gcc versions 4.8 to 4.9, changing CFLAGS

[Massimo B.]

Hi,

while comparing the different -march keywords on different gcc versions here...

• https://gcc.gnu.org/onlinedocs/gcc-4.9.1/gcc/i386-and-x86-64-Options.html
  
• https://gcc.gnu.org/onlinedocs/gcc-4.8.4/gcc/i386-and-x86-64-Options.html
  
• https://gcc.gnu.org/onlinedocs/gcc-4.7.4/gcc/i386-and-x86-64-Options.html
  
• https://gcc.gnu.org/onlinedocs/gcc-4.1.2/gcc/i386-and-x86_002d64-Options.html
  

...I found some major differences:
Lot of marchs are dropped from 4.8 to 4.9. Currently using 4.8.4 and facing the next 4.9.2 upgrade, I wonder why all "core*" marchs have been dropped for these new items:

• ‘core2’
  
• ‘nehalem’
  
• ‘westmere’
  
• ‘sandybridge’
  
• ‘ivybridge’
  
• ‘haswell’
  
• ‘broadwell’
  

Are there still aliases? Beside the "native" item, do I need to adapt fixed -march CFLAGS?
_________________
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[xaviermiller]

I you don't use distcc, using native for -march and -mtune works for all GCC versions and architectures, from your microwave to your local server

If you don't like native, find the new native flags with

[Massimo B.]

For some VMware ESX virtual machine I just need to stay compatible with different archs in order to move the VM from server to local archs. There I set the compatible -march together with -mtune=native. Usually these machine are building themselves with the option to be moved temporarily to a different VMware Workstation without rebuilding all.

btw. for the -march=native machines, what does it mean to set -march and -mtune to the same arch? Isn't -mtune redundant or even worse? If architecture features are natively built in why to add some more runtime checks the the same optimizations?
_________________
HP ZBook Power 15.6" G8 i7-11800H|HP EliteDesk 800G1 i7-4790|HP Compaq Pro 6300 i7-3770

[chithanh]

The compiler flags are still supported, just the man page no longer mentions them.

[krinn]

march is the familly opitmize, mtune is the cpu
if you don't set one or the other, the missing one met the other (so redundant in this case), but mtune can let you tune more your cpu optimization than its familly.

a coreX familly may say : a coreX must have sse, but some coreX may not have sse2
so if you have a coreX-33 cpu: setting only familly (-march=coreX) will not enable sse2 because not all cpu of the familly have sse2
and setting -mtune=coreX-33 will enable coreX set + the set only coreX-33 cpu could handle (in the example, sse2)

[Massimo B.]

So as final conclusion on native platforms not being portable, setting both -march=native and -mtune=native is the optimum?
_________________
HP ZBook Power 15.6" G8 i7-11800H|HP EliteDesk 800G1 i7-4790|HP Compaq Pro 6300 i7-3770

[mreff555]

[Hu]