From 021d23428bdbae032294e8f4a29cb53cb50ae71c Mon Sep 17 00:00:00 2001 From: Wende Tan Date: Tue, 17 Oct 2023 15:21:04 -0700 Subject: RISC-V: build: Allow LTO to be selected Allow LTO to be selected for RISC-V, only when LLD >= 14, since there is an issue [1] in prior LLD versions that prevents LLD to generate proper machine code for RISC-V when writing `nop`s. To avoid boot failures in QEMU [2], '-mattr=+c' and '-mattr=+relax' need to be passed via '-mllvm' to ld.lld, as there appears to be an issue with LLVM's target-features and LTO [3], which can result in incorrect relocations to branch targets [4]. Once this is fixed in LLVM, it can be made conditional on affected ld.lld versions. Disable LTO for arch/riscv/kernel/pi, as llvm-objcopy expects an ELF object file when manipulating the files in that subfolder, rather than LLVM bitcode. [1] https://github.com/llvm/llvm-project/issues/50505, resolved by LLVM commit e63455d5e0e5 ("[MC] Use local MCSubtargetInfo in writeNops") [2] https://github.com/ClangBuiltLinux/linux/issues/1942 [3] https://github.com/llvm/llvm-project/issues/59350 [4] https://github.com/llvm/llvm-project/issues/65090 Tested-by: Wende Tan Signed-off-by: Wende Tan Co-developed-by: Nathan Chancellor Signed-off-by: Nathan Chancellor Reviewed-by: Conor Dooley Link: https://lore.kernel.org/r/20231017-riscv-lto-v4-1-e7810b24e805@kernel.org Signed-off-by: Palmer Dabbelt --- arch/riscv/Makefile | 5 +++++ 1 file changed, 5 insertions(+) (limited to 'arch/riscv/Makefile') diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile index 0b7d109258e7..252d63942f34 100644 --- a/arch/riscv/Makefile +++ b/arch/riscv/Makefile @@ -50,6 +50,11 @@ ifndef CONFIG_AS_IS_LLVM KBUILD_CFLAGS += -Wa,-mno-relax KBUILD_AFLAGS += -Wa,-mno-relax endif +# LLVM has an issue with target-features and LTO: https://github.com/llvm/llvm-project/issues/59350 +# Ensure it is aware of linker relaxation with LTO, otherwise relocations may +# be incorrect: https://github.com/llvm/llvm-project/issues/65090 +else ifeq ($(CONFIG_LTO_CLANG),y) + KBUILD_LDFLAGS += -mllvm -mattr=+c -mllvm -mattr=+relax endif ifeq ($(CONFIG_SHADOW_CALL_STACK),y) -- cgit