On Fri, 30 Apr 2010 15:20:02 +0200, Nikos Chantziaras wrote about
[gentoo-user] Re: Compiling 32 bit library on x86_64:
On 04/30/2010 03:09 PM, David W Noon wrote:
On Fri, 30 Apr 2010 12:10:02 +0200, Roger Mason wrote about
[gentoo-user] Compiling 32 bit library on x86_64:
Hello,
I need to compile a 32 bit version of libtermcap on an x86_64
(multilib) system. Can someone tell me how to set up CFLAGS? This
is what I have at the moment:
CFLAGS=-O2 -m32 -march=native -msse3 -pipe
CXXFLAGS=-O2 -m32 -march=native -msse3 -pipe
The -march=native will shoot you in the foot. Pick a 32-bit
architecture and use that instead; e.g. -march=i686
Then, -msse3 could also be problematic, unless the target is a very
late model Pentium 4. I would ditch that too.
None of those options are problematic. -march=native has nothing to
do with 32/64 bit. Every 64-bit CPU is 32-bit compatible and has zero
consequence.
I think you fell into the logical trap that 32-bit CPUs are not 64-bit
compatible but it's OK vice versa :) Meaning you can't use -m64
-march=i686. But you *can* and *should* use -m32 -march=core2.
No, I stand by what I wrote.
The -march=native option tells the compiler to issue the CPUID
instruction to determine the architecture. This means that on an amd64
box it will return data for either an AMD K8 or an Intel Pentium D
architecture. This, in turn, allows the compiler to generate K8
instructions that are not valid on IA32 processors. It even allows
the compiler to use 64-bit registers, including the additional
registers that were not in an IA32 processor.
The -m32 option instructs the compiler to generate code with 32-bit
pointers and relocation dictionary. It does not constrain the compiler
to generate code that will definitely run on an IA32 processor, but it
does ensure that the code can be linked with 32-bit libraries.
So, if one is compiling on, say, a Core2 Duo and one uses -march=native
and -m32, the compiler can use all kinds of instructions valid on the
Core2 Duo, but limits addressing to 32-bit.
From the info pages for GCC:
3.17.14 Intel 386 and AMD x86-64 Options
These `-m' options are defined for the i386 and x86-64 family of
computers:
`-mtune=CPU-TYPE'
Tune to CPU-TYPE everything applicable about the generated code,
except for the ABI and the set of available instructions. The
choices for CPU-TYPE are:
_generic_
Produce code optimized for the most common IA32/AMD64/EM64T
processors. If you know the CPU on which your code will run,
then you should use the corresponding `-mtune' option instead
of `-mtune=generic'. But, if you do not know exactly what
CPU users of your application will have, then you should use
this option.
As new processors are deployed in the marketplace, the
behavior of this option will change. Therefore, if you
upgrade to a newer version of GCC, the code generated option
will change to reflect the processors that were most common
when that version of GCC was released.
There is no `-march=generic' option because `-march'
indicates the instruction set the compiler can use, and there
is no generic instruction set applicable to all processors.
In contrast, `-mtune' indicates the processor (or, in this
case, collection of processors) for which the code is
optimized.
_native_
This selects the CPU to tune for at compilation time by
determining the processor type of the compiling machine.
Using `-mtune=native' will produce code optimized for the
local machine under the constraints of the selected
instruction set. Using `-march=native' will enable all
instruction subsets supported by the local machine (hence the
result might not run on different machines).
_i386_
Original Intel's i386 CPU.
_i486_
Intel's i486 CPU. (No scheduling is implemented for this
chip.)
_i586, pentium_
Intel Pentium CPU with no MMX support.
_pentium-mmx_
Intel PentiumMMX CPU based on Pentium core with MMX
instruction set support.
_pentiumpro_
Intel PentiumPro CPU.
_i686_
Same as `generic', but when used as `march' option, PentiumPro
instruction set will be used, so the code will run on all
i686 family chips.
_pentium2_
Intel Pentium2 CPU based on PentiumPro core with MMX
instruction set support.
_pentium3, pentium3m_
Intel Pentium3 CPU based on PentiumPro core with MMX and SSE
instruction set support.
_pentium-m_
Low power version of Intel Pentium3 CPU with MMX, SSE and
SSE2 instruction set support. Used by Centrino notebooks.
_pentium4, pentium4m_
Intel
