On 3 Nov 2017, at 13:42, Will Deacon [off-list ref] wrote:

Hi Jason,

[+Ard]

quoted

On Thu, Nov 02, 2017 at 06:43:22PM +0100, Jason A. Donenfeld wrote:
Versions of gcc prior to gcc 5 emitted a __multi3 function call when
dealing with TI types, resulting in failures when trying to link to
libgcc, and more generally, bad performance. However, since gcc 5,
the compiler supports actually emitting fast instructions, which means
we can at long last enable this option and receive the speedups.

The gcc commit that added proper Aarch64 support is:
https://gcc.gnu.org/git/?p=gcc.git;a=commitdiff;h=d1ae7bb994f49316f6f63e6173f2931e837a351d
This commit appears to be part of the gcc 5 release.

There are still a few instructions, such as __ashlti3 and __ashrti3,
that require libgcc, which is fine. So, we also link to libgcc for these
functions when needed, which is what several other architectures already
have been doing for a long time.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
arch/arm64/Makefile | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)

We used to link against libgcc way back when, but that dependency was
removed in commit d67703a8a69e ("arm64: kill off the libgcc dependency")
and I'm really not keen to add it back. I also think that there might
be licensing concerns if you link against it and make use of GCC plugins,
but IANAL.

Is it guaranteed that libgcc doesn?t use neon/vfp? We can?t use it in the kernel otherwise.

Shouldn't we just provide our own implementations of __ashlti3 and
__ashrti3 instead?

Cheers,

Will

quoted

diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
index 939b310913cf..70c7c0c1bccb 100644
--- a/arch/arm64/Makefile
+++ b/arch/arm64/Makefile

@@ -53,6 +53,8 @@ KBUILD_AFLAGS    += $(lseinstr) $(brokengasinst)

KBUILD_CFLAGS    += $(call cc-option,-mabi=lp64)
KBUILD_AFLAGS    += $(call cc-option,-mabi=lp64)

+KBUILD_CFLAGS    += $(call cc-ifversion, -ge, 0500, -DCONFIG_ARCH_SUPPORTS_INT128)
+
ifeq ($(CONFIG_CPU_BIG_ENDIAN), y)
KBUILD_CPPFLAGS    += -mbig-endian
CHECKFLAGS    += -D__AARCH64EB__

@@ -108,7 +110,7 @@ core-$(CONFIG_NET) += arch/arm64/net/

core-$(CONFIG_KVM) += arch/arm64/kvm/
core-$(CONFIG_XEN) += arch/arm64/xen/
core-$(CONFIG_CRYPTO) += arch/arm64/crypto/
-libs-y        := arch/arm64/lib/ $(libs-y)
+libs-y        := arch/arm64/lib/ $(shell $(CC) $(KBUILD_CFLAGS) -print-libgcc-file-name) $(libs-y)
core-$(CONFIG_EFI_STUB) += $(objtree)/drivers/firmware/efi/libstub/lib.a

# Default target when executing plain make
-- 
2.14.2