Messages in this thread | | | Date | Fri, 3 Nov 2017 13:42:24 +0000 | From | Will Deacon <> | Subject | Re: [PATCH v2] arm64: support __int128 on gcc 5+ |
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Hi Jason,
[+Ard]
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.
Shouldn't we just provide our own implementations of __ashlti3 and __ashrti3 instead?
Cheers,
Will
> 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 >
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