[cfe-dev] Fwd: SIGILL with clang

[Csaba Raduly via cfe-dev]

Hi,
foo() has undefined behavior if i==0 (lack of return statement)

6.6.3 para 2:
Flowing off the end of a function is equivalent to a return with no
value; this results in undefined behavior
in a value-returning function.

clang generates an invalid instruction for undefined behavior.
This is not ARM-specific.

https://godbolt.org/#g:!((g:!((g:!((h:codeEditor,i:(j:1,source:'int+foo(int+i)+%7B%0A+if(i)+%7B%0A+++return+1%3B%0A+%7D%0A%7D%0A%0Aint+main()+%7B%0A+foo(0)%3B%0A%7D%0A'),l:'5',n:'0',o:'C%2B%2B+source+%231',t:'0')),k:50,l:'4',n:'0',o:'',s:0,t:'0'),(g:!((h:compiler,i:(compiler:clang400,filters:(___0:(),b:'0',commentOnly:'0',directives:'0',intel:'0',jquery:'3.2.1',length:1,prevObject:(___0:(),length:1,prevObject:(___0:(jQuery3210202339019067484751:(display:'')),length:1)),trim:'0'),options:'-O0',source:1),l:'5',n:'0',o:'x86-64+clang+4.0.0+(Editor+%231,+Compiler+%231)',t:'0')),k:50,l:'4',n:'0',o:'',s:0,t:'0')),l:'2',n:'0',o:'',t:'0')),version:4

That ud2 instruction is the machine code for undefined behavior.

Csaba


On Sat, Aug 26, 2017 at 12:32 PM, Raphael Isemann via cfe-dev
<cfe-dev at lists.llvm.org> wrote:
> Glad to help. CCing ML for the record.
>
> On Aug 26, 2017 12:24 PM, "ROHIT KUMAR" <rohitpoddar92 at gmail.com> wrote:
>
> It is as you mentioned. The missing return statement in a function very much
> similar to your example. Adding it fixed the issue
>
> So, it seems Clang is a little too strict as compared to GCC. I faced many
> issues earlier and all of them ended up being related to flags, I didn't
> expect the issue to be this simple.
>
> Thanks for your help
>
>
>
>
>
> On Sat, Aug 26, 2017 at 3:14 PM, Raphael Isemann <teemperor at gmail.com>
> wrote:
>>
>> On a side note that is probably too simple for the ML: Did you check
>> for missing return statements in the called/callee function? For a
>> missing return clang generates SIGILL while GCC is returning something
>> ABI-dependent IIRC.
>>
>> E.g. this causes SIGILL on clang and runs just fine on GCC:
>> ```
>> int foo(int i) {
>>  if(i) {
>>    return 1;
>>  }
>> }
>>
>> int main() {
>>  foo(0);
>> }
>> ```
>>
>> - Raphael
>>
>>
>> 2017-08-26 10:34 GMT+02:00 ROHIT KUMAR via cfe-dev
>> <cfe-dev at lists.llvm.org>:
>> > Hey folks,
>> >
>> > I am migrating our huge Android code base from GCC(Eclipse IDE) to
>> > clang(Android Studio).
>> >
>> > Error: SIGILL (signal SIGILL: illegal instruction)
>> >
>> > At a weird line in the code, I am getting this error. which points to a
>> > closing curly bracket of an if condition. After googling, I am assuming
>> > it
>> > is some kind of architecture flag issue. I am using same flags which
>> > were
>> > used for GCC.
>> >
>> > Note: I am currently working on Arm build.
>> >
>> > Here are the architecture flags for ARM7:
>> >
>> > -march=armv7-a -mfloat-abi=softfp -mfpu=vfpv3-d16 -mtune=cortex-a8
>> > -mthumb
>> > -DTARGET_THUMB2
>> >
>> > Can anyone help me debug this issue? Is there any issue with the flags
>> > with
>> > clang?
>> >
>> > Thanks,
>> > Rohit Kumar
>> >
>> >
>> >
>> > _______________________________________________
>> > cfe-dev mailing list
>> > cfe-dev at lists.llvm.org
>> > http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>> >
>
>
>
>
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