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
+ && 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 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..1a426b1328b
--- /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 "-O -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" } }
--
2.35.1.455.g1a4874565f
