Re: [google] Patch to support calling multi-versioned functions via new GCC builtin. (issue4440078)

[Xinliang David Li <davidxl at google dot com>]

Ok. There may be more correctness related comments -- but those can be
addressed when available. For trunk, you need to address issues such
as multi-way dispatch.

Thanks,

David

On Mon, May 2, 2011 at 12:11 PM, Sriraman Tallam <tmsriram@google.com> wrote:
> Hi,
>
> ?I want to submit this patch to google/main to make sure it is
> available for our internal use at Google in order to materialize some
> optimization opportunities. Let us continue this dicussion as I make
> changes and submit this for review for trunk.
>
> Thanks,
> -Sri.
>
>
> On Mon, May 2, 2011 at 9:41 AM, Xinliang David Li <davidxl@google.com> wrote:
>> On Mon, May 2, 2011 at 2:11 AM, Richard Guenther
>> <richard.guenther@gmail.com> wrote:
>>> On Fri, Apr 29, 2011 at 6:23 PM, Xinliang David Li <davidxl@google.com> wrote:
>>>> Here is the background for this feature:
>>>>
>>>> 1) People relies on function multi-version to explore hw features and
>>>> squeeze performance, but there is no standard ways of doing so, either
>>>> a) using indirect function calls with function pointers set at program
>>>> initialization; b) using manual dispatch at each callsite; b) using
>>>> features like IFUNC. ?The dispatch mechanism needs to be promoted to
>>>> the language level and becomes the first class citizen;
>>>
>>> You are not doing that, you are inventing a new (crude) GCC extension.
>>
>> To capture the high level semantics and prevent user from lowering the
>> dispatch calls into forms compiler can not recognize, language
>> extension is the way to go.
>>
>>>
>>
>>>> See above. Multi-way dispatch can be added similarly.
>>>
>>> Not with the specified syntax. ?So you'd end up with _two_
>>> language extensions. ?That's bad (and unacceptable, IMHO).
>>
>> This part can be improved.
>>
>>>
>>>>
>>>>>
>>>>> That's a nice idea, but why not simply process all functions with
>>>>> a const attribute and no arguments this way? ?IMHO
>>>>>
>>>>> int testsse (void) __attribute__((const));
>>>>>
>>>>> is as good and avoids the new attribute completely. ?The pass would
>>>>> also benefit other uses of this idiom (giving a way to have global
>>>>> dynamic initializers in C). ?The functions may not be strictly 'const'
>>>>> in a way the compiler autodetects this attribute but it presents
>>>>> exactly the promises to the compiler that allow this optimization.
>>>>>
>>>>> Thus, please split this piece out into a separate patch and use
>>>>> the const attribute.
>>>>
>>>>
>>>> Sounds good.
>>>>
>>
>>>>
>>>> 1) the function selection may happen in a different function;
>>>
>>> Well, sure. ?I propose to have the above as lowered form. ?If people
>>> deliberately obfuscate code it's their fault. ?Your scheme simply
>>> makes that obfuscation impossible (or less likely possible). ?So
>>> 1) is not a valid argument.
>>
>> Lowering too early may defeat the purpose because
>>
>> 1) the desired optimization may not happen subject to many compiler
>> heuristic changes;
>> 2) it has other side effects such as wrong estimation of function size
>> which may impact inlining
>> 3) it limits the lowering into one form which may not be ideal ?--
>> with builtin_dispatch, after hoisting optimization, the lowering can
>> use more efficient IFUNC scheme, for instance.
>>
>>>
>>>
>>> My point is that such optimization is completely independent of
>>> that dispatch thing. ?The above could as well be a selection to
>>> use different input arrays, one causing alias analysis issues
>>> and one not.
>>>
>>> Thus, a __builtin_dispatch centric optimization pass is the wrong
>>> way to go.
>>
>> I agree that many things can implemented in different ways, but a high
>> level standard builtin_dispatch mechanism doing hoisting
>> interprocedcurally is cleaner and simpler and targeted for function
>> multi-versioning -- it does not depend on/rely on later phase's
>> heuristic tunings to make the right things to happen. Function MV
>> deserves this level of treatment as it will become more and more
>> important for some users (e.g., Google).
>>
>>>
>>> Now, with FDO I'd expect the foo is inlined into bar (because foo
>>> is deemed hot),
>>
>> That is a myth -- the truth is that there are other heuristics which
>> can prevent this from happening.
>>
>>> then you only need to deal with loop unswitching,
>>> which should be easy to drive from FDO.
>>
>> Same here -- the loop body may not be well formed/recognized. The loop
>> nests may not be perfectly nested, or other unswitching heuristics may
>> block it from happening. ?This is the common problem form many other
>> things that get lowered too early. It is cleaner to make the high
>> level transformation first in IPA, and let unswitching dealing with
>> intra-procedural optimization.
>>
>> Thanks,
>>
>> David
>>
>>>
>>> Richard.
>>>
>>>>
>>>>
>>>> Thanks,
>>>>
>>>> David
>>>>
>>>>
>>>>
>>>>>
>>>>> Note that we already have special FDO support for indirect to direct
>>>>> call promotion, so that might work already.
>>>>>
>>>>> Thus, with all this, __builtin_dispatch would be more like syntactic
>>>>> sugar to avoid writing above switch statement yourself. ?If you restrict
>>>>> that sugar to a constant number of candidates it can be replaced by
>>>>> a macro (or several ones, specialized for the number of candidates).
>>>>>
>>>>> Richard.
>>>>>
>>>>>> ?For the call graph that looks like this before cloning :
>>>>>>
>>>>>> func_cold ----> func_hot ----> findOnes ----> IsPopCntSupported ----> popcnt
>>>>>> ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?|
>>>>>> ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?-----> no_popcnt
>>>>>>
>>>>>> ?where popcnt and no_popcnt are the multi-versioned functions, then after cloning :
>>>>>>
>>>>>> func_cold ---->IsPopCntSupported ----> func_hot_clone0 ----> findOnes_clone0 ----> popcnt
>>>>>> ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?|
>>>>>> ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ----> func_hot_clone1 ----> findOnes_clone1 ----> no_popcnt
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>> ?Flags : -fclone-hot-version-paths does function unswitching via cloning.
>>>>>> ? ? ? ? ?--param=num-mversn-clones=<num> allows to specify the number of
>>>>>> ? ? ? ? ?functions that should be cloned.
>>>>>> ? ? ? ? ?--param=mversn-clone-depth=<num> allows to specify the length of
>>>>>> ? ? ? ? ?the call graph path that should be cloned. ?num = 0 implies only
>>>>>> ? ? ? ? ?leaf node that contains the __builtin_dispatch statement must be
>>>>>> ? ? ? ? ?cloned.
>>>>>> ============================================================================================
>>>>>>
>>>>>> Google ref 45947, 45986
>>>>>>
>>>>>> 2011-04-28 ?Sriraman Tallam ?<tmsriram@google.com>
>>>>>>
>>>>>> ? ? ? ?* c-family/c-common.c ? (revision 173122) (handle_version_selector_attribute): New function.
>>>>>> ? ? ? ?(c_common_attribute_table): New attribute "version_selector".
>>>>>> ? ? ? ?* tree-pass.h ? (revision 173122) (pass_tree_convert_builtin_dispatch): New pass.
>>>>>> ? ? ? ?(pass_ipa_multiversion_dispatch): New pass.
>>>>>> ? ? ? ?* testsuite/gcc.dg/mversn7.c ? ?(revision 0): New test case.
>>>>>> ? ? ? ?* testsuite/gcc.dg/mversn4.c ? ?(revision 0): New test case.
>>>>>> ? ? ? ?* testsuite/gcc.dg/mversn4.h ? ?(revision 0): New test case.
>>>>>> ? ? ? ?* testsuite/gcc.dg/mversn4a.c ? (revision 0): New test case.
>>>>>> ? ? ? ?* testsuite/gcc.dg/torture/mversn1.c ? ?(revision 0): New test case.
>>>>>> ? ? ? ?* testsuite/gcc.dg/mversn2.c ? ?(revision 0): New test case.
>>>>>> ? ? ? ?* testsuite/gcc.dg/mversn6.c ? ?(revision 0): New test case.
>>>>>> ? ? ? ?* testsuite/gcc.dg/mversn3.c ? ?(revision 0): New test case.
>>>>>> ? ? ? ?* testsuite/g++.dg/mversn8.C ? ?(revision 0): New test case.
>>>>>> ? ? ? ?* testsuite/g++.dg/mversn10a.C ?(revision 0): New test case.
>>>>>> ? ? ? ?* testsuite/g++.dg/mversn14a.C ?(revision 0): New test case.
>>>>>> ? ? ? ?* testsuite/g++.dg/tree-prof/mversn13.C (revision 0): New test case.
>>>>>> ? ? ? ?* testsuite/g++.dg/tree-prof/mversn15.C (revision 0): New test case.
>>>>>> ? ? ? ?* testsuite/g++.dg/tree-prof/mversn15a.C ? ? ? ?(revision 0): New test case.
>>>>>> ? ? ? ?* testsuite/g++.dg/mversn9.C ? ?(revision 0): New test case.
>>>>>> ? ? ? ?* testsuite/g++.dg/mversn10.C ? (revision 0): New test case.
>>>>>> ? ? ? ?* testsuite/g++.dg/mversn12.C ? (revision 0): New test case.
>>>>>> ? ? ? ?* testsuite/g++.dg/mversn14.C ? (revision 0): New test case.
>>>>>> ? ? ? ?* testsuite/g++.dg/mversn16.C ? (revision 0): New test case.
>>>>>> ? ? ? ?* testsuite/g++.dg/torture/mversn11.C ? (revision 0): New test case.
>>>>>> ? ? ? ?* testsuite/g++.dg/torture/mversn5.C ? ?(revision 0): New test case.
>>>>>> ? ? ? ?* testsuite/g++.dg/torture/mversn5.h ? ?(revision 0): New test case.
>>>>>> ? ? ? ?* testsuite/g++.dg/torture/mversn5a.C ? (revision 0): New test case.
>>>>>> ? ? ? ?* builtin-types.def ? ? (revision 173122) (BT_PTR_FN_INT): New pointer type.
>>>>>> ? ? ? ?(BT_FN_INT_PTR_FN_INT_PTR_PTR_VAR): New function type for __builtin_dispatch.
>>>>>> ? ? ? ?* builtins.def ?(revision 173122) (BUILT_IN_DISPATCH): New builtin to
>>>>>> ? ? ? ?support multi-version calls.
>>>>>> ? ? ? ?* mversn-dispatch.c ? ? (revision 0): New file.
>>>>>> ? ? ? ?* timevar.def ? (revision 173122) (TV_MVERSN_DISPATCH): New time var.
>>>>>> ? ? ? ?* common.opt ? ?(revision 173122) (fclone-hot-version-paths): New flag.
>>>>>> ? ? ? ?* Makefile.in ? (revision 173122) (mversn-dispatch.o): New rule.
>>>>>> ? ? ? ?* passes.c ? ? ?(revision 173122) (init_optimization_passes): Add the new
>>>>>> ? ? ? ?multi-version and dispatch passes to the pass list.
>>>>>> ? ? ? ?* params.def ? ?(revision 173122) (PARAM_NUMBER_OF_MVERSN_CLONES): Define.
>>>>>> ? ? ? ?(PARAM_MVERSN_CLONE_CGRAPH_DEPTH): Define.
>>>>>> ? ? ? ?* doc/invoke.texi ? ? ? (revision 173122) (mversn-clone-depth): Document.
>>>>>> ? ? ? ?(num-mversn-clones): Document.
>>>>>> ? ? ? ?(fclone-hot-version-paths): Document.
>>>>>
>>>>
>>>
>>
>