Re: [Linux-6.11-rc4] perf BROKEN with LLVM/Clang 19.1.0-rc3

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

On Thu, Aug 22, 2024 at 11:26:16AM -0300, Arnaldo Carvalho de Melo wrote:
> So the cast is ok, I think we should disable that
> -Wcast-function-type-mismatch for util/python.o when building with
> clang.
 
> Lemme try to cook a patch for you to test...

Can you try the patch below? Notice that there was already a patch
disabling that for !clang (gcc) for a similar reason:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=b7a313d84e853049062011d78cb04b6decd12f5c

------------------------------------------------------------------------------------------
perf tools: Fix python extension build for gcc 8
The gcc 8 compiler won't compile the python extension code with the
following errors (one example):

  python.c:830:15: error: cast between incompatible  function types from              \
  ‘PyObject * (*)(struct pyrf_evsel *, PyObject *, PyObject *)’                       \
  uct _object * (*)(struct pyrf_evsel *, struct _object *, struct _object *)’} to     \
  ‘PyObject * (*)(PyObject *, PyObject *)’ {aka ‘struct _object * (*)(struct _objeuct \
  _object *)’} [-Werror=cast-function-type]
     .ml_meth  = (PyCFunction)pyrf_evsel__open,

The problem with the PyMethodDef::ml_meth callback is that its type is
determined based on the PyMethodDef::ml_flags value, which we set as
METH_VARARGS | METH_KEYWORDS.
<SNIP>
------------------------------------------------------------------------------------------

Can you please check with both clang versions? I just checked with clang
18 as available in in Fedora 40 and it failed in the first, naïve patch,
that simply adds that -Wno- variant:

  GEN     /tmp/build/perf-tools-next/python/perf.cpython-312-x86_64-linux-gnu.so
error: unknown warning option '-Wno-cast-function-type-mismatch'; did you mean '-Wno-cast-function-type-strict'? [-Werror,-Wunknown-warning-option]
error: command '/usr/bin/clang' failed with exit code 1
cp: cannot stat '/tmp/build/perf-tools-next/python_ext_build/lib/perf*.so': No such file or directory

we need to check if clang has that option, we have infra for that, the
patch below works, please try it.

If it works with clang 19 I'll add two patches, one enhancing
clang_has_option and the other using it for this warning only present in
clang 19.

- Arnaldo

diff --git a/tools/perf/util/setup.py b/tools/perf/util/setup.py
index 142e9d447ce721e3..649550e9b7aa8c8f 100644
--- a/tools/perf/util/setup.py
+++ b/tools/perf/util/setup.py
@@ -17,7 +17,7 @@ src_feature_tests  = getenv('srctree') + '/tools/build/feature'
 
 def clang_has_option(option):
     cc_output = Popen([cc, cc_options + option, path.join(src_feature_tests, "test-hello.c") ], stderr=PIPE).stderr.readlines()
-    return [o for o in cc_output if ((b"unknown argument" in o) or (b"is not supported" in o))] == [ ]
+    return [o for o in cc_output if ((b"unknown argument" in o) or (b"is not supported" in o) or (b"unknown warning option" in o))] == [ ]
 
 if cc_is_clang:
     from sysconfig import get_config_vars
@@ -63,6 +63,8 @@ cflags = getenv('CFLAGS', '').split()
 cflags += ['-fno-strict-aliasing', '-Wno-write-strings', '-Wno-unused-parameter', '-Wno-redundant-decls' ]
 if cc_is_clang:
     cflags += ["-Wno-unused-command-line-argument" ]
+    if clang_has_option("-Wno-cast-function-type-mismatch"):
+        cflags += ["-Wno-cast-function-type-mismatch" ]
 else:
     cflags += ['-Wno-cast-function-type' ]