[llvm-dev] Malloc null checks, why sometimes are moved, and sometimes not?

[John Reagan via llvm-dev]

> On 4/14/2018 3:09 AM, 陳韋任 wrote:
>> 2018-04-14 7:51 GMT+08:00 Krzysztof Parzyszek via llvm-dev
>> <llvm-dev at lists.llvm.org>:
>>> On 4/13/2018 6:39 PM, Dávid Bolvanský via llvm-dev wrote:
>>>> Here is simple test code:
>>>> https://godbolt.org/g/mjAUpu
>>>>
>>>> LLVM generally assumes that malloc never fails.
>>>>
>>>> But I dont understand difference between these two example functions - and
>>>> why null check was not removed in f1, since in f2 it was removed.
>>> That's because the return value from malloc is discarded in f2. In that case
>>> it simply doesn't matter if the malloc happened or not, and can be assumed
>>> to have succeeded.
>> Wouldn't such assumption be too aggressive?
> Normally, yes, but the standard explicitly allows the compiler to not 
> call allocation/deallocation functions if the storage can be provided in 
> another way. If the storage that would have been allocated by malloc 
> cannot ever be accessed, we can pretend that it has been successfully 
> provided in some way.
>
> -Krzysztof
How often does that really occur in normal programs?  Why bother? I find
it quite surprising to fold away the call.  While the pointer doesn't
escape F2(), the code does test the existance of the memory with the "if
(!ptr)".  That would be enough to retain the call on all the compilers
I've ever worked on.

John

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