Re: [PATCH] c++: fold calls to std::move/forward [PR96780]

[Jason Merrill <jason@redhat.com>]

On 3/15/22 10:03, Patrick Palka wrote:
> On Mon, 14 Mar 2022, Jason Merrill wrote:
> 
>> On 3/14/22 13:13, Patrick Palka wrote:
>>> On Fri, 11 Mar 2022, Jason Merrill wrote:
>>>
>>>> On 3/10/22 11:27, Patrick Palka wrote:
>>>>> On Wed, 9 Mar 2022, Jason Merrill wrote:
>>>>>
>>>>>> On 3/1/22 18:08, Patrick Palka wrote:
>>>>>>> A well-formed call to std::move/forward is equivalent to a cast, but
>>>>>>> the
>>>>>>> former being a function call means it comes with bloated debug info,
>>>>>>> which
>>>>>>> persists even after the call has been inlined away, for an operation
>>>>>>> that
>>>>>>> is never interesting to debug.
>>>>>>>
>>>>>>> This patch addresses this problem in a relatively ad-hoc way by
>>>>>>> folding
>>>>>>> calls to std::move/forward into casts as part of the frontend's
>>>>>>> general
>>>>>>> expression folding routine.  After this patch with -O2 and a
>>>>>>> non-checking
>>>>>>> compiler, debug info size for some testcases decreases by about ~10%
>>>>>>> and
>>>>>>> overall compile time and memory usage decreases by ~2%.
>>>>>>
>>>>>> Impressive.  Which testcases?
>>>>>
>>>>> I saw the largest percent reductions in debug file object size in
>>>>> various tests from cmcstl2 and range-v3, e.g.
>>>>> test/algorithm/set_symmetric_difference4.cpp and .../rotate_copy.cpp
>>>>> (which are among their biggest tests).
>>>>>
>>>>> Significant reductions in debug object file size can be observed in
>>>>> some libstdc++ testcases too, such as a 5.5% reduction in
>>>>> std/ranges/adaptor/join.cc
>>>>>
>>>>>>
>>>>>> Do you also want to handle addressof and as_const in this patch, as
>>>>>> Jonathan
>>>>>> suggested?
>>>>>
>>>>> Yes, good idea.  Since each of their argument and return types are
>>>>> indirect types, I think we can use the same NOP_EXPR-based folding for
>>>>> them.
>>>>>
>>>>>>
>>>>>> I think we can do this now, and think about generalizing more in stage
>>>>>> 1.
>>>>>>
>>>>>>> Bootstrapped and regtested on x86_64-pc-linux-gnu, is this something
>>>>>>> we
>>>>>>> want to consider for GCC 12?
>>>>>>>
>>>>>>> 	PR c++/96780
>>>>>>>
>>>>>>> gcc/cp/ChangeLog:
>>>>>>>
>>>>>>> 	* cp-gimplify.cc (cp_fold) <case CALL_EXPR>: When optimizing,
>>>>>>> 	fold calls to std::move/forward into simple casts.
>>>>>>> 	* cp-tree.h (is_std_move_p, is_std_forward_p): Declare.
>>>>>>> 	* typeck.cc (is_std_move_p, is_std_forward_p): Export.
>>>>>>>
>>>>>>> gcc/testsuite/ChangeLog:
>>>>>>>
>>>>>>> 	* g++.dg/opt/pr96780.C: New test.
>>>>>>> ---
>>>>>>>      gcc/cp/cp-gimplify.cc              | 18 ++++++++++++++++++
>>>>>>>      gcc/cp/cp-tree.h                   |  2 ++
>>>>>>>      gcc/cp/typeck.cc                   |  6 ++----
>>>>>>>      gcc/testsuite/g++.dg/opt/pr96780.C | 24 ++++++++++++++++++++++++
>>>>>>>      4 files changed, 46 insertions(+), 4 deletions(-)
>>>>>>>      create mode 100644 gcc/testsuite/g++.dg/opt/pr96780.C
>>>>>>>
>>>>>>> diff --git a/gcc/cp/cp-gimplify.cc b/gcc/cp/cp-gimplify.cc
>>>>>>> index d7323fb5c09..0b009b631c7 100644
>>>>>>> --- a/gcc/cp/cp-gimplify.cc
>>>>>>> +++ b/gcc/cp/cp-gimplify.cc
>>>>>>> @@ -2756,6 +2756,24 @@ cp_fold (tree x)
>>>>>>>            case CALL_EXPR:
>>>>>>>            {
>>>>>>> +	if (optimize
>>>>>>
>>>>>> I think this should check flag_no_inline rather than optimize.
>>>>>
>>>>> Sounds good.
>>>>>
>>>>> Here's a patch that extends the folding to as_const and addressof (as
>>>>> well as __addressof, which I'm kind of unsure about since it's
>>>>> non-standard).  I suppose it also doesn't hurt to verify that the return
>>>>> and argument type of the function are sane before we commit to folding.
>>>>>
>>>>> -- >8 --
>>>>>
>>>>> Subject: [PATCH] c++: fold calls to std::move/forward [PR96780]
>>>>>
>>>>> A well-formed call to std::move/forward is equivalent to a cast, but the
>>>>> former being a function call means the compiler generates debug info for
>>>>> it, which persists even after the call has been inlined away, for an
>>>>> operation that's never interesting to debug.
>>>>>
>>>>> This patch addresses this problem in a relatively ad-hoc way by folding
>>>>> calls to std::move/forward and other cast-like functions into simple
>>>>> casts as part of the frontend's general expression folding routine.
>>>>> After this patch with -O2 and a non-checking compiler, debug info size
>>>>> for some testcases decreases by about ~10% and overall compile time and
>>>>> memory usage decreases by ~2%.
>>>>>
>>>>> 	PR c++/96780
>>>>>
>>>>> gcc/cp/ChangeLog:
>>>>>
>>>>> 	* cp-gimplify.cc (cp_fold) <case CALL_EXPR>: When optimizing,
>>>>> 	fold calls to std::move/forward and other cast-like functions
>>>>> 	into simple casts.
>>>>>
>>>>> gcc/testsuite/ChangeLog:
>>>>>
>>>>> 	* g++.dg/opt/pr96780.C: New test.
>>>>> ---
>>>>>     gcc/cp/cp-gimplify.cc              | 36 +++++++++++++++++++++++++++-
>>>>>     gcc/testsuite/g++.dg/opt/pr96780.C | 38
>>>>> ++++++++++++++++++++++++++++++
>>>>>     2 files changed, 73 insertions(+), 1 deletion(-)
>>>>>     create mode 100644 gcc/testsuite/g++.dg/opt/pr96780.C
>>>>>
>>>>> diff --git a/gcc/cp/cp-gimplify.cc b/gcc/cp/cp-gimplify.cc
>>>>> index d7323fb5c09..efc4c8f0eb9 100644
>>>>> --- a/gcc/cp/cp-gimplify.cc
>>>>> +++ b/gcc/cp/cp-gimplify.cc
>>>>> @@ -2756,9 +2756,43 @@ cp_fold (tree x)
>>>>>           case CALL_EXPR:
>>>>>           {
>>>>> -	int sv = optimize, nw = sv;
>>>>>     	tree callee = get_callee_fndecl (x);
>>>>>     +	/* "Inline" calls to std::move/forward and other cast-like
>>>>> functions
>>>>> +	   by simply folding them into the corresponding cast determined by
>>>>> +	   their return type.  This is cheaper than relying on the middle-end
>>>>> +	   to do so, and also means we avoid generating useless debug info for
>>>>> +	   them at all.
>>>>> +
>>>>> +	   At this point the argument has already been converted into a
>>>>> +	   reference, so it suffices to use a NOP_EXPR to express the
>>>>> +	   cast.  */
>>>>> +	if (!flag_no_inline
>>>>
>>>> In our conversation yesterday it occurred to me that we might make this a
>>>> separate flag that defaults to the value of flag_no_inline; I was thinking
>>>> of
>>>> -ffold-simple-inlines.  Then Vittorio et al can specify that explicitly at
>>>> -O0
>>>> if they'd like.
>>>
>>> Makes sense, like so?  Bootstrapped and regtested on x86_64-pc-linux-gnu.
>>>
>>> The patch defaults -ffold-simple-inlines according to the value of
>>> flag_no_inline at startup.  IIUC this means that if the flag has been
>>> defaulted to set, then e.g. an optimize("O0") function attribute won't
>>> disable -ffold-simple-inlines for that function, since we only compute
>>> its default value once.
>>>
>>> I wonder if we therefore instead want to handle defaulting the flag
>>> when it's used, e.g. check
>>>
>>>     (flag_fold_simple_inlines == -1
>>>      ? flag_no_inline
>>>      : flag_fold_simple_inlines)
>>>
>>> instead of
>>>
>>>     flag_fold_simple_inlines
>>>
>>> in cp_fold?
>>
>> I guess that makes sense, we can't add front-end options to the
>> default_options_table.  But I think let's use OPTION_SET_P instead of checking
>> for -1.
> 
> Done.
> 
>>
>>> -- >8 --
>>>
>>> Subject: [PATCH] c++: fold calls to std::move/forward [PR96780]
>>>
>>> A well-formed call to std::move/forward is equivalent to a cast, but the
>>> former being a function call means the compiler generates debug info for
>>> it, which persists even after the call has been inlined away, for an
>>> operation that's never interesting to debug.
>>>
>>> This patch addresses this problem by folding calls to std::move/forward
>>> and other cast-like functions into simple casts as part of the frontend's
>>> general expression folding routine.  This behavior is controlled by a
>>> new flag -ffold-simple-inlines which defaults to the value of -fno-inline.
>>>
>>> After this patch with -O2 and a non-checking compiler, debug info size
>>> for some testcases (e.g. from range-v3 and cmcstl2) decreases by about
>>> ~10% and overall compile time and memory usage decreases by ~2%.
>>
>> Did you compare the reduction after handling more functions?
> 
> The numbers are roughly the same, which I guess is not too surprising
> since calls to std::move/forward outnumber the other functions by about
> 10:1 in libstdc++, range-v3 and cmcstl2.
> 
> The biggest reduction in debug object file size (measured by du) I've
> observed is 14% with range-v3's test/algorithm/stable_partition.cpp.
> The biggest reduction in peak memory usage is (measured by /usr/bin/time -v)
> is 5% with cmcstl's test/algorithm/set_symmetric_difference4.cpp.  The
> biggest reduction in compile time (measured by perf stat) is about 3%,
> also from that testcase.
> 
> -- >8 --
> 
> Subject: [PATCH] c++: fold calls to std::move/forward [PR96780]
> 
> A well-formed call to std::move/forward is equivalent to a cast, but the
> former being a function call means the compiler generates debug info for
> it, which persists even after the call has been inlined away, for an
> operation that's never interesting to debug.
> 
> This patch addresses this problem by folding calls to std::move/forward
> and other cast-like functions into simple casts as part of the frontend's
> general expression folding routine.  This behavior is controlled by a
> new flag -ffold-simple-inlines, and otherwise by -fno-inline, so that
> users can enable such folding even with -O0 (which implies -fno-inline).
> 
> After this patch with -O2 and a non-checking compiler, debug info size
> for some testcases (e.g. from range-v3 and cmcstl2) decreases by about
> ~10% and overall compile time and memory usage decreases by ~2%.
> 
> 	PR c++/96780
> 
> gcc/c-family/ChangeLog:
> 
> 	* c-opts.cc (c_common_post_options): Handle defaulting of
> 	flag_fold_simple_inlines.
> 	* c.opt: Add -ffold-simple-inlines.

Looks like you still need a doc/invoke.texi change for the new flag. 
The rest of the patch looks good.

> gcc/cp/ChangeLog:
> 
> 	* cp-gimplify.cc (cp_fold) <case CALL_EXPR>: Fold calls to
> 	std::move/forward and other cast-like functions into simple
> 	casts.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/opt/pr96780.C: New test.
> ---
>   gcc/c-family/c.opt                 |  4 ++++
>   gcc/cp/cp-gimplify.cc              | 38 +++++++++++++++++++++++++++++-
>   gcc/testsuite/g++.dg/opt/pr96780.C | 38 ++++++++++++++++++++++++++++++
>   3 files changed, 79 insertions(+), 1 deletion(-)
>   create mode 100644 gcc/testsuite/g++.dg/opt/pr96780.C
> 
> diff --git a/gcc/c-family/c.opt b/gcc/c-family/c.opt
> index 9cfd2a6bc4e..9a4828ebe37 100644
> --- a/gcc/c-family/c.opt
> +++ b/gcc/c-family/c.opt
> @@ -1731,6 +1731,10 @@ Support dynamic initialization of thread-local variables in a different translat
>   fexternal-templates
>   C++ ObjC++ WarnRemoved
>   
> +ffold-simple-inlines
> +C++ ObjC++ Optimization Var(flag_fold_simple_inlines)
> +Fold calls to simple inline functions.
> +
>   ffor-scope
>   C++ ObjC++ WarnRemoved
>   
> diff --git a/gcc/cp/cp-gimplify.cc b/gcc/cp/cp-gimplify.cc
> index d7323fb5c09..41daaf18725 100644
> --- a/gcc/cp/cp-gimplify.cc
> +++ b/gcc/cp/cp-gimplify.cc
> @@ -41,6 +41,7 @@ along with GCC; see the file COPYING3.  If not see
>   #include "file-prefix-map.h"
>   #include "cgraph.h"
>   #include "omp-general.h"
> +#include "opts.h"
>   
>   /* Forward declarations.  */
>   
> @@ -2756,9 +2757,44 @@ cp_fold (tree x)
>   
>       case CALL_EXPR:
>         {
> -	int sv = optimize, nw = sv;
>   	tree callee = get_callee_fndecl (x);
>   
> +	/* "Inline" calls to std::move/forward and other cast-like functions
> +	   by simply folding them into a corresponding cast to their return
> +	   type.  This is cheaper than relying on the middle-end to do so, and
> +	   also means we avoid generating useless debug info for them at all.
> +
> +	   At this point the argument has already been converted into a
> +	   reference, so it suffices to use a NOP_EXPR to express the
> +	   cast.  */
> +	if ((OPTION_SET_P (flag_fold_simple_inlines)
> +	     ? flag_fold_simple_inlines
> +	     : !flag_no_inline)
> +	    && call_expr_nargs (x) == 1
> +	    && decl_in_std_namespace_p (callee)
> +	    && DECL_NAME (callee) != NULL_TREE
> +	    && (id_equal (DECL_NAME (callee), "move")
> +		|| id_equal (DECL_NAME (callee), "forward")
> +		|| id_equal (DECL_NAME (callee), "addressof")
> +		/* This addressof equivalent is used heavily in libstdc++.  */
> +		|| id_equal (DECL_NAME (callee), "__addressof")
> +		|| id_equal (DECL_NAME (callee), "as_const")))
> +	  {
> +	    r = CALL_EXPR_ARG (x, 0);
> +	    /* Check that the return and arguments types are sane before
> +	       folding.  */
> +	    if (INDIRECT_TYPE_P (TREE_TYPE (x))
> +		&& INDIRECT_TYPE_P (TREE_TYPE (r)))
> +	      {
> +		if (!same_type_p (TREE_TYPE (x), TREE_TYPE (r)))
> +		  r = build_nop (TREE_TYPE (x), r);
> +		x = cp_fold (r);
> +		break;
> +	      }
> +	  }
> +
> +	int sv = optimize, nw = sv;
> +
>   	/* Some built-in function calls will be evaluated at compile-time in
>   	   fold ().  Set optimize to 1 when folding __builtin_constant_p inside
>   	   a constexpr function so that fold_builtin_1 doesn't fold it to 0.  */
> diff --git a/gcc/testsuite/g++.dg/opt/pr96780.C b/gcc/testsuite/g++.dg/opt/pr96780.C
> new file mode 100644
> index 00000000000..61e11855eeb
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/opt/pr96780.C
> @@ -0,0 +1,38 @@
> +// PR c++/96780
> +// Verify calls to std::move/forward are folded away by the frontend.
> +// { dg-do compile { target c++11 } }
> +// { dg-additional-options "-ffold-simple-inlines -fdump-tree-gimple" }
> +
> +#include <utility>
> +
> +struct A;
> +
> +extern A& a;
> +extern const A& ca;
> +
> +void f() {
> +  auto&& x1 = std::move(a);
> +  auto&& x2 = std::forward<A>(a);
> +  auto&& x3 = std::forward<A&>(a);
> +
> +  auto&& x4 = std::move(ca);
> +  auto&& x5 = std::forward<const A>(ca);
> +  auto&& x6 = std::forward<const A&>(ca);
> +
> +  auto x7 = std::addressof(a);
> +  auto x8 = std::addressof(ca);
> +#if __GLIBCXX__
> +  auto x9 = std::__addressof(a);
> +  auto x10 = std::__addressof(ca);
> +#endif
> +#if __cpp_lib_as_const
> +  auto&& x11 = std::as_const(a);
> +  auto&& x12 = std::as_const(ca);
> +#endif
> +}
> +
> +// { dg-final { scan-tree-dump-not "= std::move" "gimple" } }
> +// { dg-final { scan-tree-dump-not "= std::forward" "gimple" } }
> +// { dg-final { scan-tree-dump-not "= std::addressof" "gimple" } }
> +// { dg-final { scan-tree-dump-not "= std::__addressof" "gimple" } }
> +// { dg-final { scan-tree-dump-not "= std::as_const" "gimple" } }