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Safe CFLAGS

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This article attempts to summarize 'safe' settings for CPU flags on Gentoo Linux.

Automatic CPU detection by tools

app-portage/cpuid2cpuflags tries to detect all supported CPU flags. Usage:

user $cpuid2cpuflags

Automatic CPU detection by the compiler

A recommended default choice for CFLAGS or CXXFLAGS is to use -march=native. This enables auto-detection of the CPU's architecture. A possible entry might look like:

FILE /etc/portage/make.conf
CFLAGS="-O2 -pipe -march=native"
CXXFLAGS="${CFLAGS}"
Warning
Do not use -march=native or -mtune=native in the CFLAGS or CXXFLAGS variables of make.conf when compiling with distcc. Consult the distcc page about how to set up CFLAGS/CXXFLAGS correctly.
Note
Additional information can be found at the GCC optimization page.

Manual

Finding the CPU

To identify the model of the CPU, take a look inside /proc/cpuinfo for the "cpu family" and "model" numbers like so:

user $grep -m1 -A3 "vendor_id" /proc/cpuinfo

Once this information is found match the CPU to one listed on this page in order to find out the suggested "safe" CFLAGS.

Below is a list of CFLAGS which are to be considered "safe" for the given processors. These are the settings that should be used, especially when unsure which CFLAGS the processor needs.

x86/amd64

Intel

Skylake

Core i3/i5/i7 and Xeon E3/E5 *V5 
vendor_id       : GenuineIntel
cpu family      : 6
model           : 94
model name      : Intel(R) Core(TM) i7-6700K CPU @ 4.00GHz
FILE /etc/portage/make.conf
CHOST="x86_64-pc-linux-gnu"
CFLAGS="-march=skylake -O2 -pipe"
CXXFLAGS="${CFLAGS}"
Note
Even though they fit the family and model, this set of CFLAGS does NOT work for Skylake Pentium CPUs (e. g. G4400) producing invalid code. -march=native works.

Haswell

Core i3/i5/i7 and Xeon E3/E5/E7 *V3 
vendor_id	: GenuineIntel
cpu family	: 6
model		: 60
model name	: Intel(R) Xeon(R) CPU E3-1271 v3 @ 3.60GHz
…
model           : 60
model name      : Intel(R) Core(TM) i7-4770 CPU @ 3.40GHz
FILE /etc/portage/make.conf
CHOST="x86_64-pc-linux-gnu"
CFLAGS="-march=haswell -O2 -pipe"
CXXFLAGS="${CFLAGS}"

Ivy Bridge

Core i3/i5/i7 and Xeon E3/E5/E7 *V2 
vendor_id       : GenuineIntel
cpu family      : 6
model           : 58
model name      : Intel(R) Core(TM) i7-3610QM CPU @ 2.30GHz
FILE /etc/portage/make.conf
CHOST="x86_64-pc-linux-gnu"
CFLAGS="-march=ivybridge -O2 -pipe"
CXXFLAGS="${CFLAGS}"
Pentium 
vendor_id	: GenuineIntel
cpu family	: 6
model		: 58
model name	: Intel(R) Pentium(R) CPU G2020 @ 2.90GHz
FILE /etc/portage/make.conf
CHOST="x86_64-pc-linux-gnu"
CFLAGS="-march=ivybridge -mno-avx -mno-aes -mno-rdrnd -O2 -pipe"
CXXFLAGS="${CFLAGS}"

Sandy Bridge

Core i3/i5/i7 and Xeon E3/E5/E7 
vendor_id	: GenuineIntel
cpu family	: 6
…
model		: 42
model name	: Intel(R) Core(TM) i5-2400 CPU @ 3.10GHz
…
model		: 45
model name	: Intel(R) Core(TM) i7-3930K CPU @ 3.20GHz
… 
model		: 42
model name	: Intel(R) Xeon(R) CPU E31245 @ 3.30GHz
… 
model           : 45
model name      : Intel(R) Xeon(R) CPU E5-2407 0 @ 2.20GHz
FILE /etc/portage/make.conf
CHOST="x86_64-pc-linux-gnu"
CFLAGS="-march=sandybridge -O2 -pipe"
CXXFLAGS="${CFLAGS}"
Pentium 
vendor_id	: GenuineIntel
cpu family	: 6
model		: 42
model name	: Intel(R) Pentium(R) CPU B960 @ 2.20GHz
FILE /etc/portage/make.conf
CHOST="x86_64-pc-linux-gnu"
CFLAGS="-march=sandybridge -mno-avx -mno-aes -mno-rdrnd -O2 -pipe"
CXXFLAGS="${CFLAGS}"

Nehalem

Core i3/i5/i7 
vendor_id	: GenuineIntel
cpu family	: 6
model		: 30
model name	: Intel(R) Core(TM) i5-750 CPU @ 2.66GHz
FILE /etc/portage/make.conf
CHOST="x86_64-pc-linux-gnu"
CFLAGS="-march=nehalem -O2 -pipe"
CXXFLAGS="${CFLAGS}"

Westmere

Core i3/i5/i7 
vendor_id	: GenuineIntel
cpu family	: 6
model		: 37
model name	: Intel(R) Core(TM) i5-650 CPU @ 3.20GHz
FILE /etc/portage/make.conf
CHOST="x86_64-pc-linux-gnu"
CFLAGS="-march=westmere -O2 -pipe"
CXXFLAGS="${CFLAGS}"

Intel Core

vendor_id       : GenuineIntel
cpu family      : 6
…
model		: 15
model name	: Intel(R) Core(TM)2 Duo CPU     T7500  @ 2.20GHz
…
model           : 15
model name      : Intel(R) Xeon(R) CPU            3040  @ 1.86GHz
FILE /etc/portage/make.conf
CHOST="x86_64-pc-linux-gnu"
CFLAGS="-march=core2 -O2 -pipe"
CXXFLAGS="${CFLAGS}"

Older microarchitecture

Pentium M (Dothan) 
vendor_id	: GenuineIntel
cpu family	: 6
model		: 13
model name	: Intel(R) Pentium(R) M processor 2.13GHz
FILE /etc/portage/make.conf
CHOST="i686-pc-linux-gnu"
CFLAGS="-O2 -march=pentium-m -pipe"
CXXFLAGS="${CFLAGS}"
Pentium 4 (Prescott)


64-bit capable models: 505, 505J, 506, 511, 516, 517, 519K, 521, 524, 531, 541, 551, 561, 571, 6xx and the 3.73(3)GHz Pentium 4 Extreme Edition.

Note
Check the /proc/cpuinfo for the lm flag to detect 64-bit CPUs:
user $grep lm /proc/cpuinfo
The cpuid model (4 vs 3) is not an indicator of 64-bit compatibility; there are 32-bit family15/model4 CPUs!
vendor_id	: GenuineIntel
cpu family	: 15
model		: 4
model name	: Intel(R) Pentium(R) 4 CPU XXXGHz
FILE /etc/portage/make.conf
CHOST="x86_64-pc-linux-gnu"
CFLAGS="-O2 -march=nocona -pipe"
CXXFLAGS="${CFLAGS}"
All other Prescotts 
vendor_id	: GenuineIntel
cpu family	: 15
model		: 3
model name	: Intel(R) Pentium(R) 4 CPU XXXGHz
FILE /etc/portage/make.conf
CHOST="i686-pc-linux-gnu"
CFLAGS="-O2 -march=prescott -pipe"
CXXFLAGS="${CFLAGS}"

AMD

Ryzen (Zen)

1700, 1700X, 1800X 
vendor_id	: AuthenticAMD
cpu family	: 23
model		: 1
model name	: AMD Ryzen 7 1800X Eight-Core Processor

Please see Ryzen -article for recommended CFLAGS. Pay special attention to the troubleshooting -section.

A6/A8/A9/A10/A12-8XXX/9XXX (Excavator)

Carrizo, Bristol Ridge, and Stoney Ridge 
vendor_id	: AuthenticAMD
cpu family	: 21
model		: 96 or 101 or 112
model name	: AMD A12-9800 RADEON R7, 12 COMPUTE CORES 4C+8G
FILE /etc/portage/make.conf
CHOST="x86_64-pc-linux-gnu"
CFLAGS="-O2 -march=bdver4 -pipe"
CXXFLAGS="${CFLAGS}"

A4/A6/A8/A10-7XXX/8XXX (Steamroller)

Kaveri and Godavari 
vendor_id	: AuthenticAMD
cpu family	: 21
model		: 48 or 56
model name	: AMD A10-7850K Radeon R7, 12 Compute Cores 4C+8G
Important
Various lowpower AMD APUs branded as AX-7XXX (eg. A4-7210) don't belong to cpu family 21!
FILE /etc/portage/make.conf
CHOST="x86_64-pc-linux-gnu"
CFLAGS="-O2 -march=bdver3 -pipe"
CXXFLAGS="${CFLAGS}"

E1/E2-XXXX, A4/A6/A8/A10-XXXX (Jaguar, Puma)

Kabini, Temash, Beema, Mullins, and Carrizo-L 
vendor_id	: AuthenticAMD
cpu family	: 22
model		: 0 or 48
model name	: AMD A4-5000 APU with Radeon(TM) HD Graphics
Important
Majority of AMD APUs branded as A4/A6/A8-XXXX (eg. A4-5300, A4-4000 or A6-5345M) don't belong to cpu family 22! They should use settings listed for AMD APU model given by the cpu family and model.
FILE /etc/portage/make.conf
CHOST="x86_64-pc-linux-gnu"
CFLAGS="-O2 -march=btver2 -pipe"
CXXFLAGS="${CFLAGS}"

A4/A6/A8/A10-4XXX/5XXX/6XXX (Piledriver)

Trinity and Richland 
vendor_id	: AuthenticAMD
cpu family	: 21
model		: 16 or 19
model name	: AMD A8-4500M APU with Radeon(tm) HD Graphics
Important
Various low power AMD APUs branded as AX-5XXX/6XXX (eg. A4-5000, A4-5100 or A6-6310) don't belong to CPU family 21!
FILE /etc/portage/make.conf
CHOST="x86_64-pc-linux-gnu"
CFLAGS="-O2 -march=bdver2 -pipe"
CXXFLAGS="${CFLAGS}"

FX-XXXX

Bulldozer and Piledriver 
vendor_id	: AuthenticAMD
cpu family	: 21
model		: 1 or 2
model name	: AMD FX(tm)-8150 Eight-Core Processor
Important
Make sure and check the number listed by model on the system, the -march flag should be bdverX where X is the model number.
Important
Various AMD APUs branded as FX don't match model 1 or 2! They should use settings listed for AMD APU model given by the cpu family and model.
FILE /etc/portage/make.conf
CHOST="x86_64-pc-linux-gnu"
CFLAGS="-O2 -march=bdver1 -pipe"
CXXFLAGS="${CFLAGS}"

Z-XX, C-X0, E-XX0, E1/E2-1X00, E2-2000 (Bobcat)

Ontario, Hondo, Desna, and Zacate 
vendor_id	: AuthenticAMD
cpu family	: 20
model		: 1 or 2
model name	: AMD E-350 Processor
FILE /etc/portage/make.conf
CHOST="x86_64-pc-linux-gnu"
CFLAGS="-O2 -march=btver1 -pipe"
CXXFLAGS="${CFLAGS}"

A4/A6/A8-3XXX/3XXXM (12h)

Llano 
vendor_id	: AuthenticAMD
cpu family	: 18
model		: 1
model name	: AMD A8-3500M APU with Radeon(tm) HD Graphics
FILE /etc/portage/make.conf
CHOST="x86_64-pc-linux-gnu"
CFLAGS="-O2 -march=amdfam10 -mcx16 -mpopcnt -pipe"
CXXFLAGS="${CFLAGS}"

Phenom/Phenom II, Athlon II, Sempron (10h)

Agena, Deneb, Thuban, and derivatives 
vendor_id	: AuthenticAMD
cpu family	: 16
model		: X
model name	: AMD Phenom(tm) II X6 1090T Processor
FILE /etc/portage/make.conf
CHOST="x86_64-pc-linux-gnu"
CFLAGS="-O2 -march=amdfam10 -pipe"
CXXFLAGS="${CFLAGS}"

Older microarchitectures

Important
Various AMD CPUs branded as Sempron (eg. Sempron 2200+ or Sempron 3000+) don't belong to cpu family 15!
E+ revisions - Athlon 64, Athlon 64 X2/FX, Sempron (0Fh) 
vendor_id	: AuthenticAMD
cpu family	: 15
model		: >= 39
model name	: AMD Athlon(tm) 64 X2 Dual Core Processor 4200+
FILE /etc/portage/make.conf
CHOST="x86_64-pc-linux-gnu"
CFLAGS="-O2 -march=opteron-sse3 -pipe"
CXXFLAGS="${CFLAGS}"
Geode LX 
vendor_id	: AuthenticAMD
cpu family	: 5
model		: 10
model name	: Geode(TM) Integrated Processor by AMD PCS
FILE /etc/portage/make.conf
CHOST="i486-pc-linux-gnu"
CFLAGS="-Os -pipe -march=geode -mmmx -m3dnow -fomit-frame-pointer"
CXXFLAGS="${CFLAGS}"
Pre-E revisions - Athlon 64, Athlon 64 FX, Sempron (0Fh) 
vendor_id	: AuthenticAMD
cpu family	: 15
model		: < 39
model name	: AMD Athlon(tm) 64 Processor 3200+
FILE /etc/portage/make.conf
CHOST="x86_64-pc-linux-gnu"
CFLAGS="-O2 -march=opteron -pipe"
CXXFLAGS="${CFLAGS}"

ARM

Note
To identify the respective ARM core of the SoC on your board, List of ARM microarchitectures and List of applications of ARM cores on Wikipedia may help.

Cortex-A

ARMv7-A/Cortex-A9 MPCore

with optional VFPv3 FPU 
processor       : 0
model name      : ARMv7 Processor rev 0 (v7l)
BogoMIPS        : 2.00
Features        : half thumb fastmult vfp edsp vfpv3 vfpv3d16 tls 
CPU implementer : 0x41
CPU architecture: 7
CPU variant     : 0x1
CPU part        : 0xc09
CPU revision    : 0

processor       : 1
model name      : ARMv7 Processor rev 0 (v7l)
BogoMIPS        : 2.00
Features        : half thumb fastmult vfp edsp vfpv3 vfpv3d16 tls 
CPU implementer : 0x41
CPU architecture: 7
CPU variant     : 0x1
CPU part        : 0xc09
CPU revision    : 0

Hardware        : NVIDIA Tegra SoC (Flattened Device Tree)
Revision        : 0000
Serial          : 0000000000000000
Note
This ARM core (equipped with the optional vfpv3d16 FPU but missing the NEON extension) is used in the Toshiba AC100/Dynabook AZ/Compal Paz00 Board.
FILE /etc/portage/make.conf
CHOST="armv7a-hardfloat-linux-gnueabi"
CFLAGS="-O2 -march=cortex-a9 -mfpu=vfpv3-d16 -mfloat-abi=hard -pipe -fomit-frame-pointer"
CXXFLAGS="${CFLAGS}"

ARMv8-A/BCM2837

Note
This is the Broadcom chip used in the Raspberry Pi 3 Model B.
AArch32 with neon FPU 
processor	: 0-3
model name	: ARMv7 Processor rev 4 (v7l)
BogoMIPS	: 38.40
Features	: half thumb fastmult vfp edsp neon vfpv3 tls vfpv4 idiva idivt vfpd32 lpae evtstrm crc32 
CPU implementer	: 0x41
CPU architecture: 7
CPU variant	: 0x0
CPU part	: 0xd03
CPU revision	: 4

Hardware	: BCM2709
Revision        : 0000
Serial          : 0000000000000000
FILE /etc/portage/make.conf
CHOST="armv7a-hardfloat-linux-gnueabi"
CFLAGS="-O2 -pipe -march=armv7-a -mfpu=neon-vfpv4 -mfloat-abi=hard"
CXXFLAGS="${CFLAGS}"
AArch64 
processor	: 0-3
BogoMIPS	: 38.40
Features	: fp asimd evtstrm crc32
CPU implementer	: 0x41
CPU architecture: 8
CPU variant	: 0x0
CPU part	: 0xd03
CPU revision	: 4
FILE /etc/portage/make.conf
CHOST="aarch64-unknown-linux-gnu"
CFLAGS="-march=armv8-a+crc -mtune=cortex-a53 -O2 -pipe"
CXXFLAGS="${CFLAGS}"

ARM11

ARMv6/ARM1176JZF-S

Note
This ARM core is used in the first generation of the Raspberry Pi.
processor	: 0
model name	: ARMv6-compatible processor rev 7 (v6l)
BogoMIPS	: 697.95
Features	: half thumb fastmult vfp edsp java tls 
CPU implementer	: 0x41
CPU architecture: 7
CPU variant	: 0x0
CPU part	: 0xb76
CPU revision	: 7

Hardware	: BCM2835
Revision	: 0000
Serial		: 000000000XXXXXXX
FILE /etc/portage/make.conf
CHOST="armv6j-hardfloat-linux-gnueabi"
CFLAGS="-O2 -pipe -march=arm1176jzf-s -mfpu=vfp -mfloat-abi=hard"
CXXFLAGS="${CFLAGS}"

ARMv6/ARM1136JF-S

Processor       : ARMv6-compatible processor rev 5 (v6l)
BogoMIPS        : 791.34
Features        : swp half thumb fastmult vfp edsp java 
CPU implementer : 0x41
CPU architecture: 6TEJ
CPU variant     : 0x1
CPU part        : 0xb36
CPU revision    : 5

Hardware        : IMAPX200
Revision        : 0000
Serial          : 0000000000000000
FILE /etc/portage/make.conf
CHOST="armv6j-hardfloat-linux-gnueabi"
CFLAGS="-Os -march=arm1136jf-s -mfpu=vfp -mfloat-abi=hard -pipe -fomit-frame-pointer"
CXXFLAGS="${CFLAGS}"

ppc/ppc64

Important
-march= is not supported on the PowerPC (and RS/6000) instruction set architecture (ISA). Use -mcpu= and -mtune= instead.
Note
Using -mcpu=native (and/or -mtune=native) should work on PowerPC but is not recommended due to distcc.

POWER8

processor       : 0
cpu             : POWER8E (raw), altivec supported
clock           : 3026.000000MHz
revision        : 2.1 (pvr 004b 0201)

timebase        : 512000000
platform        : pSeries
model           : IBM pSeries (emulated by qemu)
machine         : CHRP IBM pSeries (emulated by qemu)
Note
Currently Gentoo does not support POWER8, but safe CFLAGS for it would look like the following.
FILE /etc/portage/make.conf
CHOST="powerpc64le-linux-gnu"
CFLAGS="-mcpu=power8 -O2 -pipe -mabi=altivec -maltivec"
CXXFLAGS="${CFLAGS}"

Cell

processor	: 0
cpu		: Cell Broadband Engine, altivec supported
clock		: 3192.000000MHz
revision	: 5.1 (pvr 0070 0501)

processor	: 1
cpu		: Cell Broadband Engine, altivec supported
clock		: 3192.000000MHz
revision	: 5.1 (pvr 0070 0501)

timebase	: 79800000
platform	: PS3
model		: SonyPS3
FILE /etc/portage/make.conf
CHOST="powerpc-unknown-linux-gnu"
CFLAGS="-mcpu=cell -O2 -pipe -mabi=altivec -maltivec"
CXXFLAGS="${CFLAGS}"
Note
GCC's -mspe and -mabi=spe options are not targetting PS3 systems and IBM Cell. Instead, those options are dedicated to IBM e500. More info:

PPC 970 (G5)

Compatible processors are IBM PPC970, PPC970FX, PPC970MP and PPC970GX. 

cpu             : PPC970, altivec supported
clock           : 1800MHz
revision        : 2.2 (pvr 0039 0202)
bogomips        : 1127.21
machine         : PowerMac7,2
motherboard     : PowerMac7,2 MacRISC4 Power Macintosh
board revision  : 00000001
detected as     : 336 (PowerMac G5)
pmac flags      : 00000000
L2 cache        : 512K unified
pmac-generation : NewWorld
FILE /etc/portage/make.conf
CHOST="powerpc-unknown-linux-gnu"
CFLAGS="-mcpu=970 -O2 -maltivec -mabi=altivec -pipe"
CXXFLAGS="${CFLAGS}"

G4 (PPC 74xx)

PPC 7450 family

Compatible processors are Motorola/Freescale MPC7450, MPC7440, MPC7451, MPC7441, MPC7455, MPC7445, MPC7457, MPC7447, MPC7447/A, and MPC7448. 

processor	      : 0
cpu		           : 7447A, altivec supported
clock		         : 1666.666000MHz
revision	       : 1.5 (pvr 8003 0105)
bogomips	       : 33.28
timebase	       : 8320000
platform	       : PowerMac
model		         : PowerBook5,9
machine		       : PowerBook5,9
motherboard	    : PowerBook5,9 MacRISC3 Power Macintosh 
detected as	    : 287 (PowerBook G4 17")
pmac flags	     : 00000018
L2 cache	       : 512K unified
pmac-generation	: NewWorld
FILE /etc/portage/make.conf
CHOST="powerpc-unknown-linux-gnu"
CFLAGS="-mcpu=7450 -O2 -maltivec -mabi=altivec -fno-strict-aliasing -pipe"
CXXFLAGS="${CFLAGS}"

PPC 7400 family

Compatible processors are Motorola MPC7400 and MPC7410. Note: IBM manufactured the MPC7400 as 06K5319 and 10K8298 when Motorola was not able to fulfill Apple's demands. 

processor       : 0
cpu             : 7400, altivec supported
clock           : 400.000000MHz
revision        : 2.9 (pvr 000c 0209)
bogomips        : 49.66
timebase        : 24908583
platform        : PowerMac
machine         : PowerMac3,1
motherboard     : PowerMac3,1 MacRISC Power Macintosh
detected as     : 65 (PowerMac G4 AGP Graphics)
pmac flags      : 00000004
L2 cache        : 1024K unified
pmac-generation : NewWorld
FILE /etc/portage/make.conf
CHOST="powerpc-unknown-linux-gnu"
CFLAGS="-mcpu=7400 -O2 -maltivec -mabi=altivec -fno-strict-aliasing -pipe"
CXXFLAGS="${CFLAGS}"

G3 (PPC 7XX)

Compatible processors are Motorola/Freescale MPC750, MPC740, MPC755 and MPC745 as well as IBM PPC750, PPC740, PPC750L, PPC740L, PPC750CX, PPC750CXe, PPCDBK ("Gekko"), PPC750FX, PPC750GX, PPC750CXr, PPC750CL ("Broadway"), PPC750GL and PPC750FL. The BAE Systems RAD750 is a radiation hardened variant of the PPC750. The "Espresso" (following the "Gekko" and "Broadway") is also based on the PPC750.

For CPUs for embedded systems such as the Gekko (PPCDBK, used in the Nintendo GameCube) additional CFLAGS (like -meabi) will be required. 

processor       : 0
cpu             : 740/750
clock           : 400.000000MHz
revision        : 131.0 (pvr 0008 8300)
bogomips        : 49.93
timebase        : 24966218
platform        : PowerMac
model           : PowerBook3,1
machine         : PowerBook3,1
motherboard     : PowerBook3,1 MacRISC2 MacRISC Power Macintosh
detected as     : 70 (PowerBook Pismo)
pmac flags      : 0000001f
L2 cache        : 1024K unified
pmac-generation : NewWorld
FILE /etc/portage/make.conf
CHOST="powerpc-unknown-linux-gnu"
CFLAGS="-mcpu=750 -Os -pipe -fno-strict-aliasing"
CXXFLAGS="${CFLAGS}"

m68k

See also

External resources

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