fixes #25296; get CPU arch from gcc

[demotomohiro]

No description provided.

[bptato]

Long options are a GNUism so this won't work on FreeBSD etc. cut -d - -f 1 seems preferable.
Also, later down the file there's this line:

mycpu = $(shell /bin/sh -c '"$(CC)" -dumpmachine | sed "s/-.*//"')

clang supports dumpmachine too, so maybe $(CC) is better?

[demotomohiro]

Thank you! I wasn't aware there is already code calls $(CC) -dumpmachine.

[Araq]

I don't see why we would prefer GCC's ideas over the OS's ideas for OS/CPU detection. For example, what if we build with clang?

[demotomohiro]

On Raspberry Pi 5 with 32-bit OS,

$ gcc -dumpmachine
arm-linux-gnueabihf
$ uname -m
aarch64

So if uname -m is used to detect CPU, csource for 64bit aarch64 is compiled with GCC targets 32bit arm, and fail to compile.

I think using GCC for OS/CPU detection works better when you want to cross compile csource and set PATH to use GCC for cross compile.
clang also supports -dumpmachine command that works on 32-bit Raspberry Pi OS.

Other ways to detect CPU architecture without C compiler are:

• Run following Python code:

import platform

print(platform.architecture()[0])
print(platform.machine())

Output on 32bit Raspberry Pi OS:

32bit
aarch64

• Run file command with any executable to get the CPU architecture of it and parse output:

file `which bash`

Output on 32bit Raspberry Pi OS:

/usr/bin/bash: ELF 32-bit LSB executable, ARM, EABI5 version 1 (SYSV), dynamically linked, interpreter /lib/ld-linux-armhf.so.3, BuildID[sha1]=db2e74d1b1322683c707352cc7b0de9e6205acdb, for GNU/Linux 3.2.0, stripped

[Araq]

There is nothing to detect here, if uname -m is broken you go and fix your operating system instead of an endless amount of hacks. On a 32bit OS with a 32bit GCC why the fuck would it say arm64, there is no way in hell you can use these "64" bits.