On Mon, 14 Jan 2019, Geert Uytterhoeven wrote: > On Fri, Dec 21, 2018 at 7:21 AM Finn Thain <fth...@telegraphics.com.au> 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....@igel.home/ > > > > > > > > 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 -- > ge...@linux-m68k.org > > 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 >