[RFC PATCH 14/15] config.mak.dev: add a DEVOPTS=analyzer mode to use GCC's -fanalyzer

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

Add an option to compile with GCC's -fanalyzer, which as noted in [1]
and [2] has become much more useful in the recently released GCC v12
series.

Here we're quieting a few outstanding -fanalyzer issues that require
us to use -Wno-error=* on an entire file:

 * range-diff.c, because it involves loop variables and would be
   painfully verbose to instrument with the ASSERT_FOR_FANALYZER() macro
   introduced in the subsequent commit.

 * http-fetch.c and fsmonitor-settings.c, because those aren't issues
   where we're referencing NULL, and therefore we can't quiet it with an
   assert().

For non-GCC compilers I considered wrapping the DEVOPTS logic in:

	ifneq ($(filter gcc10,$(COMPILER_FEATURES)),)
	endif

Which would make it OK to specify DEVOPTS=analyzer under other
compilers, or on older GCC. But then we'd silently ignore the option
on those. Let's instead trust the compiler to error out if it doesn't
support -fanalyzer.

There are various limitations and bugs in the analyzer engine, e.g. I
filed [3] for a false positive in builtin/merge-file.c before GCC v12
was released, which was subsequently fixed in GCC 12 trunk in [4], but
many other issues remain.

1. https://developers.redhat.com/articles/2022/04/12/state-static-analysis-gcc-12-compiler
2. https://gcc.gnu.org/gcc-12/changes.html
3. https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105264
4. https://gcc.gnu.org/git/gitweb.cgi?p=gcc.git;h=a358e4b60815b41e27f3508014ceb592f86b9b45

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---
 Makefile       | 14 ++++++++++++
 config.mak.dev | 61 ++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 75 insertions(+)

diff --git a/Makefile b/Makefile
index 18ca6744a50..129d55f5992 100644
--- a/Makefile
+++ b/Makefile
@@ -507,6 +507,20 @@ include shared.mak
 #    no-pedantic:
 #
 #        Disable -pedantic compilation.
+#
+#    analyzer:
+#
+#        Compile with GCC's -fanalyzer, this analysis is much more
+#        expensive than other GCC warnings.
+#
+#        The set of analysis flags is curated based on known issues
+#        and compiler version. Known issues are made into non-fatal
+#        warnings (even "no-error" isn't set).
+#
+#    no-suppress-analyzer:
+#
+#        When using "analyzer" disable the suppression of known
+#        -fanalyzer issues.
 
 GIT-VERSION-FILE: FORCE
 	@$(SHELL_PATH) ./GIT-VERSION-GEN
diff --git a/config.mak.dev b/config.mak.dev
index c3104f400b2..d6f5be92297 100644
--- a/config.mak.dev
+++ b/config.mak.dev
@@ -72,3 +72,64 @@ DEVELOPER_CFLAGS += -Wno-error=dangling-pointer
 endif
 
 GIT_TEST_PERL_FATAL_WARNINGS = YesPlease
+
+# GCC's -fanalyzer mode
+ifeq ($(filter analyzer,$(DEVOPTS)),analyzer)
+
+ifeq ($(filter gcc1,$(COMPILER_FEATURES)),)
+$(error you must be using a new-ish version of GCC for DEVOPTS=analyzer, your \
+$(CC) is not GCC at all!)
+endif
+
+DEVELOPER_CFLAGS += -fanalyzer
+
+## -fanalyzer exists exists as of gcc10, but versions older than gcc12
+## have a lot of false positives.
+ifeq ($(filter gcc12,$(COMPILER_FEATURES)),)
+DEVELOPER_CFLAGS += -Wno-analyzer-double-free
+DEVELOPER_CFLAGS += -Wno-analyzer-free-of-non-heap
+endif
+
+## Helper templates to whitelist existing issues
+define fn_disable_analyzer_tmpl
+$(1).sp $(1).s $(1).o: EXTRA_CPPFLAGS += $(2)
+
+endef
+
+define fn_disable_analyzer
+$(foreach f,$(2),$(call fn_disable_analyzer_tmpl,$(f),$(1)))
+endef
+
+## -Wno-error=analyzer-null-dereference
+$(eval $(call fn_disable_analyzer, \
+	-Wno-error=analyzer-null-dereference, \
+	range-diff \
+))
+## -Wno-error=analyzer-malloc-leak
+$(eval $(call fn_disable_analyzer, \
+	-Wno-error=analyzer-malloc-leak, \
+	fsmonitor-settings \
+))
+## per-GCC version annotations
+### -Wno-error=analyzer-use-of-uninitialized-value: gcc >= 12
+ifneq ($(filter gcc12,$(COMPILER_FEATURES)),)
+$(eval $(call fn_disable_analyzer, \
+	-Wno-error=analyzer-use-of-uninitialized-value, \
+	http-fetch \
+))
+else # gcc < 12
+### -Wno-error=analyzer-null-dereference: gcc == 11
+ifneq ($(filter gcc11,$(COMPILER_FEATURES)),)
+$(eval $(call fn_disable_analyzer, \
+	-Wno-error=analyzer-null-dereference, \
+	merge \
+	xdiff/xemit \
+	reftable/reader \
+))
+else
+$(error Your GCC version is too old for -fanalyze, or you are using \
+gcc10 which has it, but has too many false positives!)
+endif
+endif # gcc < 12
+
+endif
-- 
2.36.1.1124.g577fa9c2ebd