[llvm-commits] [PATCH] Skip NULL pointer check for free

[Quentin Colombet]

Ping!

-Quentin


On Dec 21, 2012, at 4:04 PM, Quentin Colombet <qcolombet at apple.com> wrote:

> Hi Nick,
> 
> On Dec 21, 2012, at 3:59 PM, Dmitri Gribenko <gribozavr at gmail.com> wrote:
> 
>> On Sat, Dec 22, 2012 at 1:53 AM, Nick Lewycky <nicholas at mxc.ca> wrote:
>>> On 12/21/2012 03:43 PM, Quentin Colombet wrote:
>>>> 
>>>> Hi,
>>>> 
>>>> Here is a patch to help llvm turning a code like this:
>>>> if (foo)
>>>> free(foo)
>>>> 
>>>> into that:
>>>> free(foo)
>>>> 
>>>> It is legal and safe, since free is already checking its argument
>>>> against NULL internally (I may find the reference in the Standard if
>>>> needed :)).
>>> 
>>> It's legal and safe, but is it desirable? Calling a function is expensive
>>> even if the function doesn't do anything.
> 
> Yes, you're right, it may not be desirable but I'm tempt to think like Dmitri.
> 
> Do you think that this optimization should be controlled by a flag?
> At least, for code size (Oz and maybe Os), it may always be desirable.
> 
> I did not consider changing free(foo) into if(foo) free(foo)
> 
>> 
>> That's true, but there's another viewpoint: programmers usually write
>> 'if(!foo) free(foo);' not because they are manually inlining the fast
>> path of free(), but because they think that free(NULL) is UB.
>> 
>> Dmitri
>> 
>> -- 
>> main(i,j){for(i=2;;i++){for(j=2;j<i;j++){if(!(i%j)){j=0;break;}}if
>> (j){printf("%d\n",i);}}} /*Dmitri Gribenko <gribozavr at gmail.com>*/
> 
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