Re: [PATCH] libstdc++: Remove extern "C" from Ryu sources

[Patrick Palka <ppalka@redhat.com>]

On Tue, 11 May 2021, Patrick Palka wrote:

> floating_to_chars.cc includes the Ryu sources into an anonymous
> namespace as a convenient way to give all its symbols internal linkage.
> But an entity declared extern "C" always has external linkage, even
> from within an anonymous namespace, so this trick doesn't work in the
> presence of extern "C", and it causes the Ryu function generic_to_chars
> to be visible from libstdc++.a.
> 
> This patch removes the only use of extern "C" from our local copy of
> Ryu, along with some declarations for never-defined functions that GCC
> now warns about.
> 
> Tested on x86_64-pc-linux-gnu, and also verified that generic_to_chars
> is not visible from libstdc++.a.  Does this look OK for trunk and the 11
> branch?

Now with a testcase:

-- >8 --

Subject: [PATCH] libstdc++: Remove extern "C" from Ryu sources

floating_to_chars.cc includes the Ryu sources into an anonymous
namespace as a convenient way to give all its symbols internal linkage.
But an entity declared extern "C" always has external linkage, even
from within an anonymous namespace, so this trick doesn't work in the
presence of extern "C", and it causes the Ryu function generic_to_chars
to be visible from libstdc++.a.

This patch removes the only use of extern "C" from our local copy of
Ryu along with some declarations for never-defined functions that GCC
now warns about.

Tested on x86_64-pc-linux-gnu, and also verified that generic_to_chars
is not visible from libstdc++.a.  Does this look OK for trunk and the 11
branch?

libstdc++-v3/ChangeLog:

	* src/c++17/ryu/LOCAL_PATCHES: Update.
	* src/c++17/ryu/ryu_generic_128.h: Remove extern "C".
	Remove declarations for never-defined functions.
	* testsuite/20_util/to_chars/4.cc: New test.
---
 libstdc++-v3/src/c++17/ryu/LOCAL_PATCHES     |  1 +
 libstdc++-v3/src/c++17/ryu/ryu_generic_128.h | 21 ++----------
 libstdc++-v3/testsuite/20_util/to_chars/4.cc | 36 ++++++++++++++++++++
 3 files changed, 40 insertions(+), 18 deletions(-)
 create mode 100644 libstdc++-v3/testsuite/20_util/to_chars/4.cc

diff --git a/libstdc++-v3/src/c++17/ryu/LOCAL_PATCHES b/libstdc++-v3/src/c++17/ryu/LOCAL_PATCHES
index 51e504cb6ea..72ffad9662d 100644
--- a/libstdc++-v3/src/c++17/ryu/LOCAL_PATCHES
+++ b/libstdc++-v3/src/c++17/ryu/LOCAL_PATCHES
@@ -1,2 +1,3 @@
 r11-6248
 r11-7636
+r12-XXX
diff --git a/libstdc++-v3/src/c++17/ryu/ryu_generic_128.h b/libstdc++-v3/src/c++17/ryu/ryu_generic_128.h
index 2afbf274e11..6d988ab01eb 100644
--- a/libstdc++-v3/src/c++17/ryu/ryu_generic_128.h
+++ b/libstdc++-v3/src/c++17/ryu/ryu_generic_128.h
@@ -18,9 +18,9 @@
 #define RYU_GENERIC_128_H
 
 
-#ifdef __cplusplus
-extern "C" {
-#endif
+// NOTE: These symbols are declared extern "C" upstream, but we don't want that
+// because it'd override the internal linkage of the anonymous namespace into
+// which this header is included.
 
 // This is a generic 128-bit implementation of float to shortest conversion
 // using the Ryu algorithm. It can handle any IEEE-compatible floating-point
@@ -42,18 +42,6 @@ struct floating_decimal_128 {
   bool sign;
 };
 
-struct floating_decimal_128 float_to_fd128(float f);
-struct floating_decimal_128 double_to_fd128(double d);
-
-// According to wikipedia (https://en.wikipedia.org/wiki/Long_double), this likely only works on
-// x86 with specific compilers (clang?). May need an ifdef.
-struct floating_decimal_128 long_double_to_fd128(long double d);
-
-// Converts the given binary floating point number to the shortest decimal floating point number
-// that still accurately represents it.
-struct floating_decimal_128 generic_binary_to_decimal(
-    const uint128_t bits, const uint32_t mantissaBits, const uint32_t exponentBits, const bool explicitLeadingBit);
-
 // Converts the given decimal floating point number to a string, writing to result, and returning
 // the number characters written. Does not terminate the buffer with a 0. In the worst case, this
 // function can write up to 53 characters.
@@ -63,8 +51,5 @@ struct floating_decimal_128 generic_binary_to_decimal(
 // = 1 + 39 + 1 + 1 + 1 + 10 = 53
 int generic_to_chars(const struct floating_decimal_128 v, char* const result);
 
-#ifdef __cplusplus
-}
-#endif
 
 #endif // RYU_GENERIC_128_H
diff --git a/libstdc++-v3/testsuite/20_util/to_chars/4.cc b/libstdc++-v3/testsuite/20_util/to_chars/4.cc
new file mode 100644
index 00000000000..96f6e5d010c
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/to_chars/4.cc
@@ -0,0 +1,36 @@
+// Copyright (C) 2021 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-do link { target c++17 } }
+// { dg-require-effective-target ieee-floats }
+// { dg-require-static-libstdcxx }
+// { dg-additional-options "-static-libstdc++" }
+
+// Verify the Ryu symbol generic_to_chars doesn't inadvertently leak into
+// libstdc++.a.  If it did, this test would fail at link time with a multiple
+// definition error.
+
+#include <charconv>
+
+extern "C" void generic_to_chars (void) { __builtin_abort(); }
+
+int
+main()
+{
+  char x[64];
+  std::to_chars(x, x+64, 42.L, std::chars_format::scientific);
+}
-- 
2.31.1.527.g2d677e5b15