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;
2) Most importantly, the above non standard approaches block
interprocedural optimizations such as inlining. With the introduction
of buitlin_dispatch and the clear semantics, compiler can do more
aggressive optimization.
Multi-way dispatch will be good, but most of the use cases we have
seen is 2-way -- a fast path + a default path.
Who are the targeted consumer of the feature?
1) For developers who like to MV function manually;
2) For user directed function cloning
e.g,
int foo (...) __attribute__((clone ("sse")):
3) Automatic function cloning determined by compiler.
>
> I am not sure that combining the function choice and its invocation
> to a single builtin is good. First, you use variadic arguments for
> the actual function arguments which will cause vararg promotions
> to apply and thus it will be impossible to dispatch to functions
> taking single precision float values (and dependent on ABI details
> many more similar issues). Second, you restrict yourself to
> only two versions of a function - that looks quite limited and this
> restriction is not easy to lift with your proposed interface.
See above. Multi-way dispatch can be added similarly.
>
> 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.
>
>> What happens with cloning, -fclone-hot-version-paths ?
>> -----------------------------------------------------
>
> Now, here you lost me somewhat, because I didn't look into the
> patch details and I am missing an example on how the lowered
> IL looks before that cloning. So for now I suppose this
> -fclone-hot-version-paths
> is to expose direct calls to the IL. If you would lower __builtin_dispatch
> to a style like
>
> int sel = selector ();
> switch (sel)
> {
> case 0:
> fn = popcnt;
> break;
> case 1:
> fn = popcnt_sse4;
> break;
> }
> res = (*fn) (25);
>
> then rather than a new pass specialized for __builtin_dispatch existing
> optimization passes that are able to do tracing like VRP and DOM
> via jump-threading or the tracer pass should be able to do this
> optimization for you. If they do not use FDO in a good way it is better
> to improve them for this case instead of writing a new pass.
What you describe may not work
1) the function selection may happen in a different function;
2) Compiler will need to hoist the selection into the program
initializer to avoid overhead
As an example of why dispatch hoisting and call chain cloning is needed:
void foo();
void bar();
void doit_v1();
void doit_v2();
bool check_v () __attribute__((const));
int test();
void bar()
{
....
for (.....)
{
foo();
....
}
}
void foo()
{
...
for (...)
{
__builtin_dispatch(check_v, doit_v1, doit_v2,...);
...
}
}
int test ()
{
..
bar ();
}
The feature testing and dispatch is embedded in a 2-deep loop nest
crossing function boundaries. The call paths test --> bar --> foo
needs to be cloned. This is done by hoisting dispatch up the call
chain -- it ends up :
void bar_v1()
{
....
for (..)
{
foo_v1 ();
}
..
}
void bar_v2 ()
{
...
for (..)
{
foo_v2();
}
..
}
void foo_v1 ()
{
..
for ()
{
doit_v1()
}
}
void foo_v2 ()
{
..
for ()
{
doit_v2()
}
}
int test ()
{
__builtin_dispatch(check_v, bar_v1, bar_v2);
..
}
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 <tmsri...@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.
>
