[PATCH 1/3] RISC-V: build: Pass `-mattr` to `lld`

[Fāng-ruì Sòng]

On Thu, Aug 5, 2021 at 1:12 PM twd2 <twd2.me at gmail.com> wrote:
>
> Sure. I'll send a v2 after the LLVM issue is resolved.
>
> Thanks!
> Wende
>
>
> On 2021/8/5 4:59, Palmer Dabbelt wrote:
> > On Mon, 19 Jul 2021 14:14:59 PDT (-0700), ndesaulniers at google.com wrote:
> >> On Mon, Jul 19, 2021 at 1:53 PM Wende Tan <twd2.me at gmail.com> wrote:
> >>>
> >>> Pass `-mattr` to `lld` so that it can generate proper machine code.
> >>> By the way, it seems that `-march` does not work well now.
> >>>
> >>> Signed-off-by: Wende Tan <twd2.me at gmail.com>
> >>> ---
> >>>  arch/riscv/Makefile | 7 +++++++
> >>>  1 file changed, 7 insertions(+)
> >>>
> >>> diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
> >>> index bc74afdbf31e..bbc186a3ab45 100644
> >>> --- a/arch/riscv/Makefile
> >>> +++ b/arch/riscv/Makefile
> >>> @@ -52,9 +52,16 @@ riscv-march-$(CONFIG_ARCH_RV32I)     := rv32ima
> >>>  riscv-march-$(CONFIG_ARCH_RV64I)       := rv64ima
> >>>  riscv-march-$(CONFIG_FPU)              := $(riscv-march-y)fd
> >>>  riscv-march-$(CONFIG_RISCV_ISA_C)      := $(riscv-march-y)c
> >>> +riscv-mattr-y                          := +m,+a
> >>> +riscv-mattr-$(CONFIG_FPU)              := $(riscv-mattr-y),+f,+d
> >>> +riscv-mattr-$(CONFIG_RISCV_ISA_C)      := $(riscv-mattr-y),+c
> >>>  KBUILD_CFLAGS += -march=$(subst fd,,$(riscv-march-y))
> >>>  KBUILD_AFLAGS += -march=$(riscv-march-y)
> >>>
> >>> +ifeq ($(CONFIG_LD_IS_LLD),y)
> >>> +       KBUILD_LDFLAGS += -mllvm -mattr=$(riscv-mattr-y)
> >>
> >> We should prevent the proliferation of the compiler internal `-mllvm`
> >> flags from Kbuild Makefiles; these are indicative of LLVM IR specific
> >> issues with regards to LTO.  Can you please file a bug against LLVM
> >> that `-mattr` should be encoded in the IR for LTO's sake?
> >
> > Works for me.  I'll hold off on taking this until the LLVM bug gets sorted out and expect a v2 (ideally with the other textual changes Nick suggested).
> >
> > Thanks!

For GNU ld compatibility: -mllvm -mattr=xxx => -plugin-opt=-mattr=xxx

Which can allow us to drop the CONFIG_LD_IS_LLD condition.
(gold doesn't support RISC-V yet.)

Note: LLVM LTO + GNU ld support is experimental and has very little testing.
To actually make GNU ld work, we probably also need to teach GNU nm
and ar to specify --plugin LLVMgold.so
https://llvm.org/docs/GoldPlugin.html


> >>
> >> See also:
> >> commit 0236526d76b8 ("Makefile: lto: Pass -warn-stack-size only on LLD
> >> < 13.0.0")
> >> commit 2398ce80152a ("x86, lto: Pass -stack-alignment only on LLD < 13.0.0")
> >>
> >>> +endif
> >>> +
> >>>  KBUILD_CFLAGS += -mno-save-restore
> >>>  KBUILD_CFLAGS += -DCONFIG_PAGE_OFFSET=$(CONFIG_PAGE_OFFSET)
> >>>
> >>> --
>
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