On Mon, 14 Jan 2019, Geert Uytterhoeven wrote:
On Fri, Dec 21, 2018 at 7:21 AM Finn Thain <fthain@xxxxxxxxxxxxxxxxxxx> wrote:
On Thu, 20 Dec 2018, John Paul Adrian Glaubitz wrote:
On 12/20/18 12:16 AM, Finn Thain wrote:
Andreas said the error happened when the new compiler expanded a
__bultin_strcmp call to a strcmp call.
https://lore.kernel.org/lkml/87in513wbt.fsf@xxxxxxxxx/
The new compiler seems to assume that a strcmp symbol exists in
the final link. I don't see how that kind of assumption is valid
here.
So I think the real bug is the lack of -ffreestanding. That
omission allows the compiler to assume that libc is available in
the final link. (At least, I imagine that's what the compiler
authors had in mind.) Anyway, the use of -ffreestanding certainly
avoids this optimization.
Yeah, during the kernel history, several other architectures started
using -ffreestanding, to avoid similar replacements behind our back. So
there is definitely prior art for starting to use that.
These commits are interesting.
72fbfb260197 [MIPS] Fix optimization for size build.
fec468b0c9e0 [PATCH] uml: add -ffreestanding to CFLAGS
6edfba1b33c7 [PATCH] x86_64: Don't define string functions to builtin
b2444d34a0e7 [XTENSA] Add freestanding option to CFLAGS
I get the impression that people were reluctant to use -ffreestanding...
This patch seems to work fine but may have implications for both 680x0
and Coldfire. It will make some optimizations unavailable but I
haven't tried to measure that effect. It could be a hard sell if it
harms performance.
With gcc 7.3.0, it increases the kernel image size for atari_defconfig
by ca. 300 bytes (which is an 0.01% increase). But -ffreestanding allows
to switch to gcc 8.2.0, which reduces the same kernel by ca 7.5 KiB
again.
If there is a performance drop caused by -ffreestanding, perhaps some
optimizations be done using semantic patches instead. I could imagine that
most sensible optimizations involving libc calls would also make sense
when written as high-level program transformations.
Finn, can you please submit your patch with a proper SoB?
OK. -ffreestanding implies -fno-builtin, so we may need more of these:
#define foo(z) __builtin_foo(z)
...depending on the optimizations available in gcc 8.
--
Thanks!
Gr{oetje,eeting}s,
Geert
--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx
In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds
