linker issue undefined reference to __lshrti3

[wbx-github]

Hi,

I am trying to compile Linux 4.15 branch and targeting Qemu. But the linking step fails with:

• /Users/wbrodkorb/openadk/toolchain_qemu-riscv64_glibc/usr/bin/riscv64-openadk-linux-gnu-ld -melf64lriscv --build-id -o vmlinux -T ./arch/riscv/kernel/vmlinux.lds --whole-archive built-in.o --no-whole-archive --start-group lib/lib.a arch/riscv/lib/lib.a --end-group
  kernel/sched/fair.o: In function .L21': fair.c:(.text+0xdc): undefined reference to __lshrti3'
  kernel/time/timekeeping.o: In function read_persistent_clock64': timekeeping.c:(.text+0x1b0c): undefined reference to __lshrti3'

Looks like gcc emits some symbols not provided by the internal libgcc copy?

Self compiled toolchain with gcc 7.3.0. Happens with gcc 8.1.0, too. Binutils 2.30 + glib 2.27.

./toolchain_qemu-riscv64_glibc/usr/bin/riscv64-openadk-linux-gnu-gcc -v
Using built-in specs.
COLLECT_GCC=./toolchain_qemu-riscv64_glibc/usr/bin/riscv64-openadk-linux-gnu-gcc
COLLECT_LTO_WRAPPER=/Users/wbrodkorb/openadk/toolchain_qemu-riscv64_glibc/usr/libexec/gcc/riscv64-openadk-linux-gnu/7.3.0/lto-wrapper
Target: riscv64-openadk-linux-gnu
Configured with: /Users/wbrodkorb/openadk/toolchain_build_qemu-riscv64_glibc/w-gcc-7.3.0-1/gcc-7.3.0/configure --prefix=/Users/wbrodkorb/openadk/toolchain_qemu-riscv64_glibc/usr --with-bugurl=https://openadk.org --build=x86_64-apple-darwin17.5.0 --host=x86_64-apple-darwin17.5.0 --target=riscv64-openadk-linux-gnu --with-gmp=/Users/wbrodkorb/openadk/host_x86_64-apple-darwin17.5.0/usr --with-mpfr=/Users/wbrodkorb/openadk/host_x86_64-apple-darwin17.5.0/usr --disable-__cxa_atexit --with-system-zlib --with-gnu-ld --with-gnu-as --disable-libsanitizer --disable-install-libiberty --disable-libitm --disable-libmudflap --disable-libgomp --disable-libcc1 --disable-libmpx --disable-libcilkrts --disable-libquadmath --disable-libquadmath-support --disable-decimal-float --disable-libstdcxx-pch --disable-ppl-version-check --disable-cloog-version-check --without-ppl --without-cloog --without-isl --disable-werror --disable-nls --disable-lto --with-arch=rv64imafdc --with-abi=lp64d --enable-tls --enable-threads --enable-libatomic --enable-shared --enable-cxx-flags=-fPIC --disable-libssp --disable-biarch --disable-multilib --enable-languages=c --with-build-sysroot='/../../target_qemu-riscv64_glibc' --with-sysroot='/../../target_qemu-riscv64_glibc'
Thread model: posix
gcc version 7.3.0 (GCC)

Any ideas?

[palmer-dabbelt]

I'm not sure what's going on here. I'd expect that symbol to only end up necessary on a 32-bit kernel, as 64-bit builds can do 64-bit shifts with a single instruction. Our 32-bit port selects GENERIC_LSHRDI3 to get the symbol, and while that might make this go away you'll probably have some other problem.

[jim-wilson]

That is a 128-bit logical right shift. But it should still be expanded inline by a 64-bit compiler.

rohan:2050$ cat tmp.c
__uint128_t
sub (__uint128_t i)
{
  return i >> 10;
}
rohan:2051$ ./xgcc -B./ -O -S tmp.c
rohan:2052$ cat tmp.s
	.file	"tmp.c"
	.option nopic
	.text
	.align	1
	.globl	sub
	.type	sub, @function
sub:
	slli	a5,a1,54
	srli	a0,a0,10
	or	a0,a5,a0
	srli	a1,a1,10
	ret
	.size	sub, .-sub
	.ident	"GCC: (GNU) 9.0.0 20180518 (experimental) [trunk revision 260382]"

[wbx-github]

Looks like it only happens when compiling the kernel with -Os.
The problem disappears for me, when changing to -O2 and disable optimize for size in the kernel configuration.

[aswaterman]

When right-shifting by a variable instead of a constant, it takes 10 instructions, so GCC emits a function call when optimizing for size. Same thing happens on AArch64. So the resolution is that this routine needs to be added to Linux...?

…

On Fri, May 25, 2018 at 10:48 AM, Waldemar Brodkorb < ***@***.***> wrote: Looks like it only happens when compiling the kernel with -Os. The problem disappears for me, when changing to -O2 and disable optimize for size in the kernel configuration. — You are receiving this because you are subscribed to this thread. Reply to this email directly, view it on GitHub <#136 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AA-7woKA2RWQ7rwzZJ4Fv3SkREUpN6rHks5t2EPygaJpZM4T6THl> .

[wbx-github]

I think so, same issue happened some time ago for mips64 and then the missing routine was added to libgcc inside the Linux kernel.
See here for the problematic object file:
https://debug.openadk.org/riscv/timekeeping.o

[xfguo]

Any update?

Still need -O2 instead of -Os.

[xfguo]

xfguo@c666657 this works for me.

[palmer-dabbelt]

@xfguo Do you want to submit that upstream, or should I?

[xfguo]

@palmer-dabbelt Sure, you can submit it to upstream. The implementation is from libgcc, you may need to check it manually.