llvm dev - Jan 2017 - RFC: Allow readnone and readonly functions to throw exceptions

On 01/04/2017 10:35 PM, Sanjoy Das via llvm-dev wrote:
> I just realized that there's an annoying corner case to this scheme --
> I can't DSE stores across readnone maythrow function calls because the
> exception handler could read memory. That is, in:
>
> try {
>    *a = 10;
>    call void @readnone_mayunwind_fn();
>    *a = 20;
> } catch (...) {
>    assert(*a == 10);
> }
>
> I can't DSE the `*a = 10` store.
>
> As far as I can tell, the most restrictive memory attribute for a
> potentially throwing function is readonly.  "readnone may-unwind"
does
> not make sense.
Why not? I've not followed this thread in detail, but it seems like 
you're discussing allowing the modeling of EH schemes that don't access 
accessible memory. In that case, a may-unwind readnone function is just 
one that makes its decision about if/what to throw based only on its 
arguments.

  -Hal
>   "readonly may-unwind" is fine because even if the EH
> handler writes to memory, the code in the normal execution path does
> not have worry about the memory clobbers.
>
> I thought I had this figured out, but now it looks like I gotta think more.
:)
>
> @Danny: I agree with your assessment of the example; unless the
> compiler knows that `cos` won't throw (which it may very well know
> since it is the standard library function, but I don't know GCC
> internals), the transform is wrong.
>
> -- Sanjoy
>
>
> On Tue, Jan 3, 2017 at 11:52 AM, Daniel Berlin <dberlin at
dberlin.org> wrote:
>>
>> On Tue, Jan 3, 2017 at 10:47 AM, Michael Kuperstein via llvm-dev
>> <llvm-dev at lists.llvm.org> wrote:
>>>
>>>
>>> On Tue, Jan 3, 2017 at 9:59 AM, Sanjoy Das via llvm-dev
>>> <llvm-dev at lists.llvm.org> wrote:
>>>> Hi Michael,
>>>>
>>>> On Mon, Jan 2, 2017 at 11:49 PM, Michael Kuperstein
>>>> <michael.kuperstein at gmail.com> wrote:
>>>>> This sounds right to me.
>>>>>
>>>>> IIUC, historically, readonly and readnone are meant to
model the "pure"
>>>>> and
>>>>> "const" GCC attributes. These attributes make
pretty strong guarantees:
>>>>>
>>>>> "[a pure] function can be subject to common
subexpression elimination
>>>>> and
>>>>> loop optimization just as an arithmetic operator would be.
These
>>>>> functions
>>>>> should be declared with the attribute pure [...]
Interesting non-pure
>>>>> functions are functions with infinite loops or those
depending on
>>>>> volatile
>>>>> memory or other system resource, that may change between
two
>>>>> consecutive
>>>>> calls (such as feof in a multithreading environment)."
>>>>>
>>>>> In particular, pure/const imply termination - something
that's not
>>>>> entirely
>>>>> clear w.r.t readonly. However, apparently, they don't
imply nothrow.
>>>>> I've
>>>>> actually always thought they *do* imply it - and said so
on-list :-) -
>>>>> but
>>>>> it looks like GCC itself doesn't interpret them that
way. E.g. see John
>>>>> Regher's example here: https://t.co/REzy5m1tT3
>>>>> So there's at least one use-case for possibly throwing
>>>>> readonly/readnone.
>>>> One important thing to note then: clang marks const and pure
functions
>>>> as nounwind *explicitly*.  That needs to be fixed.
>>>>
>>>> https://godbolt.org/g/SMF4C9
>>>>
>>> Hah. So it does.
>>> Eric, this was originally your change. Do I understand GCC's
behavior
>>> incorrectly?
>>
>> No, but I was in the office when Kenny wrote ipa-pure-const, which is
the
>> equivalent to llvm's pass to find function attributions, and it
mostly
>> wasn't thought about.
>>
>> GCC isn't as consistent as one may think here.
>>
>>             /* Non-looping const functions always return normally.
>>            Otherwise the call might not return or have side-effects
>>            that forbids hoisting possibly trapping expressions
>>            before it.  */
>>             int flags = gimple_call_flags (stmt);
>>             if (!(flags & ECF_CONST)
>>             || (flags & ECF_LOOPING_CONST_OR_PURE))
>>           BB_MAY_NOTRETURN (block) = 1;
>>           }
>>
>> It also, for example, will do this:
>>   double cos (double) __attribute__ ((const));
>>   double sin (double) __attribute__ ((const));
>>   double f(double a)
>>   {
>>     double b;
>>     double c,d;
>>     double (*fp) (double) __attribute__ ((const));
>>     /* Partially redundant call */
>>     if (a < 2.0)
>>       {
>>         fp = sin;
>>         c = fp (a);
>>       }
>>     else
>>       {
>>         c = 1.0;
>>         fp = cos;
>>       }
>>     d = fp (a);
>>     return d + c;
>>   }
>>
>>
>> into
>>   double cos (double) __attribute__ ((const));
>>   double sin (double) __attribute__ ((const));
>>   double f(double a)
>>   {
>>     double b;
>>     double c,d;
>>     double (*fp) (double) __attribute__ ((const));
>>     /* Partially redundant call */
>>     if (a < 2.0)
>>       {
>>         fp = sin;
>>         c = fp (a);
>>       }
>>     else
>>       {
>>         c = 1.0;
>>         fp = cos;
>>         temp = fp(a)
>>       }
>>     d = phi(c, temp)
>>     return d + c;
>>   }
>>
>> This only works if the second call to sin is guaranteed not to throw,
no?
>>
>> In any case, optimizations check throwing separately from pure/const,
but
>> i'm not sure it's well thought out here.
>>
>> Note that GCC also has a notion of possibly infinite looping pure/const
as
>> well:)
>>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory

Hi Hal,

On Thu, Jan 5, 2017 at 6:12 AM, Hal Finkel <hfinkel at anl.gov>
wrote:
>
> On 01/04/2017 10:35 PM, Sanjoy Das via llvm-dev wrote:
>>
>> I just realized that there's an annoying corner case to this scheme
--
>> I can't DSE stores across readnone maythrow function calls because
the
>> exception handler could read memory. That is, in:
>>
>> try {
>>    *a = 10;
>>    call void @readnone_mayunwind_fn();
>>    *a = 20;
>> } catch (...) {
>>    assert(*a == 10);
>> }
>>
>> I can't DSE the `*a = 10` store.
>>
>> As far as I can tell, the most restrictive memory attribute for a
>> potentially throwing function is readonly.  "readnone
may-unwind" does
>> not make sense.
>
>
> Why not? I've not followed this thread in detail, but it seems like
you're
> discussing allowing the modeling of EH schemes that don't access
accessible
> memory. In that case, a may-unwind readnone function is just one that makes
> its decision about if/what to throw based only on its arguments.
If the call to @readnone_mayunwind_fn throws and I've DSE'ed the
"*a 10" store, the exception handler will fail the *a == 10 assert
(assume
*a is not 10 to begin with).  The function call itself is readnone,
but its exceptional continuation may read any part of the heap.

This isn't a big deal, but it means that "readnone may-unwind"
will
effectively have to be treated as "readonly may-unwind" -- I don't
see
any optimization that would be applicable to one and not the other.
Maybe we should just move ahead with that (that readnone may-unwind is
allowed, but if you want readnone-like optimizations then you need to
also mark it as nounwind)?

-- Sanjoy
>
>  -Hal
>
>>   "readonly may-unwind" is fine because even if the EH
>> handler writes to memory, the code in the normal execution path does
>> not have worry about the memory clobbers.
>>
>> I thought I had this figured out, but now it looks like I gotta think
>> more. :)
>>
>> @Danny: I agree with your assessment of the example; unless the
>> compiler knows that `cos` won't throw (which it may very well know
>> since it is the standard library function, but I don't know GCC
>> internals), the transform is wrong.
>>
>> -- Sanjoy
>>
>>
>> On Tue, Jan 3, 2017 at 11:52 AM, Daniel Berlin <dberlin at
dberlin.org>
>> wrote:
>>>
>>>
>>> On Tue, Jan 3, 2017 at 10:47 AM, Michael Kuperstein via llvm-dev
>>> <llvm-dev at lists.llvm.org> wrote:
>>>>
>>>>
>>>>
>>>> On Tue, Jan 3, 2017 at 9:59 AM, Sanjoy Das via llvm-dev
>>>> <llvm-dev at lists.llvm.org> wrote:
>>>>>
>>>>> Hi Michael,
>>>>>
>>>>> On Mon, Jan 2, 2017 at 11:49 PM, Michael Kuperstein
>>>>> <michael.kuperstein at gmail.com> wrote:
>>>>>>
>>>>>> This sounds right to me.
>>>>>>
>>>>>> IIUC, historically, readonly and readnone are meant to
model the
>>>>>> "pure"
>>>>>> and
>>>>>> "const" GCC attributes. These attributes make
pretty strong
>>>>>> guarantees:
>>>>>>
>>>>>> "[a pure] function can be subject to common
subexpression elimination
>>>>>> and
>>>>>> loop optimization just as an arithmetic operator would
be. These
>>>>>> functions
>>>>>> should be declared with the attribute pure [...]
Interesting non-pure
>>>>>> functions are functions with infinite loops or those
depending on
>>>>>> volatile
>>>>>> memory or other system resource, that may change
between two
>>>>>> consecutive
>>>>>> calls (such as feof in a multithreading
environment)."
>>>>>>
>>>>>> In particular, pure/const imply termination - something
that's not
>>>>>> entirely
>>>>>> clear w.r.t readonly. However, apparently, they
don't imply nothrow.
>>>>>> I've
>>>>>> actually always thought they *do* imply it - and said
so on-list :-) -
>>>>>> but
>>>>>> it looks like GCC itself doesn't interpret them
that way. E.g. see
>>>>>> John
>>>>>> Regher's example here: https://t.co/REzy5m1tT3
>>>>>> So there's at least one use-case for possibly
throwing
>>>>>> readonly/readnone.
>>>>>
>>>>> One important thing to note then: clang marks const and
pure functions
>>>>> as nounwind *explicitly*.  That needs to be fixed.
>>>>>
>>>>> https://godbolt.org/g/SMF4C9
>>>>>
>>>> Hah. So it does.
>>>> Eric, this was originally your change. Do I understand
GCC's behavior
>>>> incorrectly?
>>>
>>>
>>> No, but I was in the office when Kenny wrote ipa-pure-const, which
is the
>>> equivalent to llvm's pass to find function attributions, and it
mostly
>>> wasn't thought about.
>>>
>>> GCC isn't as consistent as one may think here.
>>>
>>>             /* Non-looping const functions always return normally.
>>>            Otherwise the call might not return or have side-effects
>>>            that forbids hoisting possibly trapping expressions
>>>            before it.  */
>>>             int flags = gimple_call_flags (stmt);
>>>             if (!(flags & ECF_CONST)
>>>             || (flags & ECF_LOOPING_CONST_OR_PURE))
>>>           BB_MAY_NOTRETURN (block) = 1;
>>>           }
>>>
>>> It also, for example, will do this:
>>>   double cos (double) __attribute__ ((const));
>>>   double sin (double) __attribute__ ((const));
>>>   double f(double a)
>>>   {
>>>     double b;
>>>     double c,d;
>>>     double (*fp) (double) __attribute__ ((const));
>>>     /* Partially redundant call */
>>>     if (a < 2.0)
>>>       {
>>>         fp = sin;
>>>         c = fp (a);
>>>       }
>>>     else
>>>       {
>>>         c = 1.0;
>>>         fp = cos;
>>>       }
>>>     d = fp (a);
>>>     return d + c;
>>>   }
>>>
>>>
>>> into
>>>   double cos (double) __attribute__ ((const));
>>>   double sin (double) __attribute__ ((const));
>>>   double f(double a)
>>>   {
>>>     double b;
>>>     double c,d;
>>>     double (*fp) (double) __attribute__ ((const));
>>>     /* Partially redundant call */
>>>     if (a < 2.0)
>>>       {
>>>         fp = sin;
>>>         c = fp (a);
>>>       }
>>>     else
>>>       {
>>>         c = 1.0;
>>>         fp = cos;
>>>         temp = fp(a)
>>>       }
>>>     d = phi(c, temp)
>>>     return d + c;
>>>   }
>>>
>>> This only works if the second call to sin is guaranteed not to
throw, no?
>>>
>>> In any case, optimizations check throwing separately from
pure/const, but
>>> i'm not sure it's well thought out here.
>>>
>>> Note that GCC also has a notion of possibly infinite looping
pure/const
>>> as
>>> well:)
>>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
> --
> Hal Finkel
> Lead, Compiler Technology and Programming Languages
> Leadership Computing Facility
> Argonne National Laboratory
>

On 01/05/2017 10:55 AM, Sanjoy Das wrote:
> Hi Hal,
>
> On Thu, Jan 5, 2017 at 6:12 AM, Hal Finkel <hfinkel at anl.gov>
wrote:
>> On 01/04/2017 10:35 PM, Sanjoy Das via llvm-dev wrote:
>>> I just realized that there's an annoying corner case to this
scheme --
>>> I can't DSE stores across readnone maythrow function calls
because the
>>> exception handler could read memory. That is, in:
>>>
>>> try {
>>>     *a = 10;
>>>     call void @readnone_mayunwind_fn();
>>>     *a = 20;
>>> } catch (...) {
>>>     assert(*a == 10);
>>> }
>>>
>>> I can't DSE the `*a = 10` store.
>>>
>>> As far as I can tell, the most restrictive memory attribute for a
>>> potentially throwing function is readonly.  "readnone
may-unwind" does
>>> not make sense.
>>
>> Why not? I've not followed this thread in detail, but it seems like
you're
>> discussing allowing the modeling of EH schemes that don't access
accessible
>> memory. In that case, a may-unwind readnone function is just one that
makes
>> its decision about if/what to throw based only on its arguments.
> If the call to @readnone_mayunwind_fn throws and I've DSE'ed the
"*a > 10" store, the exception handler will fail the *a == 10
assert (assume
> *a is not 10 to begin with).  The function call itself is readnone,
> but its exceptional continuation may read any part of the heap.
>
> This isn't a big deal, but it means that "readnone
may-unwind" will
> effectively have to be treated as "readonly may-unwind" -- I
don't see
> any optimization that would be applicable to one and not the other.
> Maybe we should just move ahead with that (that readnone may-unwind is
> allowed, but if you want readnone-like optimizations then you need to
> also mark it as nounwind)?
Yes, I think that makes sense. The attribute only applies to the 
function anyway, so what exception handlers might do (which is assumed 
to be reading/writing any memory that might be available to them) must 
be reasoned about separately.

  -Hal
>
> -- Sanjoy
>
>>   -Hal
>>
>>>    "readonly may-unwind" is fine because even if the EH
>>> handler writes to memory, the code in the normal execution path
does
>>> not have worry about the memory clobbers.
>>>
>>> I thought I had this figured out, but now it looks like I gotta
think
>>> more. :)
>>>
>>> @Danny: I agree with your assessment of the example; unless the
>>> compiler knows that `cos` won't throw (which it may very well
know
>>> since it is the standard library function, but I don't know GCC
>>> internals), the transform is wrong.
>>>
>>> -- Sanjoy
>>>
>>>
>>> On Tue, Jan 3, 2017 at 11:52 AM, Daniel Berlin <dberlin at
dberlin.org>
>>> wrote:
>>>>
>>>> On Tue, Jan 3, 2017 at 10:47 AM, Michael Kuperstein via
llvm-dev
>>>> <llvm-dev at lists.llvm.org> wrote:
>>>>>
>>>>>
>>>>> On Tue, Jan 3, 2017 at 9:59 AM, Sanjoy Das via llvm-dev
>>>>> <llvm-dev at lists.llvm.org> wrote:
>>>>>> Hi Michael,
>>>>>>
>>>>>> On Mon, Jan 2, 2017 at 11:49 PM, Michael Kuperstein
>>>>>> <michael.kuperstein at gmail.com> wrote:
>>>>>>> This sounds right to me.
>>>>>>>
>>>>>>> IIUC, historically, readonly and readnone are meant
to model the
>>>>>>> "pure"
>>>>>>> and
>>>>>>> "const" GCC attributes. These attributes
make pretty strong
>>>>>>> guarantees:
>>>>>>>
>>>>>>> "[a pure] function can be subject to common
subexpression elimination
>>>>>>> and
>>>>>>> loop optimization just as an arithmetic operator
would be. These
>>>>>>> functions
>>>>>>> should be declared with the attribute pure [...]
Interesting non-pure
>>>>>>> functions are functions with infinite loops or
those depending on
>>>>>>> volatile
>>>>>>> memory or other system resource, that may change
between two
>>>>>>> consecutive
>>>>>>> calls (such as feof in a multithreading
environment)."
>>>>>>>
>>>>>>> In particular, pure/const imply termination -
something that's not
>>>>>>> entirely
>>>>>>> clear w.r.t readonly. However, apparently, they
don't imply nothrow.
>>>>>>> I've
>>>>>>> actually always thought they *do* imply it - and
said so on-list :-) -
>>>>>>> but
>>>>>>> it looks like GCC itself doesn't interpret them
that way. E.g. see
>>>>>>> John
>>>>>>> Regher's example here: https://t.co/REzy5m1tT3
>>>>>>> So there's at least one use-case for possibly
throwing
>>>>>>> readonly/readnone.
>>>>>> One important thing to note then: clang marks const and
pure functions
>>>>>> as nounwind *explicitly*.  That needs to be fixed.
>>>>>>
>>>>>> https://godbolt.org/g/SMF4C9
>>>>>>
>>>>> Hah. So it does.
>>>>> Eric, this was originally your change. Do I understand
GCC's behavior
>>>>> incorrectly?
>>>>
>>>> No, but I was in the office when Kenny wrote ipa-pure-const,
which is the
>>>> equivalent to llvm's pass to find function attributions,
and it mostly
>>>> wasn't thought about.
>>>>
>>>> GCC isn't as consistent as one may think here.
>>>>
>>>>              /* Non-looping const functions always return
normally.
>>>>             Otherwise the call might not return or have
side-effects
>>>>             that forbids hoisting possibly trapping expressions
>>>>             before it.  */
>>>>              int flags = gimple_call_flags (stmt);
>>>>              if (!(flags & ECF_CONST)
>>>>              || (flags & ECF_LOOPING_CONST_OR_PURE))
>>>>            BB_MAY_NOTRETURN (block) = 1;
>>>>            }
>>>>
>>>> It also, for example, will do this:
>>>>    double cos (double) __attribute__ ((const));
>>>>    double sin (double) __attribute__ ((const));
>>>>    double f(double a)
>>>>    {
>>>>      double b;
>>>>      double c,d;
>>>>      double (*fp) (double) __attribute__ ((const));
>>>>      /* Partially redundant call */
>>>>      if (a < 2.0)
>>>>        {
>>>>          fp = sin;
>>>>          c = fp (a);
>>>>        }
>>>>      else
>>>>        {
>>>>          c = 1.0;
>>>>          fp = cos;
>>>>        }
>>>>      d = fp (a);
>>>>      return d + c;
>>>>    }
>>>>
>>>>
>>>> into
>>>>    double cos (double) __attribute__ ((const));
>>>>    double sin (double) __attribute__ ((const));
>>>>    double f(double a)
>>>>    {
>>>>      double b;
>>>>      double c,d;
>>>>      double (*fp) (double) __attribute__ ((const));
>>>>      /* Partially redundant call */
>>>>      if (a < 2.0)
>>>>        {
>>>>          fp = sin;
>>>>          c = fp (a);
>>>>        }
>>>>      else
>>>>        {
>>>>          c = 1.0;
>>>>          fp = cos;
>>>>          temp = fp(a)
>>>>        }
>>>>      d = phi(c, temp)
>>>>      return d + c;
>>>>    }
>>>>
>>>> This only works if the second call to sin is guaranteed not to
throw, no?
>>>>
>>>> In any case, optimizations check throwing separately from
pure/const, but
>>>> i'm not sure it's well thought out here.
>>>>
>>>> Note that GCC also has a notion of possibly infinite looping
pure/const
>>>> as
>>>> well:)
>>>>
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org
>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>> --
>> Hal Finkel
>> Lead, Compiler Technology and Programming Languages
>> Leadership Computing Facility
>> Argonne National Laboratory
>>
-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory