https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68008
Bug ID: 68008
Summary: Pessimization of simple non-tail-recursive functions
Product: gcc
Version: 5.1.1
URL: http://stackoverflow.com/questions/32974625
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: gcc-bugzilla at mkarcher dot dialup.fu-berlin.de
Target Milestone: ---
Created attachment 36537
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=36537&action=edit
preprocessed source code
As discussed on Stack Overflow in http://stackoverflow.com/questions/32974625
(Misleading title: "Is gcc's asm volatile equivalent to the gfortran default
setting for recursions?"), gcc pessimizes the example fibonacci function, which
can be avoided by "-fno-optimize-sibling-calls" (which is the reason I assigned
the bug to tree-optimization)
The non-optimal code is generated at least by
gcc 4.5.3 (Debian 4.5.3-12)
gcc version 4.9.2 (Debian 4.9.2-10)
gcc version 5.1.1 20150507 (Debian 5.1.1-5)
I will focus on gcc 5.1.1 in the remaining report.
Configured with: ../src/configure -v --with-pkgversion='Debian 5.1.1-5'
--with-bugurl=file:///usr/share/doc/gcc-5/README.Bugs
--enable-languages=c,ada,c++,java,go,d,fortran,objc,obj-c++ --prefix=/usr
--program-suffix=-5 --enable-shared --enable-linker-build-id
--libexecdir=/usr/lib --without-included-gettext --enable-threads=posix
--libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu
--enable-libstdcxx-debug --enable-libstdcxx-time=yes
--with-default-libstdcxx-abi=c++98 --disable-libstdcxx-dual-abi
--enable-gnu-unique-object --disable-vtable-verify --enable-libmpx
--enable-plugin --with-system-zlib --disable-browser-plugin
--enable-java-awt=gtk --enable-gtk-cairo
--with-java-home=/usr/lib/jvm/java-1.5.0-gcj-5-amd64/jre --enable-java-home
--with-jvm-root-dir=/usr/lib/jvm/java-1.5.0-gcj-5-amd64
--with-jvm-jar-dir=/usr/lib/jvm-exports/java-1.5.0-gcj-5-amd64
--with-arch-directory=amd64 --with-ecj-jar=/usr/share/java/eclipse-ecj.jar
--enable-objc-gc --enable-multiarch --with-arch-32=i586 --with-abi=m64
--with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic
--enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu
--target=x86_64-linux-gnu
gcc is invoked as: g++-5 -O3 -march=core2 -std=c++0x
stack-overflow-32974625.cc; ./a.out
compared to g++-5 -O3 -march=core2 -std=c++0x -fno-optimize-sibling-calls
stack-overflow-32974625.cc; ./a.out
There are no error or warning messages printed by the compiler or linker (even
if warning would be enabled by -Wall -Wextra)
The file stack-overflow-32974625.ii is attached.
Timing with g++ 5.1.1 on a Core2Duo T7200 under 64-bit linux (cpu frequency
scaling set to "performance"):
With -fno-optimize-sibling-calls: 34.5us/iteration
Without -fno-optimize-sibling-calls: 68us/iteration
