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Re: Patch for config.guess for "cross-compiling" x86_64 -> powerpc on Ma

From: Mojca Miklavec
Subject: Re: Patch for config.guess for "cross-compiling" x86_64 -> powerpc on Mac OS X <= 10.6
Date: Tue, 18 Apr 2017 20:06:28 +0200


On 18 April 2017 at 18:47, Jan Engelhardt wrote:
> On Tuesday 2017-04-18 17:44, Mojca Miklavec wrote:
>>On 18 April 2017 at 17:27, Jan Engelhardt wrote:
>>> That will invoke /usr/bin/powerpc-unknown-linux-gnu-gcc etc., which can 
>>> either
>>> be the PPC compiler, or a shell script invoking an existing 
>>> native-by-default
>>> gcc with more arguments.
>>The problem of this approach is that the configure scripts etc. then
>>assume that the computer where you run the configuration is not able
>>to run the resulting binaries.
> Yes, that is a bit unfortunate -- but usually points to a problem
> *elsewhere*. If a software package needs, say, an utility to generate
> more source code from an IDL [think protobuf], the package in
> question should ensure it uses the native compiler for these things.
> I also acknowledge that autotools does not expose the native CC very
> well.

No, the idea is not to use the native compiler.

>From a random example using AC_TRY_RUN([...]) the configure script
compiles the code for the host platform, runs it and checks whether
the resulting binary crashes. The idea is to check the binary for
*host* platform, not the guest.

The autotools need to know whether the will actually run the code or
not. When cross-compiling, the autotools cannot simply run the
resulting binary and the result has to be guessed or a database of all
features of all platforms needs to be present. When compiling
natively, the code can easily run.

If I use --host=powerpc-<whatever> on x86_64, how can autotools decide
whether or not powerpc binaries can run or not?

>>Building for i386 or ppc is not really cross-compiling in the
>>traditional sense. An x86_64 machine can still run the ppc binaries
>>without problems
> Last time I tried, x86_64 chips only support the i686 and AMD64 ISAs
> natively. For something like ppc, one would need a binfmt_misc(5)
> entry with some qemu magic attached.

On Macs (10.4 - 10.5) it works automatically without any qemu magic
(there is some rosetta-something in the background, but that's fully
transparent for users).

But in order not to get too off-topic: is anything about my proposed
patch actually problematic?


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