[SOLVED] CPU_FLAGS, -march, or both?

[ShrekMovieFreeLegit]

Hello. I’m new to both Gentoo and the forums so I apologise if this isn’t the right place to post

What I’m asking is if I should use either CPU_FLAGS or -march in my make.conf?
To clarify, I’ve read somewhere that using both could potentially break some packages (don’t remember exactly which but I’m pretty sure it’s either ffmpeg or something related to mozilla). I wanted to know which of these are better for more optimised packages (i. e. compiles faster), or if I could just use both safely without any breakage.

Thanks!

[soundrolf]

My make.conf

Code: Select all

# Compiler Settings
COMMON_FLAGS="-march=native -O2 -pipe"
CFLAGS="${COMMON_FLAGS}"
CXXFLAGS="${COMMON_FLAGS}"
FCFLAGS="${COMMON_FLAGS}"
FFLAGS="${COMMON_FLAGS}"

# CPU FLAGS: AMD FX(tm)-6100 Six-Core Processor CPU Family: 21
CPU_FLAGS_X86="aes avx fma4 mmx mmxext pclmul popcnt sse sse2 sse3 sse4_1 sse4_2 sse4a ssse3 xop"
CHOST="x86_64-pc-linux-gnu"

# USE flags
USE="truetype elogind -consolekit -ipv6 -kde -gnome -systemd -pulseaudio 
smp usb alsa v4l -aim -infiniband -cjk -qt4 icu -oci8 -oscar -joystick 
-libav ffmpeg dbus dvd dvb cdr nls sockets twolame usb X udev udisks 
libnotify thunar xinerama qt5 -xscreensaver vdpau ${CPU_FLAGS_X86}"

# USE_EXPAND flags
PORTDIR="/usr/portage"
PORTAGE_LOGDIR="/var/log/portage_log"
DISTDIR="${PORTDIR}/distfiles"
PKGDIR="${PORTDIR}/packages"
MAKEOPTS="-j6"
GENTOO_MIRRORS="http://mirror.netcologne.de/gentoo/ https:ftp.halifax.rwth-aachen.de/gentoo/"
ACCEPT_LICENSE="*"
ACCEPT_KEYWORDS="amd64"
LINGUAS="de"
L10N="de"
LC_MESSAGES="C"
GRUB_PLATFORMS="pc"
LLVM_TARGETS="X86-64 AMDGPU"
VIDEO_CARDS="fbdev radeon nouveau v4l vesa amdgpu"
ALSA_CARDS="hda-intel"
INPUT_DEVICES="evdev synaptics"
KDIR="/usr/src/linux"
XFCE_PLUGINS="menu clock trash"
RUBY_TARGETS="ruby26 ruby27"
# EMERGE_DEFAULT_OPTS="--quiet-build"
# EMERGE_DEFAULT_OPTS="--with-bdeps=y"

Regards

soundrolf

[fedeliallalinea]

The CPU_FLAGS_X86 is used by portage as use flag for some package that allow you to enable or disable certain cpu flags and you can use app-portage/cpuid2cpuflags for set correct values.
With -march=native gcc automagically enable correct flag for your cpu and them are used in all package when compiling (unless a package forces something else but for the user it is transparent).
I don't know where you read that these two settings can break some packages but if it happens it's definitely a bug to report.

[ShrekMovieFreeLegit]

With -march=native gcc automagically enable correct flag for your cpu

So basically, -march=native should do the same as CPU_FLAGS_X86. Thanks a lot!

[figueroa]

Use them both. Emerge and run app-portage/cpuid2cpuflags.

[Hu]

So basically, -march=native should do the same as CPU_FLAGS_X86. Thanks a lot!

No. -march=native tells gcc to use whatever natively available instructions it wants when generating assembly from C/C++. This generally happens on a fairly granular level, like using xmm instructions for bulk copying of small structures. It will usually not have a large scope view, such as completely changing how a function is implemented. CPU_FLAGS_X86 is used to advise particular packages to enable whole optional features. Such features are dozens or hundreds of lines long, and rely on a given CPU feature throughout. The complexity of the tool feature precludes trusting gcc to just guess that a few statements here and there ought to use the CPU feature. That is why you should use both: -march lets gcc opportunistically use advanced CPU features when specific C statements can be done better. CPU_FLAGS_X86 lets a tool build an entirely different code path, which presumably is more efficient than the generic alternative. (In some cases, the tool's maintainers may have decided not to write the generic version at all, so you don't even get the tool's feature, even in a slow inefficient version, if you don't enable the right CPU_FLAGS_X86.)

[CaptainBlood]

Use them both. Emerge and run app-portage/cpuid2cpuflags.

+1
Thks 4 ur attention, interest & support.