Messages in this thread | ![/](/images/icornerl.gif) | | Subject | Re: [PATCH v2] arm64: support __int128 on gcc 5+ | From | Ard Biesheuvel <> | Date | Fri, 3 Nov 2017 14:02:41 +0000 |
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> On 3 Nov 2017, at 13:42, Will Deacon <will.deacon@arm.com> wrote: > > 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. >
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 > >> 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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