llvm dev - Dec 2015 - RFC: New function attribute HasInaccessibleState

>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
>>>
>>>
>>>
>>>
>>
>>
>
>
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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
>
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>What if you had bitcode versions of your C library
Then the definitions for these functions would be available. Would we still
set function attributes to these functions in FunctionAttrs.cpp if their
definitions were available?
>This also seems a bit tailored to malloc/free, and can't work for
user-defined allocation functions
I don't think so. For example, printf would have the flag set, preventing
two calls to printfs (without the return value being used) from being
interchanged. In the case of user-defined allocation functions, the
definitions for those functions are available, and what state they modify
is directly visible. I'm not sure I understand how malloc alone would be
more powerful. This point however reminds me to add, functions that
transitively call functions with HasInaccessibleState must also have the
flag set.

  - Vaivaswatha

On Fri, Dec 4, 2015 at 10:06 PM, James Molloy <james at jamesmolloy.co.uk>
wrote:
> 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
>>
>
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