[cfe-dev] C sequence-point analysis

[Chad Rosier]

Lukas,
FYI: -fcatch-undefined-behavior and -fsanitize=underfined are synonyms for
the time being.  IIRC, the -fcatch-undefined-behavior flag will be
deprecated at some point.  I was really just trying to point out that
David and I are suggesting the same thing.

 Chad

> 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>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
>> *****************************
>> _______________________________________________
>> cfe-dev mailing list
>> cfe-dev at cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev
>>
> _______________________________________________
> cfe-dev mailing list
> cfe-dev at cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev
>


-- 
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
hosted by The Linux Foundation