[cfe-dev] C sequence-point analysis

[Lukas Hellebrandt]

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Now, if I understood correctly, Clang Static Analyzer is just a set of
libraries in Clang - which means, it's what I already tried while
doing the Clang plugin. Am I correct?

If yes, I don't know of any way I could do static analysis in Clang
(given I am not willing to write one myself) and would need to somehow
send the data to LLVM (Do some changes in AST? Save to external file?
I don't know, never done this before.) and do alias analysis in LLVM,
as I suggested in the original message.

Did I miss something? Particularly some alias analysis in Clang?

*****************************
Lukas Hellebrandt
kamikazecz at gmail.com
*****************************

On 11/15/2013 07:01 PM, David Blaikie wrote:
> 
> 
> 
> On Fri, Nov 15, 2013 at 9:52 AM, Lukas Hellebrandt
> <kamikazecz at gmail.com <mailto:kamikazecz at gmail.com>> wrote:
> 
> Hi, David, thanks for your reply!
> 
> If I understood UBSan correctly, it just adds run-time checks, so 
> there is no error informing about possible undefined behavior
> before running the program (and even then, it might never occur).
> 
> I don't really think there is a reasonable way to check this type
> of errors during runtime and even if there was, my goal is to issue
> a warning during compilation.
> 
> What I can do in Clang is: check for this kind of error if I 
> completely forget about pointers - I don't do any alias analysis
> and just assume no two different variables alias. So if I could
> just check in Clang whether they do alias or not, my problem would
> be solved.
> 
> As I don't know about any way I could achieve this, I think about 
> using LLVM for this (or combination of both, as i wrote: Clang to
> make a set of must-not-alias rules and LLVM to check these rules)
> which would still warn during compilation.
> 
> As far as I understood, sanitizers are of no use here, am I
> correct?
> 
> 
>> If you specifically want static checking, then no, the sanitizers
>> aren't the right tool.
> 
>> For static checking you have either Clang warnings and CFG,
>> including the existing -Wsequence-point warning which catches
>> some cases of unsequenced operations, or you have the Full Power
>> of the Clang Static Analyzer which might be where you could get
>> some alias information to add more accuracy (in the case of
>> reduced false negatives that the Clang warning necessarily has to
>> have to be cheap enough to run at compile time).
> 
>> - David
> 
> 
> 
> 
> ***************************** Lukas Hellebrandt 
> kamikazecz at gmail.com <mailto:kamikazecz at gmail.com> 
> *****************************
> 
> On 11/15/2013 06:29 PM, David Blaikie wrote:
>> You might want to look into the implementation of Clang's UBSan 
>> feature (-fsanitize=undefined). Like the other sanitizers
>> (address, memory, and thread) UBSan works by adding extra checks
>> into the LLVM IR from the Clang frontend. LLVM compiles those
>> checks as it would any other IR and they are used to verify that
>> the behavior is correct.
> 
>> The simplest example of this might be bounds checking of a
>> static array (and this is one of the things UBSan can check for)
>> or overflow of a signed integer. The frontend simply adds the
>> checks a programmer might write if they were coding defensively
>> against such a circumstance. Then LLVM just compiles the code as
>> normal and when you execute the program, if you trigger the check
>> to fail, an error message is printed.
> 
>> For the particular check you're interested in implementing...
>> I'm not sure exactly how you'll go about implementing that check
>> or how you'd avoid false positives, but UBSan is probably the
>> first place to look and the ideal place for this to live, if
>> possible.
> 
> 
>> On Fri, Nov 15, 2013 at 9:21 AM, Lukas Hellebrandt 
>> <kamikazecz at gmail.com <mailto:kamikazecz at gmail.com>
> <mailto:kamikazecz at gmail.com <mailto:kamikazecz at gmail.com>>>
> wrote:
> 
>> Hi all,
> 
>> I'm trying to write a tool for detecting undefined behavior in C 
>> regarding sequence points and side effects.
> 
>> I'm not sure whether it should be a Clang plugin or LLVM run or 
>> something completely different (I'm really new to both Clang and 
>> LLVM) and that's what I need advice with.
> 
>> For my work, I need to use both AST and alias analysis
> 
>> Clang plugin: +relatively easy to use +access to AST with all
>> the needed info EXCEPT alias analysis (right?) -no alias
>> analysis, I'd need to write one myself
> 
>> LLVM run: +built-in alias analysis (I'd like to use it, writing
>> my own alias analysis is not really what my work is all about) -I
>> do NOT have access to AST -I don't know it at all (but I'm ready
>> to learn it if it shows up to be the best option)
> 
>> The big PROBLEM is: a behavior that is undefined in C (and which 
>> Clang has access to) might be (and in my case WILL be) well
>> defined in LLVM (for example, i=i++; is undefined in C but in
>> LLVM code it will be already well defined and the result will
>> depend on Clang behavior).
> 
>> So I thought I could use both, in Clang create a list of rules, 
>> for example "On line L, there is an undefined behavior if X
>> aliases with Y" and then SOMEHOW dig this info from LLVM run.
> 
>> Is this a good idea? Is there a way (other than output to file 
>> from Clang and then read it in LLVM) to give this set of rules
>> to LLVM? I'd also be glad for any other idea, not necessarily 
>> including LLVM and Clang, to solve this problem.
> 
>> Thanks in advance!
> 
>> -- ***************************** Lukas Hellebrandt 
>> kamikazecz at gmail.com <mailto:kamikazecz at gmail.com>
> <mailto:kamikazecz at gmail.com <mailto:kamikazecz at gmail.com>>
>> ***************************** 
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> 
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