[llvm-dev] RFC: New function attribute HasInaccessibleState
James Molloy via llvm-dev
llvm-dev at lists.llvm.org
Fri Dec 4 08:36:46 PST 2015
Hi,
I'm still a bit dubious about this, I don't think it's bombproof. How does
this fit with LTO? What if you had bitcode versions of your C library (not
entirely crazy - it might allow a lot of LTO) - you'd collapse those two
domains into one in a rather messy way.
This also seems a bit tailored to malloc/free, and can't work for
user-defined allocation functions. Our current attributes mechanism has the
ability to infer noalias on such functions, so here you'll be making malloc
more powerful than user-defined functions.
All in all it just smells a bit specialist. I'd welcome it if we could
bombproof the semantics and extend the scope somewhat somehow.
Cheers,
James
On Fri, 4 Dec 2015 at 16:31 Vaivaswatha Nagaraj via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> >there would be two disjoint global states
> In some sense yes, but technically not disjoint. Functions marked with
> this attribute should still be able to access the globals within the
> program under compilation, if its not marked with ReadNone.
>
>
> >If malloc and free can both use global variables (there is no notion of
> library in the compiler)
> Inaccessible state here refers to any global that is not visible to the
> program under compilation. The key idea (behind the new attribute) is to
> convey that these external functions do things inside that the compiler
> cannot know about, and hence deal with them conservatively.
>
> - Vaivaswatha
>
> On Fri, Dec 4, 2015 at 9:51 PM, Mehdi Amini <mehdi.amini at apple.com> wrote:
>
>>
>> On Dec 4, 2015, at 8:09 AM, Vaivaswatha Nagaraj <vn at compilertree.com>
>> wrote:
>>
>> >what is a non-public state that no-one but you can access? (I’d call
>> that private).
>> malloc and free could both use global variables that are defined in libc,
>> but are inaccessible to the program under compilation.
>>
>>
>>
>> If malloc and free can both use global variables (there is no notion of
>> library in the compiler), then from what I understand you are actually
>> creating another global state: i.e. there would be two disjoint global
>> states: the usual default one and another one that is only accessed by
>> function having this attribute.
>>
>> —
>> Mehdi
>>
>>
>>
>>
>> >if you’re attribute is saying they have some internal state, then
>> malloc() cannot access the state of free() and vice versa.
>> Which is why it would be preferable to call it "inaccessible" state
>> rather than "internal".
>>
>> >It would prevent to swap two mallocs but not moving freely a malloc with
>> respect to a free.
>> No, it would also prevent interchanging the order of malloc and free,
>> since they both maintain states (which can be shared, but not accessible to
>> the program under compilation) and the swapping order could result in a
>> different final state.
>>
>> >At the cost of some redundancy, I think a new attribute is needed.
>> @hal. I'm not sure what this implies. Does the semantics of the attribute
>> in the first mail sound right to you?
>>
>> > that'd be redefining the semantics of ReadNone. ReadNone allows elision
>> of a call if its result is unused,
>> @James. That right. Optimizations should hereafter (if the proposed
>> attribute is accepted) be more careful in interpreting ReadNone. If the
>> call also has HasInaccessibleState, it shouldn't remove the call, even if
>> the call takes no arguments or its return value isn't, because it could be
>> modifying some internal state.
>>
>>
>> - Vaivaswatha
>>
>> On Fri, Dec 4, 2015 at 9:10 PM, Mehdi Amini <mehdi.amini at apple.com>
>> wrote:
>>
>>>
>>> On Dec 3, 2015, at 11:58 PM, Vaivaswatha Nagaraj <vn at compilertree.com>
>>> wrote:
>>>
>>> >is this "internal state” supposed to be private to the function?
>>> It could be private or not. Hence the name "inaccessible", to mean that
>>> the program under compilation has no access to the state. So while printf
>>> and malloc (for example) could share state in libc, the program under
>>> compilation cannot access this state.
>>>
>>>
>>> This is still not clear to me, you’re saying “it could be private or
>>> not”: what is a non-public state that no-one but you can access? (I’d call
>>> that private).
>>>
>>> Now, from the point of view of the compiler, malloc and free are two
>>> separate functions, if you’re attribute is saying they have some internal
>>> state, then malloc() cannot access the state of free() and vice versa.
>>>
>>> >how this flag would prevent the last “optimization” you’re illustrating
>>> Assuming you are referring to the quoted examples, currently these
>>> optimizations are not happening anyway (from what I understand). The issue
>>> is that, after malloc/free are tagged with "ReadNone", such transforms may
>>> happen. Hence to prevent that, the additional flag denoting that these
>>> functions maintain an internal state.
>>>
>>>
>>> I’m questioning why would this flag solve that, it does not seem to to
>>> me. It would prevent to swap two mallocs but not moving freely a malloc
>>> with respect to a free.
>>>
>>> —
>>> Mehdi
>>>
>>>
>>>
>>>
>>> - Vaivaswatha
>>>
>>> On Fri, Dec 4, 2015 at 12:20 PM, Mehdi Amini <mehdi.amini at apple.com>
>>> wrote:
>>>
>>>> Hi,
>>>>
>>>> On Dec 3, 2015, at 10:33 PM, Vaivaswatha Nagaraj via llvm-dev <
>>>> llvm-dev at lists.llvm.org> wrote:
>>>>
>>>> Hi,
>>>>
>>>> This email is in continuation to the mail thread
>>>> http://lists.llvm.org/pipermail/llvm-dev/2015-December/092996.html, to
>>>> propose a new function attribute that can convey that a function maintains
>>>> state, but this state is inaccessible to the rest of the program under
>>>> compilation.
>>>>
>>>> Such a flag could be added to most libc/system calls such as
>>>> printf/malloc/free. (libc and system calls do access/modify internal
>>>> variables such as errno).
>>>>
>>>> Example attributes (in addition to what are already set):
>>>> malloc/free: HasInaccessibleState, ReadNone
>>>> printf: HasInaccessibleState, ArgMemOnly
>>>> realloc: HasInaccessibleState, ReadOnly (not sure).
>>>>
>>>> The intention behind introducing this attribute is to relax the
>>>> conditions in GlobalsAA as below:
>>>> (this code is in GlobalsAAResult::AnalyzeCallGraph)
>>>>
>>>> if (F->isDeclaration()) {
>>>> // Try to get mod/ref behaviour from function attributes.
>>>> - if (F->doesNotAccessMemory()) {
>>>> + if (F->doesNotAccessMemory() || F->onlyAccessesArgMemory()) {
>>>> // Can't do better than that!
>>>> } else if (F->onlyReadsMemory()) {
>>>> FunctionEffect |= Ref;
>>>> if (!F->isIntrinsic())
>>>> // This function might call back into the module and read a global -
>>>> // consider every global as possibly being read by this function.
>>>> FR.MayReadAnyGlobal = true;
>>>> } else {
>>>> FunctionEffect |= ModRef;
>>>> // Can't say anything useful unless it's an intrinsic - they don't
>>>> // read or write global variables of the kind considered here.
>>>> KnowNothing = !F->isIntrinsic();
>>>> }
>>>> continue;
>>>> }
>>>>
>>>> This relaxation allows functions that (transitively) call library
>>>> functions (such as printf/malloc) to still maintain and propagate GlobalsAA
>>>> info. In general, this adds more precision to the description of these
>>>> functions.
>>>>
>>>> Concerns regarding impact on other optimizations (I'm repeating a few
>>>> examples that Hal mentioned earlier).
>>>>
>>>> 1.
>>>> >A readnone function is one whose output is a function only of its
>>>> inputs, and if you have this:
>>>> >
>>>> > int *x = malloc(4);
>>>> > *x = 2;
>>>> > int *y = malloc(4);
>>>> > *y = 4;
>>>> > you certainly don't want EarlyCSE to replace the second call to
>>>> malloc with the result of the first (which it will happily do if you mark
>>>> malloc as readnone).
>>>>
>>>> For malloc, even though ReadNone is set now (as proposed above),
>>>> EarlyCSE should be taught to respect the HasInaccessibleState and not
>>>> combine functions that maintain internal states. Similarly other
>>>> optimizations (such as DCE) must be taught to respect the flag.
>>>>
>>>> 2.
>>>> >void foo(char * restrict s1, char * restrict s2) {
>>>> > printf(s1);
>>>> > printf(s2);
>>>> >}
>>>>
>>>> >If printf is argmemonly, then we could interchange the two printf
>>>> calls.
>>>>
>>>> In this example too, printf maintains an internal state, preventing the
>>>> calls from being internchanged. Also, it is now correct to add
>>>> ArgMemOnly to printf as it does not access any other program memory.
>>>>
>>>> 3.
>>>> >For malloc this is still a problem, in the following sense, if we have:
>>>> >
>>>> > p1 = malloc(really_big);
>>>> > ...
>>>> > free(p1);
>>>> >
>>>> > p2 = malloc(really_big);
>>>> > ...
>>>> > free(p2);
>>>> >allowing a transformation into:
>>>> > p1 = malloc(really_big);
>>>> > p2 = malloc(really_big);
>>>> > ...
>>>> > free(p1); free(p2);
>>>> >could be problematic.
>>>>
>>>> Both free and malloc would be marked with having an internal state.
>>>> This should prevent this kind of an optimization. Note that having the
>>>> ReadNone attribute should not cause problems anymore.
>>>>
>>>>
>>>> Something is not clear to me: is this "internal state” supposed to be
>>>> private to the function?
>>>> How does it deal with malloc/free which can be seen as sharing a state?
>>>> Especially it is not clear to me how this flag would prevent the last
>>>> “optimization” you’re illustrating.
>>>>
>>>> —
>>>> Mehdi
>>>>
>>>>
>>>>
>>>>
>>>
>>>
>>
>>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20151204/7ee13a5e/attachment.html>
More information about the llvm-dev
mailing list