How to tune gcc for different x86 host and target cpus?

How to tune gcc for different x86 host and target cpus?

[Michael Roth]

Hello,

as far as I understand, when building gcc, I can use --with-arch=pentium 
to tell configure that the resulting compiler should emit code optimized 
for pentium processors.

But how can I tell the configure script that the compiler itself should 
be optimized for a different cpu architecture?

Example #1: Host and build are athlon-xp, target should be pentium.

Example #2: Build is pentium4, Host should be pentium-mmx, target should 
be 386.

What configure options should I use? (Host, build and target are all 
linux systems, but I think this apply to every system with x86 processor 
variants?)

Michael Roth

Re: How to tune gcc for different x86 host and target cpus?

[Ian Lance Taylor]

Michael Roth <mroth@nessie.de> writes:

> as far as I understand, when building gcc, I can use
> --with-arch=pentium to tell configure that the resulting compiler
> should emit code optimized for pentium processors.
> 
> But how can I tell the configure script that the compiler itself
> should be optimized for a different cpu architecture?

One way is to set CFLAGS when you compile.  The default value for
CFLAGS is "-g -O2" (when compiling with gcc).

> Example #1: Host and build are athlon-xp, target should be pentium.

make CFLAGS="-g -O2 -march=athlon"

> Example #2: Build is pentium4, Host should be pentium-mmx, target
> should be 386.

make CFLAGS="-g -O2 -march=pentium-mmx"

If you really want to optimize the build programs, you can use
CFLAGS_FOR_BUILD=-march=pentium4.  That will only affect the time it
takes to build the compiler itself, though, and is unlikely to make
any noticeable difference at that.

> What configure options should I use? (Host, build and target are all
> linux systems, but I think this apply to every system with x86
> processor variants?)

I think that if you set the environment variable CFLAGS when you run
configure, it will be picked up and stored in the Makefile.  Try it by
looking at the definition of CFLAGS in the top level Makefile after
you run configure.

Ian

Re: How to tune gcc for different x86 host and target cpus?

[Michael Roth]

Ian Lance Taylor wrote:

> Michael Roth <mroth@nessie.de> writes:
> 
>>But how can I tell the configure script that the compiler itself
>>should be optimized for a different cpu architecture?
> 
> 
> One way is to set CFLAGS when you compile.  The default value for
> CFLAGS is "-g -O2" (when compiling with gcc).
> 
> 
>>Example #1: Host and build are athlon-xp, target should be pentium.
> 
> 
> make CFLAGS="-g -O2 -march=athlon"

Thank you. This did the job.


> I think that if you set the environment variable CFLAGS when you run
> configure, it will be picked up and stored in the Makefile.  Try it by
> looking at the definition of CFLAGS in the top level Makefile after
> you run configure.

I'm not sure, I tried this before I posted to this list, and I think it 
didn't worked well. But I could be wrong.


Michael