https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85012
Bug ID: 85012
Summary: GCC does not inline extern "C" functions defined in a
different namespace
Product: gcc
Version: 7.3.1
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: m101010a at gmail dot com
Target Milestone: ---
$ cat x.cpp
namespace foo { extern "C" void f(); }
extern "C" void f() {}
void f1()
{
foo::f();
foo::f();
}
void f2()
{
f();
f();
}
$ g++ -v -S -O1 x.cpp
Using built-in specs.
COLLECT_GCC=g++
Target: x86_64-pc-linux-gnu
Configured with: /build/gcc/src/gcc/configure --prefix=/usr --libdir=/usr/lib
--libexecdir=/usr/lib --mandir=/usr/share/man --infodir=/usr/share/info
--with-bugurl=https://bugs.archlinux.org/
--enable-languages=c,c++,ada,fortran,go,lto,objc,obj-c++ --enable-shared
--enable-threads=posix --enable-libmpx --with-system-zlib --with-isl
--enable-__cxa_atexit --disable-libunwind-exceptions --enable-clocale=gnu
--disable-libstdcxx-pch --disable-libssp --enable-gnu-unique-object
--enable-linker-build-id --enable-lto --enable-plugin
--enable-install-libiberty --with-linker-hash-style=gnu
--enable-gnu-indirect-function --enable-multilib --disable-werror
--enable-checking=release --enable-default-pie --enable-default-ssp
Thread model: posix
gcc version 7.3.1 20180312 (GCC)
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-S' '-O1' '-shared-libgcc'
'-mtune=generic' '-march=x86-64'
/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.1/cc1plus -E -quiet -v -D_GNU_SOURCE
x.cpp -mtune=generic -march=x86-64 -O1 -fpch-preprocess -o x.ii
ignoring nonexistent directory
"/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.1/../../../../x86_64-pc-linux-gnu/include"
#include "..." search starts here:
#include <...> search starts here:
/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.1/../../../../include/c++/7.3.1
/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.1/../../../../include/c++/7.3.1/x86_64-pc-linux-gnu
/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.1/../../../../include/c++/7.3.1/backward
/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.1/include
/usr/local/include
/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.1/include-fixed
/usr/include
End of search list.
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-S' '-O1' '-shared-libgcc'
'-mtune=generic' '-march=x86-64'
/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.1/cc1plus -fpreprocessed x.ii -quiet
-dumpbase x.cpp -mtune=generic -march=x86-64 -auxbase x -O1 -version -o x.s
GNU C++14 (GCC) version 7.3.1 20180312 (x86_64-pc-linux-gnu)
compiled by GNU C version 7.3.1 20180312, GMP version 6.1.2, MPFR
version 4.0.1, MPC version 1.1.0, isl version isl-0.18-GMP
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
GNU C++14 (GCC) version 7.3.1 20180312 (x86_64-pc-linux-gnu)
compiled by GNU C version 7.3.1 20180312, GMP version 6.1.2, MPFR
version 4.0.1, MPC version 1.1.0, isl version isl-0.18-GMP
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: 7db9d5d3f13a1b145edda774ba2b8b66
COMPILER_PATH=/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.1/:/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.1/:/usr/lib/gcc/x86_64-pc-linux-gnu/:/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.1/:/usr/lib/gcc/x86_64-pc-linux-gnu/
LIBRARY_PATH=/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.1/:/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.1/../../../../lib/:/lib/../lib/:/usr/lib/../lib/:/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.1/../../../:/lib/:/usr/lib/
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-S' '-O1' '-shared-libgcc'
'-mtune=generic' '-march=x86-64'
The generated assembly for f2 contains only a return. The generated assembly
for f1 contains 2 calls to f. Since ::f and foo::f refer to the same
function^[1], the same optimizations should be applied to calls using either
name.
[1] n4659 section 10.5 [dcl.link] paragraph 6: Two declarations for a function
with C language linkage with the same function name (ignoring the namespace
names that qualify it) that appear in different namespace scopes refer to the
same function.
