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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 24/11/2025 2:18 am, 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.
> - 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

While you're improving these comments, please drop -fcondition-coverage
(as it's no longer accurate), and expand MC/DC for the benefit of people
who don't know what it is.

>  config CC_HAS_MCDC

Also, # GCC >= 14, or Clang >= 18

It's important for toolchain versions to be given in comments, so we can
figure out what to clean up when upgrading the toolchain baselines.

> diff --git a/xen/common/coverage/llvm.c b/xen/common/coverage/llvm.c
> index 532889c857..a8c7e7e8d2 100644
> --- a/xen/common/coverage/llvm.c
> +++ b/xen/common/coverage/llvm.c
> @@ -120,6 +120,10 @@ extern const char __start___llvm_prf_names[];
>  extern const char __stop___llvm_prf_names[];
>  extern uint64_t __start___llvm_prf_cnts[];
>  extern uint64_t __stop___llvm_prf_cnts[];
> +#ifdef CONFIG_CONDITION_COVERAGE
> +extern const char __start___llvm_prf_bits[];
> +extern const char __stop___llvm_prf_bits[];
> +#endif

No need for these to be #ifdef'd.  In turn, it lets you do ...

>  
>  #define START_DATA      ((const void *)__start___llvm_prf_data)
>  #define END_DATA        ((const void *)__stop___llvm_prf_data)
> @@ -127,16 +131,25 @@ extern uint64_t __stop___llvm_prf_cnts[];
>  #define END_NAMES       ((const void *)__stop___llvm_prf_names)
>  #define START_COUNTERS  ((void *)__start___llvm_prf_cnts)
>  #define END_COUNTERS    ((void *)__stop___llvm_prf_cnts)
> +#define START_BITMAP    ((void *)__start___llvm_prf_bits)
> +#define END_BITMAP      ((void *)__stop___llvm_prf_bits)
>  
>  static void cf_check reset_counters(void)
>  {
>      memset(START_COUNTERS, 0, END_COUNTERS - START_COUNTERS);
> +#ifdef CONFIG_CONDITION_COVERAGE
> +    memset(START_BITMAP, 0, END_BITMAP - START_BITMAP);
> +#endif

... this:

    if ( IS_ENABLED(CONFIG_CONDITION_COVERAGE) )
        memset(START_BITMAP, 0, END_BITMAP - START_BITMAP);

>  }
>  
>  static uint32_t cf_check get_size(void)
>  {
> -    return ROUNDUP(sizeof(struct llvm_profile_header) + END_DATA - 
> START_DATA +
> +    uint32_t size = ROUNDUP(sizeof(struct llvm_profile_header) + END_DATA - 
> START_DATA +
>                     END_COUNTERS - START_COUNTERS + END_NAMES - START_NAMES, 
> 8);
> +#ifdef CONFIG_CONDITION_COVERAGE
> +    size += ROUNDUP(END_BITMAP - START_BITMAP, 8);
> +#endif

and similar here.

> +    return size;
>  }
>  
>  static int cf_check dump(
> @@ -147,11 +160,17 @@ static int cf_check dump(
>          .version = LLVM_PROFILE_VERSION,
>          .num_data = DIV_ROUND_UP(END_DATA - START_DATA, sizeof(struct 
> llvm_profile_data)),
>          .num_counters = DIV_ROUND_UP(END_COUNTERS - START_COUNTERS, 
> sizeof(uint64_t)),
> +#if defined(CONFIG_CONDITION_COVERAGE) && LLVM_PROFILE_VERSION >= 9
> +        .num_bitmap_bytes = END_BITMAP - START_BITMAP,
> +#endif
>          .names_size = END_NAMES - START_NAMES,
>  #if LLVM_PROFILE_VERSION >= 8
>          .counters_delta = START_COUNTERS - START_DATA,
>  #else
>          .counters_delta = (uintptr_t)START_COUNTERS,
> +#endif
> +#if defined(CONFIG_CONDITION_COVERAGE) && LLVM_PROFILE_VERSION >= 9
> +        .bitmap_delta = START_BITMAP - START_DATA,
>  #endif
>          .names_delta = (uintptr_t)START_NAMES,
>          .value_kind_last = LLVM_PROFILE_NUM_KINDS - 1,

With structure initialisation, you do not need to have the fields in
declaration order.  Therefore, you want to do something like this:

         .value_kind_last = LLVM_PROFILE_NUM_KINDS - 1,
+#if defined(CONFIG_CONDITION_COVERAGE) && LLVM_PROFILE_VERSION >= 9
+        .num_bitmap_bytes = END_BITMAP - START_BITMAP,
+        .bitmap_delta = START_BITMAP - START_DATA,
+#endif
 };


to keep the ifdefary more simple.

~Andrew