[PATCH v5] xen: debug: gcov: add condition coverage support

[Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>]

Condition coverage, also known as MC/DC (modified condition/decision
coverage) is a coverage metric that tracks separate outcomes in
boolean expressions.

This patch adds CONFIG_CONDITION_COVERAGE option to enable MC/DC for
GCC. Clang is not supported right now because Xen can't emit version
10 of LLVM profile data, where MC/DC support was added.

Also, use the opportunity to convert COV_FLAGS to cov-cflags-y, which
reduces amount of ifeqs in Rules.mk. Otherwise this patch had to add
another nesting level with "ifeq ($(CONFIG_CONDITION_COVERAGE),y)".

Signed-off-by: Volodymyr Babchuk <volodymyr_babchuk@xxxxxxxx>

---
Changes in v5:
 - cov-flags-y -> cov-cflags-y
 - Clarified why Clang is not supported right now

Changes in v4:
 - Slight formatting fixes
 - COV_FLAGS  -> cov-flags-y

Changes in v3:
 - Introduced CC_HAS_MCDC that checks if compiler supports
   required feature

Changes in v2:
 - Move gcc version check from .c file to Rules.mk (I can't find
   an easy way to check GCC version at Kconfig level)
 - Check for gcc 14, not gcc 14.1
---
 xen/Kconfig       |  4 ++++
 xen/Kconfig.debug |  9 +++++++++
 xen/Rules.mk      | 14 +++++++-------
 3 files changed, 20 insertions(+), 7 deletions(-)

diff --git a/xen/Kconfig b/xen/Kconfig
index 2128f0ccfc..3a723db8ea 100644
--- a/xen/Kconfig
+++ b/xen/Kconfig
@@ -41,6 +41,10 @@ config CC_SPLIT_SECTIONS
 config CC_HAS_UBSAN
        def_bool $(cc-option,-fsanitize=undefined)
 
+# Compiler supports -fcondition-coverage aka MC/DC
+config CC_HAS_MCDC
+       def_bool $(cc-option,-fcondition-coverage)
+
 # Set code alignment.
 #
 # Allow setting on a boolean basis, and then convert such selection to an
diff --git a/xen/Kconfig.debug b/xen/Kconfig.debug
index f7cc5ffaab..f89cbd823b 100644
--- a/xen/Kconfig.debug
+++ b/xen/Kconfig.debug
@@ -44,6 +44,15 @@ config COVERAGE
 
          If unsure, say N here.
 
+config CONDITION_COVERAGE
+       bool "Condition coverage support"
+       depends on COVERAGE && CC_HAS_MCDC
+       help
+         Enable condition coverage support. Used for collecting MC/DC
+         (Modified Condition/Decision Coverage) metrics.
+
+         If unsure, say N here.
+
 config DEBUG_LOCK_PROFILE
        bool "Lock Profiling"
        select DEBUG_LOCKS
diff --git a/xen/Rules.mk b/xen/Rules.mk
index d759cccee3..da21850926 100644
--- a/xen/Rules.mk
+++ b/xen/Rules.mk
@@ -29,6 +29,7 @@ targets :=
 subdir-y :=
 CFLAGS-y :=
 AFLAGS-y :=
+cov-cflags-y :=
 nocov-y :=
 noubsan-y :=
 
@@ -133,19 +134,18 @@ $(filter %.init.o,$(obj-y) $(obj-bin-y) $(extra-y)): 
CFLAGS-y += -DINIT_SECTIONS
 
 non-init-objects = $(filter-out %.init.o, $(obj-y) $(obj-bin-y) $(extra-y))
 
-ifeq ($(CONFIG_COVERAGE),y)
 ifeq ($(CONFIG_CC_IS_CLANG),y)
-    COV_FLAGS := -fprofile-instr-generate -fcoverage-mapping
+    cov-cflags-$(CONFIG_COVERAGE) := -fprofile-instr-generate 
-fcoverage-mapping
 else
-    COV_FLAGS := -fprofile-arcs -ftest-coverage
+    cov-cflags-$(CONFIG_COVERAGE) := -fprofile-arcs -ftest-coverage
+    cov-cflags-$(CONFIG_CONDITION_COVERAGE) += -fcondition-coverage
 endif
 
-# Reset COV_FLAGS in cases where an objects has another one as prerequisite
+# Reset cov-cflags-y in cases where an objects has another one as prerequisite
 $(nocov-y) $(filter %.init.o, $(obj-y) $(obj-bin-y) $(extra-y)): \
-    COV_FLAGS :=
+    cov-cflags-y :=
 
-$(non-init-objects): _c_flags += $(COV_FLAGS)
-endif
+$(non-init-objects): _c_flags += $(cov-cflags-y)
 
 ifeq ($(CONFIG_UBSAN),y)
 # Any -fno-sanitize= options need to come after any -fsanitize= options
-- 
2.48.1