On Thu, Jan 14, 2021 at 8:20 AM Sedat Dilek [off-list ref] wrote:

On Thu, Jan 14, 2021 at 12:27 AM Nick Desaulniers
[off-list ref] wrote:

quoted

Sedat,
Thanks for testing, and congrats on https://lwn.net/Articles/839772/.
I always appreciate you taking the time to help test my work, and
other Clang+Linux kernel patches!

Hi Nick,

cool, again in the top 15 :-).

I should ask Mr. Corbet for a LWN subscription.

quoted

On Wed, Jan 13, 2021 at 1:24 PM Sedat Dilek [off-list ref] wrote:

quoted

On Wed, Jan 13, 2021 at 1:32 AM Nick Desaulniers
[off-list ref] wrote:

quoted

--- a/Makefile
+++ b/Makefile

@@ -826,12 +826,16 @@ else
 DEBUG_CFLAGS   += -g
 endif

-ifneq ($(LLVM_IAS),1)
-KBUILD_AFLAGS  += -Wa,-gdwarf-2
+dwarf-version-$(CONFIG_DEBUG_INFO_DWARF2) := 2
+dwarf-version-$(CONFIG_DEBUG_INFO_DWARF4) := 4
+DEBUG_CFLAGS   += -gdwarf-$(dwarf-version-y)

^ DEBUG_CFLAGS are set for everyone (all toolchains) if
CONFIG_DEBUG_INFO is defined.

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+ifneq ($(dwarf-version-y)$(LLVM_IAS),21)

^ "If not using dwarf 2 and LLVM_IAS=1", ie. CONFIG_DEBUG_INFO_DWARF5
&& CONFIG_CC_IS_GCC

OK, I know DWARF v2 and LLVM_IAS=1 is broken.

Looks like DWARF v5 with GCC v10.2.1 and binutils v2.35.1 is currently
(here) no good choice.

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+# Binutils 2.35+ required for -gdwarf-4+ support.
+dwarf-aflag    := $(call as-option,-Wa$(comma)-gdwarf-$(dwarf-version-y))
+ifdef CONFIG_CC_IS_CLANG

^ "if clang"

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+DEBUG_CFLAGS   += $(dwarf-aflag)
 endif

Why is that "ifdef CONFIG_CC_IS_CLANG"?

That's what Arvind requested on v2, IIUC:
https://lore.kernel.org/lkml/X8psgMuL4jMjP%2FOy@rani.riverdale.lan/ (local)

quoted

When I use GCC v10.2.1 DEBUG_CFLAGS are not set.

You should have -gdwarf-4 (and not -Wa,-gwarf-4) set for DEBUG_CFLAGS
when compiling with GCC and enabling CONFIG_DEBUG_INFO_DWARF4. Can you
please confirm? (Perhaps you may have accidentally disabled
CONFIG_DEBUG_INFO by rerunning `make defconfig`?)

$ egrep 'CC_IS_|LD_IS|BTF|DWARF'
config-5.11.0-rc3-5-amd64-gcc10-llvm11 | grep ^CONFIG
CONFIG_CC_IS_GCC=y
CONFIG_LD_IS_LLD=y
CONFIG_DEBUG_INFO_DWARF4=y
CONFIG_DEBUG_INFO_BTF=y
CONFIG_DEBUG_INFO_BTF_MODULES=y

$ grep '\-Wa,-gdwarf-4' build-log_5.11.0-rc3-5-amd64-gcc10-llvm11.txt
| wc -l
156

I wonder why I see GNU/as here (see above CONFIG_LD_IS_LLD=y)?

$ llvm-dwarfdump-11 vmlinux | head -20 | egrep
'debug_info|format|version|DW_AT_producer'
vmlinux:        file format elf64-x86-64
.debug_info contents:
0x00000000: Compile Unit: length = 0x0000001e, format = DWARF32,
version = 0x0004, abbr_offset = 0x0000, addr_size = 0x08 (next unit at
0x00000022)
             DW_AT_producer    ("GNU AS 2.35.1")
0x00000022: Compile Unit: length = 0x0000c1d2, format = DWARF32,
version = 0x0004, abbr_offset = 0x0012, addr_size = 0x08 (next unit at
0x0000c1f8)
             DW_AT_producer    ("GNU C89 10.2.1 20210110 -mno-sse
-mno-mmx -mno-sse2 -mno-3dnow -mno-avx -m64 -mno-80387
-mno-fp-ret-in-387 -mpreferred-stack-boundary
=3 -mskip-rax-setup -mtune=generic -mno-red-zone -mcmodel=kernel
-mindirect-branch=thunk-extern -mindirect-branch-register
-mrecord-mcount -mfentry -march=x86-64 -g -g
dwarf-4 -O2 -std=gnu90 -fno-strict-aliasing -fno-common -fshort-wchar
-fno-PIE -falign-jumps=1 -falign-loops=1
-fno-asynchronous-unwind-tables -fno-jump-tables -fno-de
lete-null-pointer-checks -fno-allow-store-data-races
-fstack-protector-strong -fno-strict-overflow -fstack-check=no
-fconserve-stack -fcf-protection=none -fno-stack-pr
otector")

Maybe, I should set all LLVM utils and linker manually, not using LLVM=1.

- Sedat -