invalidation of any variables during conservative evaluation (when we
>>>> evaluate a pure function). There are cases when we get false positives
>>>> exactly because of the too conservative invalidation.
>>>> Some further ideas to use info from the IR:
>>>> - We should invalidate only the arg regions for functions with
>>>> "argmemonly" attribute.
>>>> - Use the smarter invalidation in cross translation unit analysis too.
>>>> We can get the IR for the other TUs as well.
>>>> - Run the Attributor
>>>> <https://llvm.org/doxygen/structllvm_1_1Attributor.html> passes on the
>>>> IR. We could get range values for return values or for arguments. These
>>>> range values then could be fed to StdLibraryFunctionsChecker to make the
>>>> proper assumptions. And we could do this in CTU mode too, these attributes
>>>> could form some sort of a summary of these functions. Note that I don't
>>>> expect a meaningful summary for more than a few percent of all the
>>>> available functions.
>>>> Please let me know if you have any further ideas about how we could use
>>>> IR attributes (or anything else) during the symbolic execution.
>>>> There are some concerns as well. There may be some source code that we
>>>> cannot CodeGen, but we can still analyse with the current CSA. That is why
>>>> I suppress CodeGen diagnostics in the prototype. But in the worst case we
>>>> may run into assertions in the CodeGen and this may cause regression in the
>>>> whole analysis experience. This may be the case especially when we get a
>>>> compile_commands.json from a project that is compiled only with e.g. GCC.
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