From: Ingo Molnar
Date: Mon May 06 2019 - 03:06:27 EST

* Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx> wrote:

> On 09/04/2019 23.25, Rasmus Villemoes wrote:
> > While refreshing these patches, which were orignally just targeted at
> > x86-64, it occured to me that despite the implementation relying on
> > inline asm, there's nothing x86 specific about it, and indeed it seems
> > to work out-of-the-box for ppc64 and arm64 as well, but those have
> > only been compile-tested.
> So, apart from the Clang build failures for non-x86, I now also got a
> report that gcc 4.8 miscompiles this stuff in some cases [1], even for
> x86 - gcc 4.9 does not seem to have the problem. So, given that the 5.2
> merge window just opened, I suppose this is the point where I should
> pull the plug on this experiment :(
> Rasmus
> [1] Specifically, the problem manifested in net/ipv4/tcp_input.c: Both
> uses of the static inline inet_csk_clear_xmit_timer() pass a
> compile-time constant 'what', so the ifs get folded away and both uses
> are completely inlined. Yet, gcc still decides to emit a copy of the
> final 'else' branch of inet_csk_clear_xmit_timer() as its own
> inet_csk_reset_xmit_timer.part.55 function, which is of course unused.
> And despite the asm() that defines the ddebug descriptor being an "asm
> volatile", gcc thinks it's fine to elide that (the code path is
> unreachable, after all....), so the entire asm for that function is
> .section .text.unlikely
> .type inet_csk_reset_xmit_timer.part.55, @function
> inet_csk_reset_xmit_timer.part.55:
> movq $.LC1, %rsi #,
> movq $__UNIQUE_ID_ddebug160, %rdi #,
> xorl %eax, %eax #
> jmp __dynamic_pr_debug #
> .size inet_csk_reset_xmit_timer.part.55,
> .-inet_csk_reset_xmit_timer.part.55
> which of course fails to link since the symbol __UNIQUE_ID_ddebug160 is
> nowhere defined.

It's sad to see such nice data footprint savings go the way of the dodo
just because GCC 4.8 is buggy.

The current compatibility cut-off is GCC 4.6:

GNU C 4.6 gcc --version

Do we know where the GCC bug was fixed, was it in GCC 4.9?

According to the GCC release dates:

4.8.0 was released in early-2013, while 4.9.0 was released in early-2014.
So the tooling compatibility window for latest upstream would narrow from
~6 years to ~5 years.