linux-kernel - Re: [PATCH v1] perf build: Revert "enable -fno-strict-aliasing"

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Message-ID: <20250904205924.GO4068168@noisy.programming.kicks-ass.net>
Date: Thu, 4 Sep 2025 22:59:24 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Ian Rogers <irogers@...gle.com>
Cc: Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Namhyung Kim <namhyung@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Jiri Olsa <jolsa@...nel.org>,
	Adrian Hunter <adrian.hunter@...el.com>,
	Kan Liang <kan.liang@...ux.intel.com>,
	linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
	Eric Biggers <ebiggers@...gle.com>
Subject: Re: [PATCH v1] perf build: Revert "enable -fno-strict-aliasing"

On Thu, Sep 04, 2025 at 10:41:17PM +0200, Peter Zijlstra wrote:
> On Thu, Sep 04, 2025 at 10:44:30AM -0700, Ian Rogers wrote:
> > This reverts commit 55a18d2f3ff7 ("perf build: enable
> > -fno-strict-aliasing"). With (get|put)_unaligned_* using memcpy
> > -fno-strict-aliasing is no longer necessary as memcpys are assumed to
> > possibly alias.
> 
> I don't think this is a good idea. Much of tools/ includes kernel
> headers and various kernel code, all of which is written in the
> understanding that this (often called broken) C language feature does
> not exist.
> 
> As such, I would strongly suggest all of tools is built with
> -fno-strict-aliasing.

Similarly I would strongly suggest having -fwrapv on all code that
includes kernel headers.

> > Signed-off-by: Ian Rogers <irogers@...gle.com>
> > ---
> > This patch needs:
> > https://lore.kernel.org/lkml/20250722215754.672330-1-irogers@google.com/
> > which have been merged into the tip timers/vdso branch.
> > ---
> >  tools/perf/Makefile.config | 4 ----
> >  1 file changed, 4 deletions(-)
> > 
> > diff --git a/tools/perf/Makefile.config b/tools/perf/Makefile.config
> > index 5a5832ee7b53..306b8334b788 100644
> > --- a/tools/perf/Makefile.config
> > +++ b/tools/perf/Makefile.config
> > @@ -19,10 +19,6 @@ detected_var = $(shell echo "$(1)=$($(1))" >> $(OUTPUT).config-detected)
> >  CFLAGS := $(EXTRA_CFLAGS) $(filter-out -Wnested-externs,$(EXTRA_WARNINGS))
> >  HOSTCFLAGS := $(filter-out -Wnested-externs,$(EXTRA_WARNINGS))
> >  
> > -# This is required because the kernel is built with this and some of the code
> > -# borrowed from kernel headers depends on it, e.g. put_unaligned_*().
> > -CFLAGS += -fno-strict-aliasing
> > -
> >  # Enabled Wthread-safety analysis for clang builds.
> >  ifeq ($(CC_NO_CLANG), 0)
> >    CFLAGS += -Wthread-safety
> > -- 
> > 2.51.0.355.g5224444f11-goog
> >