Re: [ptxdist] libgmp cflags

[Michael Olbrich <m.olbrich@pengutronix.de>]

Hi,

On Tue, Dec 10, 2024 at 05:11:59PM +0100, Alexander Dahl wrote:
> I'm using nftables and on an armv5te target (microchip at91 sam9x60)
> the command `nft list ruleset` crashes with 'illegal instruction'.
> 
> BSP is on ptxdist-2024.05.0 with the latest libgmp 6.3 backported from
> master.  Toolchain is precompiled
> OSELAS.Toolchain-2023.07.1/arm-v5te-linux-gnueabi from
> debian.pengutronix.de, so overall nothing special.
> 
> `ptxdist prepare libgmp` gives this (somewhere in between):
> 
>     using ABI="32"
>           CC="arm-v5te-linux-gnueabi-gcc"
>           CFLAGS="-O2 -pedantic -fomit-frame-pointer -march=armv4 -mfloat-abi=softfp"
>           CPPFLAGS=""
>           MPN_PATH=" arm generic"
> 
> After staring at configure.ac and configure I guess they try to be
> smart here, but do not detect the actual arch.
> 
> Analzing the coredump with cgdb suggests it fails in __gmp_doprnt()
> shows this:
> 
>       1│ Dump of assembler code for function __gmp_doprnt:
>       2│    0xb6d69840 <+0>:     push    {r4, r5, r6, r7, r8, r9, r10, r11, lr}
>       3├──> 0xb6d69844 <+4>:     vpush   {d8}
>       4│    0xb6d69848 <+8>:     sub     sp, sp, #132    @ 0x84
> 
> That instruction seems to be some VFP related ARM floating point thing?
> 
> I tried adding this to rules/libgmp.make:
> 
> LIBGMP_CONF_ENV     := \
>     CFLAGS='-O2 -pedantic -g'
> 
> This gives almost the same output of prepare, but with "my" cflags
> only, so without -march or -mfloat-abi and then nft does not crash
> anymore.
> 
> I could use that as a workaround, but it seems to circumvent libgmp
> arch detection entirely, and so I'm not sure if that's the correct way
> here?
> 
> btw, you can see libgmp autoconf is not so super smart, because this is the
> output of `ptxdist prepare libgmp` on v7a target:
> 
>     using ABI="32"
>           CC="arm-v7a-linux-gnueabihf-gcc"
>           CFLAGS="-O2 -pedantic -fomit-frame-pointer -march=armv4 -mfloat-abi=hard"
>           CPPFLAGS=""
>           MPN_PATH=" arm generic"
> 
> So it's still -march=armv4 here.  But is that really necessary with
> the cross compiler at all?

Wow that detection is really confusing. And I'm not seeing anything there
that should be added to the CFLAGS. And somewhere in there I find: "Any
user CFLAGS, even an empty string, takes precedence".

So setting the CFLAGS works by design and I think it's the correct fix. But
please add a comment why this is necessary.

Michael

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