[PATCH] Remove unreachable gcc_unreachable () at the end of functions

[Jeff Law]

On 11/25/2021 6:33 AM, Richard Biener via Gcc-patches wrote:
> It seems to be a style to place gcc_unreachable () after a
> switch that handles all cases with every case returning.
> Those are unreachable (well, yes!), so they will be elided
> at CFG construction time and the middle-end will place
> another __builtin_unreachable "after" them to note the
> path doesn't lead to a return when the function is not declared
> void.
>
> So IMHO those explicit gcc_unreachable () serve no purpose,
> if they could be replaced by a comment.  But since all cases
> cover switches not handling a case or not returning will
> likely cause some diagnostic to be emitted which is better
> than running into an ICE only at runtime.
>
> Bootstrapped and tested on x86_64-unknown-linux-gnu - any
> comments?
>
> Thanks,
> Richard.
>
> 2021-11-24  Richard Biener  <rguenther@suse.de>
>
> 	* tree.h (reverse_storage_order_for_component_p): Remove
> 	spurious gcc_unreachable.
> 	* cfganal.c (dfs_find_deadend): Likewise.
> 	* fold-const-call.c (fold_const_logb): Likewise.
> 	(fold_const_significand): Likewise.
> 	* gimple-ssa-store-merging.c (lhs_valid_for_store_merging_p):
> 	Likewise.
>
> gcc/c-family/
> 	* c-format.c (check_format_string): Remove spurious
> 	gcc_unreachable.
They would be a check if someone added a case to the switch that didn't 
return.  But we'd get a return-value warning if that happened.  So I 
don't see that they serve much purpose.


> ---
>   gcc/c-family/c-format.c        | 2 --
>   gcc/cfganal.c                  | 2 --
>   gcc/fold-const-call.c          | 2 --
>   gcc/gimple-ssa-store-merging.c | 2 --
>   gcc/tree.h                     | 2 --
>   5 files changed, 10 deletions(-)
>
> diff --git a/gcc/c-family/c-format.c b/gcc/c-family/c-format.c
> index e735e092043..617fb5ea626 100644
> --- a/gcc/c-family/c-format.c
> +++ b/gcc/c-family/c-format.c
> @@ -296,8 +296,6 @@ check_format_string (const_tree fntype, unsigned HOST_WIDE_INT format_num,
>         *no_add_attrs = true;
>         return false;
>       }
> -
> -  gcc_unreachable ();
>   }
>   
>   /* Under the control of FLAGS, verify EXPR is a valid constant that
> diff --git a/gcc/cfganal.c b/gcc/cfganal.c
> index 0cba612738d..48598e55c01 100644
> --- a/gcc/cfganal.c
> +++ b/gcc/cfganal.c
> @@ -752,8 +752,6 @@ dfs_find_deadend (basic_block bb)
>   	  next = e ? e->dest : EDGE_SUCC (bb, 0)->dest;
>   	}
>       }
> -
> -  gcc_unreachable ();
>   }
>   
>   
> diff --git a/gcc/fold-const-call.c b/gcc/fold-const-call.c
> index d6cb9b11a31..c542e780a18 100644
> --- a/gcc/fold-const-call.c
> +++ b/gcc/fold-const-call.c
> @@ -429,7 +429,6 @@ fold_const_logb (real_value *result, const real_value *arg,
>   	}
>         return false;
>       }
> -  gcc_unreachable ();
>   }
>   
>   /* Try to evaluate:
> @@ -463,7 +462,6 @@ fold_const_significand (real_value *result, const real_value *arg,
>   	}
>         return false;
>       }
> -  gcc_unreachable ();
>   }
>   
>   /* Try to evaluate:
> diff --git a/gcc/gimple-ssa-store-merging.c b/gcc/gimple-ssa-store-merging.c
> index e7c90ba8b59..13413ca4cd6 100644
> --- a/gcc/gimple-ssa-store-merging.c
> +++ b/gcc/gimple-ssa-store-merging.c
> @@ -4861,8 +4861,6 @@ lhs_valid_for_store_merging_p (tree lhs)
>       default:
>         return false;
>       }
> -
> -  gcc_unreachable ();
>   }
>   
>   /* Return true if the tree RHS is a constant we want to consider
> diff --git a/gcc/tree.h b/gcc/tree.h
> index f0e72b55abe..094501bd9b1 100644
> --- a/gcc/tree.h
> +++ b/gcc/tree.h
> @@ -5110,8 +5110,6 @@ reverse_storage_order_for_component_p (tree t)
>       default:
>         return false;
>       }
> -
> -  gcc_unreachable ();
>   }
>   
>   /* Return true if T is a storage order barrier, i.e. a VIEW_CONVERT_EXPR