Hi Felix,
On 2017-02-28 07:11, Felix Fietkau wrote:
On 2017-02-20 15:10, Mehrtens, Hauke wrote:
When I compile MIPS malta with the LEDE patches on kernel 4.9 I get
this error in the kernel build:
CC arch/mips/boot/compressed/decompress.o
OBJCOPY arch/mips/boot/compressed/vmlinux.bin
XZKERN arch/mips/boot/compressed/vmlinux.bin.z
OBJCOPY arch/mips/boot/compressed/piggy.o
LD vmlinuz
arch/mips/boot/compressed/decompress.o: In function `lzma2_lzma':
/tmp2/mehrtens/linux/arch/mips/boot/compressed/../../../../lib/xz/xz_dec_lzma2.c:884:
undefined reference to `memmove'
make[1]: *** [vmlinuz] Error 1
make: *** [vmlinuz] Error 2
This is caused by this option selected in the MIPS Malta target:
SYS_SUPPORTS_ZBOOT
This is caused by this patch from LEDE:
https://git.lede-project.org/?p=source.git;a=blob;f=target/linux/generic/patches-4.9/306-mips_mem_functions_performance.patch
This was initially committed here:
https://git.lede-project.org/?p=source.git;a=commitdiff;h=07e59c7bc7f375f792ec9734be42fe4fa391a8bb
I've pushed a fix to my staging tree, please test.
I tested it and this commit fixes my problem:
https://git.lede-project.org/?p=lede/nbd/staging.git;a=commitdiff;h=1cb69f5542b1a17b46d2c0849a35ece52ef20fb5
Thank you for looking into this.
When I remove the "#define memmove ..." from the string.h it works
for me, I also checked the GCC documentation and it does not list
__builtin_memmove() as an official interface here:
https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html
There's nothing wrong with __builtin_memmove or the function references
that it generates. The problem lies within decompress_unxz.c:
#ifndef memmove
/* Not static to avoid a conflict with the prototype in the Linux
headers. */
void *memmove(void *dest, const void *src, size_t size)
{
...
}
#endif
Since the patch adds a #define to string.h, the the custom memmove
function no longer gets emitted, and gcc generates references to it
through __builtin_memmove
I haven't seen this #ifndef memmove in the code, that explains the
behavior.
I would like to remove the optimization for memmove as it uses an
unofficial GCC API.
It is used in quite a few places, and I it's safe to rely on it.
The problem is that I do not fully understand this patch and it is not
good documented.
Why are the __buildin_ functions only called when the length is *not*
a __builtin_constant_p() or smaller than 64 bytes?
The length >= 64 check (to force gcc not to inline larger copies) can
only be guaranteed to run at compile time if the pointer is constant.
If it's not constant, the decision on whether to inline it or not is
left to gcc.
Would be nice to have this in the patch comment, if you do not add it I
will do it later.
Doesn't the compiler automatically replaces a call to memcpy() and
similar functions with the build in version when he things it is good
to do so?
When I first added the patch, there were some issues with that. I don't
know if there are any corner cases with current gcc, but the code
should
be safe the way it is.
- Felix
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