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

Artem Dergachev via cfe-dev cfe-dev at lists.llvm.org
Wed Aug 5 12:17:24 PDT 2020

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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