Stop encoding absolute paths in symbols

GCC & Clang encode the current working directory into the symbol files.
Even if the binaries are stripped before being installed, the build-id
hash will be different. This means that two different checkouts will
produce different installed binaries, even if the sources are identical.

To prevent this, set PWD=/proc/self/cwd, so that it replaces the
absolute path with the static path /proc/self/cwd. With GCC, use
-fdebug-prefix-map to remove that static path. Our clang does not yet
support this flag, but it was merged in r250094.

This does change the behavior when using gdb -- you now need to be in
$ANDROID_BUILD_TOP before gdb will load the sources.

Bug: 24204119
Change-Id: I792d65b042b7a3fdc78d567c5a6e93cdf0141b9b

core/clang/config.mk

@@ -78,7 +78,8 @@ CLANG_CONFIG_UNKNOWN_CFLAGS := \
   -Wno-unused-local-typedefs \
   -Wunused-but-set-parameter \
   -Wunused-but-set-variable \
-  -fdiagnostics-color
+  -fdiagnostics-color \
+  -fdebug-prefix-map=/proc/self/cwd=
 
 # Clang flags for all host rules
 CLANG_CONFIG_HOST_EXTRA_ASFLAGS :=

core/config.mk

@@ -176,6 +176,19 @@ include $(BUILD_SYSTEM)/envsetup.mk
 # See envsetup.mk for a description of SCAN_EXCLUDE_DIRS
 FIND_LEAVES_EXCLUDES := $(addprefix --prune=, $(OUT_DIR) $(SCAN_EXCLUDE_DIRS) .repo .git)
 
+# ---------------------------------------------------------------
+# We run gcc/clang with PWD=/proc/self/cwd to remove the $TOP
+# from the debug output. That way two builds in two different
+# directories will create the same output.
+# /proc doesn't exist on Darwin.
+ifeq ($(HOST_OS),linux)
+RELATIVE_PWD := PWD=/proc/self/cwd
+# Remove this useless prefix from the debug output.
+COMMON_GLOBAL_CFLAGS += -fdebug-prefix-map=/proc/self/cwd=
+else
+RELATIVE_PWD :=
+endif
+
 # ---------------------------------------------------------------
 # Allow the C/C++ macros __DATE__ and __TIME__ to be set to the
 # build date and time, so that a build may be repeated.

core/definitions.mk

@@ -1049,7 +1049,7 @@ endef
 define transform-cpp-to-o
 @echo "target $(PRIVATE_ARM_MODE) C++: $(PRIVATE_MODULE) <= $<"
 @mkdir -p $(dir $@)
-$(hide) $(PRIVATE_CXX) \
+$(hide) $(RELATIVE_PWD) $(PRIVATE_CXX) \
 	$(addprefix -I , $(PRIVATE_C_INCLUDES)) \
 	$(shell cat $(PRIVATE_IMPORT_INCLUDES)) \
 	$(addprefix -isystem ,\
@@ -1081,7 +1081,7 @@ endef
 # $(1): extra flags
 define transform-c-or-s-to-o-no-deps
 @mkdir -p $(dir $@)
-$(hide) $(PRIVATE_CC) \
+$(hide) $(RELATIVE_PWD) $(PRIVATE_CC) \
 	$(addprefix -I , $(PRIVATE_C_INCLUDES)) \
 	$(shell cat $(PRIVATE_IMPORT_INCLUDES)) \
 	$(addprefix -isystem ,\
@@ -1156,7 +1156,7 @@ endef
 define transform-host-cpp-to-o
 @echo "$($(PRIVATE_PREFIX)DISPLAY) C++: $(PRIVATE_MODULE) <= $<"
 @mkdir -p $(dir $@)
-$(hide) $(PRIVATE_CXX) \
+$(hide) $(RELATIVE_PWD) $(PRIVATE_CXX) \
 	$(addprefix -I , $(PRIVATE_C_INCLUDES)) \
 	$(shell cat $(PRIVATE_IMPORT_INCLUDES)) \
 	$(addprefix -isystem ,\
@@ -1186,7 +1186,7 @@ endef
 # $(1): extra flags
 define transform-host-c-or-s-to-o-no-deps
 @mkdir -p $(dir $@)
-$(hide) $(PRIVATE_CC) \
+$(hide) $(RELATIVE_PWD) $(PRIVATE_CC) \
 	$(addprefix -I , $(PRIVATE_C_INCLUDES)) \
 	$(shell cat $(PRIVATE_IMPORT_INCLUDES)) \
 	$(addprefix -isystem ,\