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

[Patrick Palka <ppalka@redhat.com>]

On Tue, 15 Mar 2022, Jason Merrill wrote:

> 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.

Like his perhaps?  I opted to document the current scope of the flag
(which only cares about a fixed set of functions) as opposed to its
future scope (folding all sufficiently simple inline functions).

-- >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 this folding with -O0 (which implies -fno-inline).

After this patch with -O2 and a non-checking compiler, debug info size
for some testcases from range-v3 and cmcstl2 decreases by as much as ~10%
and overall compile time and memory usage decreases by ~2%.

	PR c++/96780

gcc/c-family/ChangeLog:

	* c.opt: Add -ffold-simple-inlines.

gcc/ChangeLog:

	* doc/invoke.texi (C++ Dialect Options): Document
	-ffold-simple-inlines.

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/doc/invoke.texi                | 10 ++++++++
 gcc/testsuite/g++.dg/opt/pr96780.C | 38 ++++++++++++++++++++++++++++++
 4 files changed, 89 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..e4c2644af15 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 argument 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/doc/invoke.texi b/gcc/doc/invoke.texi
index 2a14e1a9472..d65979bba3f 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -3124,6 +3124,16 @@ On targets that support symbol aliases, the default is
 @option{-fextern-tls-init}.  On targets that do not support symbol
 aliases, the default is @option{-fno-extern-tls-init}.
 
+@item -ffold-simple-inlines
+@itemx -fno-fold-simple-inlines
+@opindex ffold-simple-inlines
+@opindex fno-fold-simple-inlines
+Permit the C++ frontend to fold calls to @code{std::move}, @code{std::forward},
+@code{std::addressof} and @code{std::as_const}.  In contrast to inlining, this
+means no debug information will be generated for such calls.  Since these
+functions are rarely interesting to debug, this flag is enabled by default
+unless @option{-fno-inline} is active.
+
 @item -fno-gnu-keywords
 @opindex fno-gnu-keywords
 @opindex fgnu-keywords
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" } }
-- 
2.35.1.500.gb896f729e2