[cfe-dev] [analyzer][RFC] Get info from the LLVM IR for precision

Thu Aug 6 02:06:34 PDT 2020

> 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
could).
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, right?
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> wrote:

> 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:
>
> Hi,
>
> I have been working on a prototype that makes it possible to access the IR
> from the components of the Clang Static Analyzer.
> https://reviews.llvm.org/D85319
>
> 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.
>
> Thanks,
> Gabor
>
>
>
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