Re: [PATCH] tcg/optimize: Add fallthrough annotations

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

On 10/29/20 1:28 PM, Thomas Huth wrote:
> To be able to compile this file with -Werror=implicit-fallthrough,
> we need to add some fallthrough annotations to the case statements
> that might fall through. Unfortunately, the typical "/* fallthrough */"
> comments do not work here as expected since some case labels are
> wrapped in macros and the compiler fails to match the comments in
> this case. But using __attribute__((fallthrough)) seems to work fine,
> so let's use that instead.
> 
> Signed-off-by: Thomas Huth <thuth@redhat.com>
> ---
>  tcg/optimize.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/tcg/optimize.c b/tcg/optimize.c
> index 220f4601d5..c2768c9770 100644
> --- a/tcg/optimize.c
> +++ b/tcg/optimize.c
> @@ -26,6 +26,12 @@
>  #include "qemu/osdep.h"
>  #include "tcg/tcg-op.h"
>  
> +#if __has_attribute(fallthrough)
> +# define fallthrough() __attribute__((fallthrough))
> +#else
> +# define fallthrough() do {} while (0)
> +#endif

This looks something we can reuse, what about adding it
as QEMU_FALLTHROUGH in "qemu/compiler.h"?

> +
>  #define CASE_OP_32_64(x)                        \
>          glue(glue(case INDEX_op_, x), _i32):    \
>          glue(glue(case INDEX_op_, x), _i64)
> @@ -855,6 +861,7 @@ void tcg_optimize(TCGContext *s)
>              if ((arg_info(op->args[1])->mask & 0x80) != 0) {
>                  break;
>              }
> +            fallthrough();
>          CASE_OP_32_64(ext8u):
>              mask = 0xff;
>              goto and_const;
> @@ -862,6 +869,7 @@ void tcg_optimize(TCGContext *s)
>              if ((arg_info(op->args[1])->mask & 0x8000) != 0) {
>                  break;
>              }
> +            fallthrough();
>          CASE_OP_32_64(ext16u):
>              mask = 0xffff;
>              goto and_const;
> @@ -869,6 +877,7 @@ void tcg_optimize(TCGContext *s)
>              if ((arg_info(op->args[1])->mask & 0x80000000) != 0) {
>                  break;
>              }
> +            fallthrough();
>          case INDEX_op_ext32u_i64:
>              mask = 0xffffffffU;
>              goto and_const;
> @@ -886,6 +895,7 @@ void tcg_optimize(TCGContext *s)
>              if ((arg_info(op->args[1])->mask & 0x80000000) != 0) {
>                  break;
>              }
> +            fallthrough();
>          case INDEX_op_extu_i32_i64:
>              /* We do not compute affected as it is a size changing op.  */
>              mask = (uint32_t)arg_info(op->args[1])->mask;
>