[cfe-dev] [LLVMdev] proposal for exploiting undefined behavior much more aggressively

[Florian Weimer]

On 07/27/2012 11:45 AM, David Chisnall wrote:
> On 27 Jul 2012, at 09:35, annulen at yandex.ru wrote:
>
>> if someone has checked the code and found it UB-free he could allow aggressive UB exploiting.
>
> Surely code that contains no undefined behaviour will gain no benefits from optimisations that exploit undefined behaviour?

Relying undefined behavior, the compiler can infer conditions which must 
necessarily hold, and use this information in subsequent optimizations. 
  For instance, adding positive values to a signed integer which is zero 
initially will never yield a negative value.  If this value is later fed 
to code which can cope with negatives values through an explicit check, 
that check can be optimized away.  So obviously, you need some code 
reuse/abstraction (calling a more general routine from very specialized 
code) to trigger optimizations, but there are hypothetical wins even for 
programs which never actually trigger undefined behavior at run time.

However, I would prefer if programmers could provide the compiler with 
the necessary information in more obvious ways.  This way, similar 
properties could be specified for values of unsigned integer type as well.

-- 
Florian Weimer / Red Hat Product Security Team