linux-kernel - Re: [PATCH v2] perf build: Use -fzero-init-padding-bits=all

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Message-ID: <20250320150645.GD3249206@e132581.arm.com>
Date: Thu, 20 Mar 2025 15:06:45 +0000
From: Leo Yan <leo.yan@....com>
To: James Clark <james.clark@...aro.org>
Cc: 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>, Ian Rogers <irogers@...gle.com>,
	Adrian Hunter <adrian.hunter@...el.com>,
	"Liang, Kan" <kan.liang@...ux.intel.com>,
	linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] perf build: Use -fzero-init-padding-bits=all

On Thu, Mar 20, 2025 at 11:46:03AM +0000, James Clark wrote:

[...]

> > --- a/tools/perf/Makefile.config
> > +++ b/tools/perf/Makefile.config
> > @@ -323,6 +323,8 @@ FEATURE_CHECK_LDFLAGS-libaio = -lrt
> >   FEATURE_CHECK_LDFLAGS-disassembler-four-args = -lbfd -lopcodes -ldl
> >   FEATURE_CHECK_LDFLAGS-disassembler-init-styled = -lbfd -lopcodes -ldl
> > +# Explicitly clear padding bits with the initializer '{ 0 }'
> > +CORE_CFLAGS += $(call cc-option,-fzero-init-padding-bits=all)
> >   CORE_CFLAGS += -fno-omit-frame-pointer
> >   CORE_CFLAGS += -Wall
> >   CORE_CFLAGS += -Wextra
> 
> I don't think this makes its way to libperf. I don't have a compiler that
> supports it, but "-std=gnu11" is in CORE_CFLAGS and that's not there on any
> libperf compile commands so I'm assuming CORE_CFLAGS is only local to Perf.

Indeed.  Thanks for pointing out.

Since the commit 91009a3a9913 ("perf build: Install libperf locally when
building"), though the CORE_CFLAGS is appended to LIBPERF_CFLAGS, the
later is never populated to libperf.

I can add the compiler option into tools/lib/perf as well, and remove
the unused LIBPERF_CFLAGS variable.  This can allow the libperf to be
built independently from the perf tool.

To be honest, I am a bit concerned this is still insufficient as Perf
also links other libs (e.g. libbpf).  Another option is to place the
compiler option into tools/scripts/Makefile.include, something like:

diff --git a/tools/scripts/Makefile.include b/tools/scripts/Makefile.include
index 0aa4005017c7..86d1a318a9f6 100644
--- a/tools/scripts/Makefile.include
+++ b/tools/scripts/Makefile.include
@@ -40,6 +40,18 @@ EXTRA_WARNINGS += -Wwrite-strings
 EXTRA_WARNINGS += -Wformat
 EXTRA_WARNINGS += -Wno-type-limits

+try-run = $(shell set -e;               \
+        if ($(1)) >/dev/null 2>&1;      \
+        then echo "$(2)";               \
+        else echo "$(3)";               \
+        fi)
+
+__cc-option = $(call try-run,\
+        $(1) -Werror $(2) -c -x c /dev/null -o /dev/null,$(2),)
+cc-option = $(call __cc-option, $(CC),$(1))
+
+CFLAGS += $(call cc-option,-fzero-init-padding-bits=all)
+

Thanks,
Leo