[PATCH v2] Makefile: detect compiler and enable more warnings in DEVELOPER=1

["Nguyễn Thái Ngọc Duy" <pclouds@gmail.com>]

The set of extra warnings we enable when DEVELOPER has to be
conservative because we can't assume any compiler version the
developer may use. Detect the compiler version so we know when it's
safe to enable -Wextra and maybe more.

These warning settings are mostly from my custom config.mak a long
time ago when I tried to enable as many warnings as possible that can
still build without showing warnings. Some of them those warnings are
probably worth fixing instead of just suppressing in future.

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
---
 v2 improves a bit over v1:

 - apple clang support (though I suspect we may want to make it a
   separate compiler family since apple clang has different
   versioning, but I can't and won't work on that front)

 - support CC='ccache gcc' (yes it breaks now if you have spaces in
   your cc path)

 - allow to skip detect-compiler by setting COMPILER_FEATURES in
   config.mak

 I notice Ramsay is working on clean -Wmaybe-uninitialized, if his
 series is merged first I'll stop disabling it here.

 Interdiff

    diff --git a/Makefile b/Makefile
    index 9dfd152a1e..04b2a39bab 100644
    --- a/Makefile
    +++ b/Makefile
    @@ -434,6 +434,10 @@ all::
     #
     # When cross-compiling, define HOST_CPU as the canonical name of the CPU on
     # which the built Git will run (for instance "x86_64").
    +#
    +# Define DEVELOPER to enable more compiler warnings. Compiler version
    +# and faimily are auto detected, but could be overridden by defining
    +# COMPILER_FEATURES (see config.mak.dev)
     
     GIT-VERSION-FILE: FORCE
     	@$(SHELL_PATH) ./GIT-VERSION-GEN
    diff --git a/config.mak.dev b/config.mak.dev
    index 59aef342c4..d8beaf9347 100644
    --- a/config.mak.dev
    +++ b/config.mak.dev
    @@ -8,7 +8,9 @@ CFLAGS += -Wstrict-prototypes
     CFLAGS += -Wunused
     CFLAGS += -Wvla
     
    +ifndef COMPILER_FEATURES
     COMPILER_FEATURES := $(shell ./detect-compiler $(CC))
    +endif
     
     ifneq ($(filter clang4,$(COMPILER_FEATURES)),)
     CFLAGS += -Wtautological-constant-out-of-range-compare
    diff --git a/detect-compiler b/detect-compiler
    index bc2ea39ef5..70b754481c 100755
    --- a/detect-compiler
    +++ b/detect-compiler
    @@ -9,7 +9,7 @@ CC="$*"
     #
     # FreeBSD clang version 3.4.1 (tags/RELEASE...)
     get_version_line() {
    -	"$CC" -v 2>&1 | grep ' version '
    +	$CC -v 2>&1 | grep ' version '
     }
     
     get_family() {
    @@ -38,12 +38,15 @@ case "$(get_family)" in
     gcc)
     	print_flags gcc
     	;;
    -*clang)
    +clang)
     	print_flags clang
     	;;
     "FreeBSD clang")
     	print_flags clang
     	;;
    +"Apple LLVM")
    +	print_flags clang
    +	;;
     *)
     	: unknown compiler family
     	;;

 Makefile        | 15 +++++---------
 config.mak.dev  | 30 ++++++++++++++++++++++++++++
 detect-compiler | 53 +++++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 88 insertions(+), 10 deletions(-)
 create mode 100644 config.mak.dev
 create mode 100755 detect-compiler

diff --git a/Makefile b/Makefile
index a1d8775adb..04b2a39bab 100644
--- a/Makefile
+++ b/Makefile
@@ -434,6 +434,10 @@ all::
 #
 # When cross-compiling, define HOST_CPU as the canonical name of the CPU on
 # which the built Git will run (for instance "x86_64").
+#
+# Define DEVELOPER to enable more compiler warnings. Compiler version
+# and faimily are auto detected, but could be overridden by defining
+# COMPILER_FEATURES (see config.mak.dev)
 
 GIT-VERSION-FILE: FORCE
 	@$(SHELL_PATH) ./GIT-VERSION-GEN
@@ -442,15 +446,6 @@ GIT-VERSION-FILE: FORCE
 # CFLAGS and LDFLAGS are for the users to override from the command line.
 
 CFLAGS = -g -O2 -Wall
-DEVELOPER_CFLAGS = -Werror \
-	-Wdeclaration-after-statement \
-	-Wno-format-zero-length \
-	-Wold-style-definition \
-	-Woverflow \
-	-Wpointer-arith \
-	-Wstrict-prototypes \
-	-Wunused \
-	-Wvla
 LDFLAGS =
 ALL_CFLAGS = $(CPPFLAGS) $(CFLAGS)
 ALL_LDFLAGS = $(LDFLAGS)
@@ -1051,7 +1046,7 @@ include config.mak.uname
 -include config.mak
 
 ifdef DEVELOPER
-CFLAGS += $(DEVELOPER_CFLAGS)
+include config.mak.dev
 endif
 
 comma := ,
diff --git a/config.mak.dev b/config.mak.dev
new file mode 100644
index 0000000000..d8beaf9347
--- /dev/null
+++ b/config.mak.dev
@@ -0,0 +1,30 @@
+CFLAGS += -Werror
+CFLAGS += -Wdeclaration-after-statement
+CFLAGS += -Wno-format-zero-length
+CFLAGS += -Wold-style-definition
+CFLAGS += -Woverflow
+CFLAGS += -Wpointer-arith
+CFLAGS += -Wstrict-prototypes
+CFLAGS += -Wunused
+CFLAGS += -Wvla
+
+ifndef COMPILER_FEATURES
+COMPILER_FEATURES := $(shell ./detect-compiler $(CC))
+endif
+
+ifneq ($(filter clang4,$(COMPILER_FEATURES)),)
+CFLAGS += -Wtautological-constant-out-of-range-compare
+endif
+
+ifneq ($(or $(filter gcc6,$(COMPILER_FEATURES)),$(filter clang4,$(COMPILER_FEATURES))),)
+CFLAGS += -Wextra
+CFLAGS += -Wmissing-prototypes
+CFLAGS += -Wno-empty-body
+CFLAGS += -Wno-missing-field-initializers
+CFLAGS += -Wno-sign-compare
+CFLAGS += -Wno-unused-function
+CFLAGS += -Wno-unused-parameter
+ifneq ($(filter gcc6,$(COMPILER_FEATURES)),)
+CFLAGS += -Wno-maybe-uninitialized
+endif
+endif
diff --git a/detect-compiler b/detect-compiler
new file mode 100755
index 0000000000..70b754481c
--- /dev/null
+++ b/detect-compiler
@@ -0,0 +1,53 @@
+#!/bin/sh
+#
+# Probe the compiler for vintage, version, etc. This is used for setting
+# optional make knobs under the DEVELOPER knob.
+
+CC="$*"
+
+# we get something like (this is at least true for gcc and clang)
+#
+# FreeBSD clang version 3.4.1 (tags/RELEASE...)
+get_version_line() {
+	$CC -v 2>&1 | grep ' version '
+}
+
+get_family() {
+	get_version_line | sed 's/^\(.*\) version [0-9][^ ]* .*/\1/'
+}
+
+get_version() {
+	get_version_line | sed 's/^.* version \([0-9][^ ]*\) .*/\1/'
+}
+
+print_flags() {
+	family=$1
+	version=$(get_version | cut -f 1 -d .)
+
+	# Print a feature flag not only for the current version, but also
+	# for any prior versions we encompass. This avoids needing to do
+	# numeric comparisons in make, which are awkward.
+	while test "$version" -gt 0
+	do
+		echo $family$version
+		version=$((version - 1))
+	done
+}
+
+case "$(get_family)" in
+gcc)
+	print_flags gcc
+	;;
+clang)
+	print_flags clang
+	;;
+"FreeBSD clang")
+	print_flags clang
+	;;
+"Apple LLVM")
+	print_flags clang
+	;;
+*)
+	: unknown compiler family
+	;;
+esac
-- 
2.17.0.rc0.348.gd5a49e0b6f