Re: [PATCH 1/7] tools build: Only probe the compiler at parse time when it is installed

[Arnaldo Carvalho de Melo <acme@kernel.org>]

On Mon, Aug 10, 2026 at 11:07:40AM -0700, Ian Rogers wrote:
> On Mon, Aug 10, 2026 at 8:44 AM Arnaldo Carvalho de Melo <acme@kernel.org> wrote:
> > Two parse-time probes still invoke $(CC) unconditionally:

> > - LP64 in tools/scripts/Makefile.arch, probing with
> >   $(CC) -E -x c, pulled in twice by tools/perf/Makefile.perf;
> > - CC_NO_CLANG in tools/scripts/Makefile.include, probing with
> >   $(CC) -dM -E -x c /dev/null.

> > In the corner case where gcc is not yet installed, the very setup the
> > install-build-deps target, added in the next patch of this series, is
> > meant for, these probes make even targets that never compile parse-time
> > spew errors like:

> >     /bin/sh: 1: gcc: not found
> >     /bin/sh: 1: gcc: not found
> >     /bin/sh: 1: gcc: not found

> > Guard both probes with 'command -v' using the first word of CC so a
> > missing compiler is handled silently with the same result as a failing
> > probe (CC_NO_CLANG and LP64 unset/0), and with no behavior change when
> > the compiler is installed.

> > Only the first word is consulted because CC may carry arguments such as
> > 'ccache gcc', and shell implementations differ in how 'command -v'
> > handles multiple words (dash only checks the first, bash any of them),
> > so validating the whole CC value would silently disable both probes on
> > some make SHELLs.

> > Assisted-by: opencode:deepseek-v4-flash-free
> > Signed-off-by: Arnaldo Carvalho de Melo <acme@kernel.org>

> > +++ b/tools/scripts/Makefile.arch
> > @@ -38,7 +38,7 @@ ifeq ($(ARCH),loongarch64)
> >         SRCARCH := loongarch
> >  endif

> > -LP64 := $(shell echo __LP64__ | ${CC} ${CFLAGS} -E -x c - | tail -n 1)
> > +LP64 := $(shell if command -v $(firstword ${CC}) >/dev/null 2>&1; then echo __LP64__ | ${CC} ${CFLAGS} -E -x c -; fi | tail -n 1)
> 
> nit: It might be worth adding a comment to explain this fairly complex
> one-liner.

Sure, will add explanations, see below.

> As there are minimum requirements to build the kernel
> (https://www.kernel.org/doc/html/latest/process/changes.html) does it
> make sense to be defensive about CC not being installed?

Well, the point of this series is to install what is needed to build
perf, gcc included, so on a freshly installed machine the user can just
do:

cd tools/perf
make install-build-deps

And the machine will have all the packages that are needed.

> Perhaps install-build-deps.sh can be part of the wider kernel build
> with flags to specify if you are building the kernel or a tool, where
> depencies are different (e.g. perf doesn't need quota-tools but
> libcapstone is useful).

I think this can be a followup series, i.e. making the kernel have that
target as well.

Having it in perf will showcase its usability and may entice the Kbuild
guys to consider accepting such a functionality.

- Arnaldo

acme@number:~/git/perf-tools-next$ git diff ca62b23f83ddfb8e..
diff --git a/tools/scripts/Makefile.arch b/tools/scripts/Makefile.arch
index e0bca6808ec44ed8..ed5f008d400ed238 100644
--- a/tools/scripts/Makefile.arch
+++ b/tools/scripts/Makefile.arch
@@ -38,6 +38,12 @@ ifeq ($(ARCH),loongarch64)
        SRCARCH := loongarch
 endif

+# Probe for __LP64__ only when the compiler is installed: this runs at
+# parse time for every target, including ones that never compile, e.g.
+# install-build-deps, and would otherwise spew "gcc: not found" when the
+# compiler is not yet installed. Only the first word of CC is consulted,
+# as CC may carry arguments, e.g. 'ccache gcc'. When the guard fails it
+# prints nothing, leaving LP64 unset, as if the probe had failed.
 LP64 := $(shell if command -v $(firstword ${CC}) >/dev/null 2>&1; then echo __LP64__ | ${CC} ${CFLAGS} -E -x c -; fi | tail -n 1)
 ifeq ($(LP64), 1)
   IS_64_BIT := 1
diff --git a/tools/scripts/Makefile.include b/tools/scripts/Makefile.include
index fff453aac187536e..378387f629603e92 100644
--- a/tools/scripts/Makefile.include
+++ b/tools/scripts/Makefile.include
@@ -98,6 +98,12 @@ else
   $(call allow-override,LLVM_STRIP,llvm-strip)
 endif

+# The CC_NO_CLANG probe also runs at parse time for targets that never
+# compile, so guard it with 'command -v': when the compiler is not yet
+# installed, e.g. the first install-build-deps run, it would otherwise
+# spew "gcc: not found". Only the first word of CC is consulted, as CC
+# may carry arguments, e.g. 'ccache gcc'. A missing compiler leaves
+# CC_NO_CLANG at 1, as if the probe had failed.
 CC_NO_CLANG := $(shell if command -v $(firstword $(CC)) >/dev/null 2>&1; then $(CC) -dM -E -x c /dev/null; fi | grep -Fq "__clang__"; echo $$?)

 # Some tools require bpftool
acme@number:~/git/perf-tools-next$