linux-kernel - Re: objtool warning, branch, patch, and .config (GCC bug)

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Date:   Thu, 26 May 2022 11:41:24 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Jakub Jelinek <jakub@...hat.com>
Cc:     "Paul E. McKenney" <paulmck@...nel.org>,
        linux-kernel@...r.kernel.org, Josh Poimboeuf <jpoimboe@...hat.com>,
        linux-toolchains@...r.kernel.org
Subject: Re: objtool warning, branch, patch, and .config (GCC bug)

On Thu, May 26, 2022 at 11:02:02AM +0200, Jakub Jelinek wrote:
> On Thu, May 26, 2022 at 10:08:49AM +0200, Peter Zijlstra wrote:
> > On Thu, May 26, 2022 at 10:03:13AM +0200, Peter Zijlstra wrote:
> > 
> > > Which is just bloody weird/broken if you ask me. What's worse, GCC-10
> > > does the same. Only when I use GCC-11 do I get sensible code again:
> > 
> > Just to clarify, I can reproduce using:
> > 
> > gcc-9 (Debian 9.4.0-5) 9.4.0
> > gcc-10 (Debian 10.3.0-15) 10.3.0
> > 
> > The good compiler is:
> > 
> > gcc-11 (Debian 11.3.0-1) 11.3.0
> 
> Such fallthrough into another function is typically the result
> of __builtin_unreachable (), either explicit somewhere in the code,
> or invoking undefined behavior somewhere and __builtin_unreachable ()
> replacing the UB code.
> I'd need preprocessed source + full gcc command line to tell more
> (as long as it is not LTO, with LTO that isn't enough of course).

 $ make O=build/ CC=gcc-9 kernel/rcu/update.o V=1

gives the compile command as:

  gcc-9 -Wp,-MMD,kernel/rcu/.update.o.d -nostdinc -I../arch/x86/include -I./arch/x86/include/generated -I../include -I./include -I../arch/x86/include/uapi -I./arch/x86/include/generated/uapi -I../include/uapi -I./include/generated/uapi -include ../include/linux/compiler-version.h -include ../include/linux/kconfig.h -include ../include/linux/compiler_types.h -D__KERNEL__ -fmacro-prefix-map=../= -Wall -Wundef -Werror=strict-prototypes -Wno-trigraphs -fno-strict-aliasing -fno-common -fshort-wchar -fno-PIE -Werror=implicit-function-declaration -Werror=implicit-int -Werror=return-type -Wno-format-security -std=gnu11 -mno-sse -mno-mmx -mno-sse2 -mno-3dnow -mno-avx -fcf-protection=none -m64 -falign-jumps=1 -falign-loops=1 -mno-80387 -mno-fp-ret-in-387 -mpreferred-stack-boundary=3 -mskip-rax-setup -mtune=generic -mno-red-zone -mcmodel=kernel -Wno-sign-compare -fno-asynchronous-unwind-tables -mindirect-branch=thunk-extern -mindirect-branch-register -fno-jump-tables -fno-delete-null-pointer-checks -Wno-frame-address -Wno-format-truncation -Wno-format-overflow -Wno-address-of-packed-member -O2 --param=allow-store-data-races=0 -Wframe-larger-than=2048 -fstack-protector-strong -Werror -Wimplicit-fallthrough=5 -Wno-main -Wno-unused-but-set-variable -Wno-unused-const-variable -fomit-frame-pointer -fno-stack-clash-protection -Wdeclaration-after-statement -Wvla -Wno-pointer-sign -Wcast-function-type -Wno-stringop-truncation -Wno-stringop-overflow -Wno-restrict -Wno-maybe-uninitialized -Wno-alloc-size-larger-than -fno-strict-overflow -fno-stack-check -fconserve-stack -Werror=date-time -Werror=incompatible-pointer-types -Werror=designated-init -Wno-packed-not-aligned -I ../kernel/rcu -I ./kernel/rcu    -DKBUILD_MODFILE='"kernel/rcu/update"' -DKBUILD_BASENAME='"update"' -DKBUILD_MODNAME='"update"' -D__KBUILD_MODNAME=kmod_update -c -o kernel/rcu/update.o ../kernel/rcu/update.c

I've attached the output of:

 $ make O=build/ CC=gcc-9 kernel/rcu/update.i



Download attachment "update.i.bz2" of type "application/octet-stream" (285348 bytes)