[cfe-dev] Understand assumptions towards uninitialized variables on stack

[Friedman, Eli via cfe-dev]

On 11/21/2018 8:58 AM, Stephan Bergmann via cfe-dev wrote:
> On 21/11/2018 16:03, via cfe-dev wrote:
>>> uninitialized variables on the stack is assumed to be zero due to
>>> some reason,
>>
>> That is not exactly what happened.  The assignment is from uninitialized
>> memory, which will have an unknown value.  Because the value is unknown,
>> the assignment can be optimized to avoid a read from memory, and
>> substitute any convenient value, without perturbing any defined property
>> of the program. The most convenient value to use here is zero.
>
> But why bother to come up with a specific value at all, why not drop 
> the "xorl %eax, %eax" completely and use whatever value is present in 
> %eax?
>
The optimizer isn't specifically trying to catch this case. It just runs 
a series of transforms which assume that the behavior is defined, and 
some of those transforms constrain the behavior of the function.  This 
eventually leads to generating an xor which wasn't necessary for the 
original function.  If you're curious about what happens in this 
particular case, you can use "-mllvm -print-after-all" to see how 
various transforms change the IR.

-Eli

-- 
Employee of Qualcomm Innovation Center, Inc.
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