[google] Patch to support calling multi-versioned functions via new GCC builtin. (issue4440078)
Sriraman Tallam
tmsriram@google.com
Mon May 2 20:38:00 GMT 2011
Thanks. I committed this patch.
-Sri.
svn commit
Sending gcc/ChangeLog.google-main
Sending gcc/Makefile.in
Sending gcc/builtin-types.def
Sending gcc/builtins.def
Sending gcc/c-family/c-common.c
Sending gcc/common.opt
Sending gcc/doc/invoke.texi
Adding gcc/mversn-dispatch.c
Sending gcc/params.def
Sending gcc/passes.c
Adding gcc/testsuite/g++.dg/mversn10.C
Adding gcc/testsuite/g++.dg/mversn10a.C
Adding gcc/testsuite/g++.dg/mversn12.C
Adding gcc/testsuite/g++.dg/mversn14.C
Adding gcc/testsuite/g++.dg/mversn14a.C
Adding gcc/testsuite/g++.dg/mversn16.C
Adding gcc/testsuite/g++.dg/mversn8.C
Adding gcc/testsuite/g++.dg/mversn9.C
Adding gcc/testsuite/g++.dg/torture/mversn11.C
Adding gcc/testsuite/g++.dg/torture/mversn5.C
Adding gcc/testsuite/g++.dg/torture/mversn5.h
Adding gcc/testsuite/g++.dg/torture/mversn5a.C
Adding gcc/testsuite/g++.dg/tree-prof/mversn13.C
Adding gcc/testsuite/g++.dg/tree-prof/mversn15.C
Adding gcc/testsuite/g++.dg/tree-prof/mversn15a.C
Adding gcc/testsuite/gcc.dg/mversn2.c
Adding gcc/testsuite/gcc.dg/mversn3.c
Adding gcc/testsuite/gcc.dg/mversn4.c
Adding gcc/testsuite/gcc.dg/mversn4.h
Adding gcc/testsuite/gcc.dg/mversn4a.c
Adding gcc/testsuite/gcc.dg/mversn6.c
Adding gcc/testsuite/gcc.dg/mversn7.c
Adding gcc/testsuite/gcc.dg/torture/mversn1.c
Sending gcc/timevar.def
Sending gcc/tree-pass.h
Transmitting file data ...................................
Committed revision 173271.
On Mon, May 2, 2011 at 12:32 PM, Xinliang David Li <davidxl@google.com> wrote:
> 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.
>>>>>>
>>>>>
>>>>
>>>
>>
>
More information about the Gcc-patches
mailing list