Re: [PATCH] XEN: enable MC/DC coverage for Clang

[Jan Beulich <jbeulich@xxxxxxxx>]

On 24.11.2025 03:18, Saman Dehghan wrote:
> Clang >= 18 supports Modified Condition/Decision Coverage (MC/DC).
> This patch enables the detection and usage of this feature when
> compiling Xen with Clang.
> 
> - Update detection logic to check for '-fcoverage-mcdc' when using Clang.

You check for ...

> - Update llvm.c to handle the profile format changes (bitmap section)
>   required for MC/DC.
> - Guard -Wno-error=coverage-too-many-conditions with CONFIG_CC_IS_GCC
>   to avoid passing a GCC-only warning option to Clang
> 
> Signed-off-by: Saman Dehghan <samaan.dehghan@xxxxxxxxx>
> ---
>  xen/Kconfig                |  2 +-
>  xen/Rules.mk               |  1 +
>  xen/arch/x86/Makefile      |  4 +++-
>  xen/common/coverage/llvm.c | 24 +++++++++++++++++++++++-
>  4 files changed, 28 insertions(+), 3 deletions(-)
> 
> diff --git a/xen/Kconfig b/xen/Kconfig
> index a5e5af3b76..5508993f02 100644
> --- a/xen/Kconfig
> +++ b/xen/Kconfig
> @@ -53,7 +53,7 @@ config CC_HAS_ASM_GOTO_OUTPUT
>  
>  # Compiler supports -fcondition-coverage aka MC/DC
>  config CC_HAS_MCDC
> -     def_bool $(cc-option,-fcondition-coverage)
> +     def_bool $(cc-option,-fcondition-coverage) || 
> $(cc-option,-fprofile-instr-generate -fcoverage-mapping -fcoverage-mcdc)

... more than that one option here. Presumably because the option alone
wouldn't be liked by the compiler? (May want mentioning in that part of the
description.)

> --- a/xen/arch/x86/Makefile
> +++ b/xen/arch/x86/Makefile
> @@ -99,7 +99,9 @@ ifneq ($(CONFIG_HVM),y)
>  $(obj)/x86_emulate.o: CFLAGS-y += -Wno-unused-label
>  endif
>  ifeq ($(CONFIG_CONDITION_COVERAGE),y)
> -$(obj)/x86_emulate.o: CFLAGS-y += -Wno-error=coverage-too-many-conditions
> +    ifeq ($(CONFIG_CC_IS_GCC),y)
> +        $(obj)/x86_emulate.o: CFLAGS-y += 
> -Wno-error=coverage-too-many-conditions
> +    endif
>  endif

Please can the two conditionals be combined, like I think we do elsewhere:

ifeq ($(CONFIG_CONDITION_COVERAGE)$(CONFIG_CC_IS_GCC),yy)

or

ifeq ($(CONFIG_CONDITION_COVERAGE)_$(CONFIG_CC_IS_GCC),y_y)

?

Jan