[LLVMdev] Exploiting 'unreachable' for optimization purposes

[Stephan Tolksdorf]

Hi Mark,

I just dug out a bug from 2006 which since 2009 seems to also have 
covered efforts to make better use of unreachable for optimization 
purposes: http://llvm.org/bugs/show_bug.cgi?id=810

I've added a comment and assigned it to you as requested.

Thanks,
   Stephan

On 24.10.13 19:05, Mark Lacey wrote:
>
> On Oct 24, 2013, at 5:22 AM, Stephan Tolksdorf <st at quanttec.com> wrote:
>
>> Hi,
>>
>> When clang/llvm compiles the following sample (with -O2) it optimizes away the second comparison in test1 but not in test2. Is this handling of 'unreachable' by purpose, or is this just a shortcoming of the current optimization passes? GCC and MSVC (with the equivalent code using the __assume intrinsic) both optimize away the comparison in test2.
>
> Hi Stephan,
>
> I think this is just a shortcoming. Can you file a bug and assign it to me? I have another limitation related to unreachable that I need to take a look at, and when I do so I will take a look at this as well.
>
> Thanks,
>
> Mark
>
>>
>>
>> void f1();
>>
>> void f2();
>>
>> void abort() __attribute__((noreturn));
>>
>> void test1(int x) {
>>   if (x < 0) abort();
>>   // the following comparison is optimized away
>>   if (x < 0) f1(); else f2();
>> }
>>
>> void test2(int x) {
>>   if (x < 0) __builtin_unreachable();
>>   // the following comparison is NOT optimized away
>>   if (x < 0) f1(); else f2();
>> }
>>
>> - Stephan
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>