[gentoo-user] Re: Compiling 32 bit library on x86_64

[Nikos Chantziaras]

On 04/30/2010 07:29 PM, David W Noon wrote:
> 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.

You are wrong.


> 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 OP was not interested whether the code runs on all IA32 processors. 
  Just on his.  Yes, if you compile with "-m32 -march=native" the code 
will not run everywhere else.  But I fail to see what this has to do 
with 32-bit vs 64-bit; it also happens if you omit "-m32".


> 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.

Yes, which is what we want.  Again, the OP was not interested whether 
the code runs on other machines.  If that was the case, he would not 
have used -march=native in the first place.

And I have proof too.  Compile whatever you want with -m32 
-march=native.  It will work just fine.