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Date: Fri, 13 Mar 2026 17:40:12 -0700
From: Nathan Chancellor <nathan@kernel.org>
To: Alice Ryhl <aliceryhl@google.com>
Cc: Miguel Ojeda <ojeda@kernel.org>, Boqun Feng <boqun.feng@gmail.com>,
	Gary Guo <gary@garyguo.net>,
	=?iso-8859-1?Q?Bj=F6rn?= Roy Baron <bjorn3_gh@protonmail.com>,
	Benno Lossin <lossin@kernel.org>,
	Andreas Hindborg <a.hindborg@kernel.org>,
	Trevor Gross <tmgross@umich.edu>,
	Danilo Krummrich <dakr@kernel.org>,
	Alexandre Courbot <acourbot@nvidia.com>,
	Will Deacon <will@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Nick Desaulniers <nick.desaulniers+lkml@gmail.com>,
	Bill Wendling <morbo@google.com>,
	Justin Stitt <justinstitt@google.com>,
	Nicolas Schier <nicolas.schier@linux.dev>,
	Andrew Morton <akpm@linux-foundation.org>,
	Uladzislau Rezki <urezki@gmail.com>, rust-for-linux@vger.kernel.org,
	linux-kernel@vger.kernel.org, llvm@lists.linux.dev,
	linux-kbuild@vger.kernel.org, linux-mm@kvack.org,
	Matthew Maurer <mmaurer@google.com>
Subject: Re: [PATCH v2 3/3] build: rust: provide an option to inline C
 helpers into Rust
Message-ID: <20260314004012.GD534169@ax162>
References: <20260203-inline-helpers-v2-0-beb8547a03c9@google.com>
 <20260203-inline-helpers-v2-3-beb8547a03c9@google.com>
 <aasPsbMEsX6iGUl8@google.com>
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In-Reply-To: <aasPsbMEsX6iGUl8@google.com>

On Fri, Mar 06, 2026 at 05:32:33PM +0000, Alice Ryhl wrote:
> On Tue, Feb 03, 2026 at 11:34:10AM +0000, Alice Ryhl wrote:
> >  $(obj)/bindings/bindings_helpers_generated.rs: $(src)/helpers/helpers.c FORCE
> >  	$(call if_changed_dep,bindgen)
> >  
> > +quiet_cmd_rust_helper = HELPER  $@
> > +      cmd_rust_helper = \
> > +	$(CC) $(filter-out $(CFLAGS_REMOVE_helpers/helpers.o), $(c_flags)) \
> > +		-c -g0 $< $(if $(filter %_module.bc,$@),-DMODULE) -emit-llvm -o $@
> > +
> > +$(obj)/helpers/helpers.bc $(obj)/helpers/helpers_module.bc: $(src)/helpers/helpers.c FORCE
> > +	+$(call if_changed_dep,rust_helper)
> 
> I think this needs:
> 
> $(obj)/helpers/helpers.bc: private part-of-builtin := y
> $(obj)/helpers/helpers_module.bc: private part-of-module := y
> 
> at least it seems to make a difference for AutoFDO.

Would

  $(obj)/helpers/helpers.bc $(obj)/helpers/helpers_module.bc: private is-kernel-object := y

work as well? Presumably, this is because c_flags only has the AutoFDO
flags when is-kernel-object is set, which is only true when
helpers/helpers.o is a part of obj-y.

Cheers,
Nathan