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

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

[Richard Biener]

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.
---
 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
-- 
2.31.1

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

[Michael Matz]

Hello,

On Thu, 25 Nov 2021, 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.

Never document in comments what you can document in code (IMO).  I think 
the code as-is clearly documents the invariants and expectations and 
removing the gcc_unreachable() leads to worse sources.

Can't you simply exempt warning on unreachable __builtin_unreachable()?
It seems an obvious thing that the warning should _not_ warn about, after 
all, quite clearly, the author is aware of that being unreachable, it says 
so, right there.


Ciao,
Michael.

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

[Richard Biener]

On Thu, 25 Nov 2021, Michael Matz wrote:

> Hello,
> 
> On Thu, 25 Nov 2021, 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.
> 
> Never document in comments what you can document in code (IMO).  I think 
> the code as-is clearly documents the invariants and expectations and 
> removing the gcc_unreachable() leads to worse sources.
> 
> Can't you simply exempt warning on unreachable __builtin_unreachable()?
> It seems an obvious thing that the warning should _not_ warn about, after 
> all, quite clearly, the author is aware of that being unreachable, it says 
> so, right there.

gcc_unreachable () is not actually __builtin_unreachable () but instead
fancy_abort (__FILE__, __LINE__, __FUNCTION__).  Yes, I agree
that the warning shouldn't warn about "this is unrechable", but if it's
not plain __builtin_unreachable () then we'd need a new function
attribute on it which in this particular case means an alternate
"fancy_abort" since in general fancy_aborts are of course reachable.

We could also handle all noreturn calls this way and not diagnose
those if they are unreachable in exchange for some false negatives.

Btw, I don't agree with "Never document in comments what you can document 
in code" in this case, but I take it as a hint that removing
gcc_unreachable in those cases should at least leave a comment in
there?

Richard.

Re: [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

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

[Richard Biener]

On Sun, 28 Nov 2021, Jeff Law wrote:

> 
> 
> 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.

I've pushed the change.

Richard.