[PATCH] diagnostics: Consistently add fixit hint for implicit builtin declaration

Martin Sebor msebor@gmail.com
Fri May 29 00:21:39 GMT 2020


On 5/28/20 5:16 PM, Mark Wielaard wrote:
> There are two warnings that might trigger when a builtin function is
> used but not declared yet. Both called through implicitly_declare in
> c-decl. The first in implicit_decl_warning does warn for builtins,
> but does not add a fixit hint for them (only for non-builtins when
> a header is suggested through lookup_name_fuzzy). This warning is
> guarded by -Wimplicit-function-declaration. The second warning, which
> does include a fixit hint if possible, is given when the implicit
> builtin declaration has an incompatible signature. This second warning
> cannot be disabled.
> 
> This setup means that you only get a fixit-hint for usage of builtin
> functions where the implicit signature is different than the actual
> signature of the builtin. No fixit hints with header suggestions
> are ever generated for builtins like abs, isdigit or putchar.
> 
> It seems more consistent to always generate a fixit-hint if possible
> for the -Wimplicit-function-declaration warning. And for the second
> warning to make it depend on -Wbuiltin-declaration-mismatch like
> other warnings about builtin declaration mismatches.
> 
> Include a new test to show we get fixit-hints for abs, isdigit and
> putchar now. And some small tweaks to existing tests to show the
> effect of -Wno-builtin-declaration-mismatch with this change.
> 
> A nice follow-up would be to merge the built-in missing headers table
> from header_for_builtin_fn in c/c-decl.c with the known headers in
> c-family/known-headers.cc so that they can also be used in the C++
> frontend unqualified_name_lookup_error through suggest_alternatives_for.

This is much more in David's domain than mine but since I promised
to look at it let me just say it seems like a nice improvement :)

Although few tests bother with it, since you add an option for
the existing warning where there was none before, an even more
exhaustive test than the one you added would also verify the same
option can be used to suppress it (e.g., via #pragma GCC diagnostic
ignored).

Martin

> 
> gcc/c/ChangeLog:
> 
> 	* c-decl.c (implicit_decl_warning): When warned and olddecl is
> 	an undeclared builtin, then add a fixit header hint, if found.
> 	(implicitly_declare): Add OPT_Wbuiltin_declaration_mismatch to
> 	warning_at about implicit builtin declaration type mismatch.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* gcc.dg/missing-header-fixit-4.c: Add
> 	-Wno-implicit-function-declaration.
> 	* gcc.dg/missing-header-fixit-4.c: Add new expected output.
> 	* gcc.dg/missing-header-fixit-5.c: New testcase.
> ---
>   gcc/c/c-decl.c                                | 30 ++++++++++++++--
>   gcc/testsuite/gcc.dg/missing-header-fixit-3.c |  2 +-
>   gcc/testsuite/gcc.dg/missing-header-fixit-4.c |  4 +++
>   gcc/testsuite/gcc.dg/missing-header-fixit-5.c | 36 +++++++++++++++++++
>   4 files changed, 68 insertions(+), 4 deletions(-)
>   create mode 100644 gcc/testsuite/gcc.dg/missing-header-fixit-5.c
> 
> diff --git a/gcc/c/c-decl.c b/gcc/c/c-decl.c
> index b3e05be0af87..81bd2ee94f02 100644
> --- a/gcc/c/c-decl.c
> +++ b/gcc/c/c-decl.c
> @@ -3368,8 +3368,30 @@ implicit_decl_warning (location_t loc, tree id, tree olddecl)
>       warned = warning_at (loc, OPT_Wimplicit_function_declaration,
>   			 G_("implicit declaration of function %qE"), id);
>   
> -  if (olddecl && warned)
> -    locate_old_decl (olddecl);
> +  if (warned)
> +    {
> +      /* Whether the olddecl is an undeclared builtin function.
> +	 locate_old_decl will not generate a diagnostic for those,
> +	 so in that case we want to look elsewhere.  */
> +      bool undeclared_builtin = (olddecl
> +				 && TREE_CODE (olddecl) == FUNCTION_DECL
> +				 && fndecl_built_in_p (olddecl)
> +				 && !C_DECL_DECLARED_BUILTIN (olddecl));
> +      if (undeclared_builtin)
> +	{
> +	  const char *header = header_for_builtin_fn (olddecl);
> +	  if (header)
> +	    {
> +	      rich_location richloc (line_table, loc);
> +	      maybe_add_include_fixit (&richloc, header, true);
> +	      inform (&richloc,
> +		      "include %qs or provide a declaration of %qE",
> +		      header, id);
> +	    }
> +	}
> +      else if (olddecl)
> +	locate_old_decl (olddecl);
> +    }
>   
>     if (!warned)
>       hint.suppress ();
> @@ -3631,7 +3653,9 @@ implicitly_declare (location_t loc, tree functionid)
>   						      (TREE_TYPE (decl)));
>   	      if (!comptypes (newtype, TREE_TYPE (decl)))
>   		{
> -		  bool warned = warning_at (loc, 0, "incompatible implicit "
> +		  bool warned = warning_at (loc,
> +					    OPT_Wbuiltin_declaration_mismatch,
> +					    "incompatible implicit "
>   					    "declaration of built-in "
>   					    "function %qD", decl);
>   		  /* See if we can hint which header to include.  */
> diff --git a/gcc/testsuite/gcc.dg/missing-header-fixit-3.c b/gcc/testsuite/gcc.dg/missing-header-fixit-3.c
> index dd53bf65d3c8..8394010c1ac1 100644
> --- a/gcc/testsuite/gcc.dg/missing-header-fixit-3.c
> +++ b/gcc/testsuite/gcc.dg/missing-header-fixit-3.c
> @@ -2,7 +2,7 @@
>      adding them to the top of the file, given that there is no
>      pre-existing #include.  */
>   
> -/* { dg-options "-fdiagnostics-show-caret -fdiagnostics-show-line-numbers" } */
> +/* { dg-options "-fdiagnostics-show-caret -fdiagnostics-show-line-numbers -Wno-implicit-function-declaration" } */
>   
>   void test (int i, int j)
>   {
> diff --git a/gcc/testsuite/gcc.dg/missing-header-fixit-4.c b/gcc/testsuite/gcc.dg/missing-header-fixit-4.c
> index 942897d8c79f..b6680563dc13 100644
> --- a/gcc/testsuite/gcc.dg/missing-header-fixit-4.c
> +++ b/gcc/testsuite/gcc.dg/missing-header-fixit-4.c
> @@ -9,6 +9,10 @@ void test (int i, int j)
>   {
>     printf ("%i of %i\n", i, j); /* { dg-line printf } */
>     /* { dg-warning "implicit declaration of function" "" { target *-*-* } printf } */
> +  /* { dg-begin-multiline-output "" }
> +   10 |   printf ("%i of %i\n", i, j);
> +      |   ^~~~~~
> +   { dg-end-multiline-output "" } */
>     /* { dg-warning "incompatible implicit declaration" "" { target *-*-* } printf } */
>     /* { dg-begin-multiline-output "" }
>      10 |   printf ("%i of %i\n", i, j);
> diff --git a/gcc/testsuite/gcc.dg/missing-header-fixit-5.c b/gcc/testsuite/gcc.dg/missing-header-fixit-5.c
> new file mode 100644
> index 000000000000..916033c689c9
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/missing-header-fixit-5.c
> @@ -0,0 +1,36 @@
> +
> +/* Forget to include any standard headers, all for built-in functions.
> +   Rely on -Wimplicit-function-declaration for fixit hints, not on
> +   -Wbuiltin-declaration-mismatch (which misses abs, isdigit, putchar).  */
> +
> +/* { dg-options "-fdiagnostics-show-caret -fdiagnostics-show-line-numbers -Wimplicit-function-declaration -Wno-builtin-declaration-mismatch" } */
> +
> +int
> +foo (char *m, int i)
> +{
> +  if (isdigit (m[0])) /* { dg-warning "implicit declaration of function" } */
> +  /* { dg-begin-multiline-output "" }
> +   11 |   if (isdigit (m[0]))
> +      |       ^~~~~~~
> +  +++ |+#include <ctype.h>
> +    1 |
> +     { dg-end-multiline-output "" } */
> +    {
> +      return abs (i); /* { dg-warning "implicit declaration of function" } */
> +  /* { dg-begin-multiline-output "" }
> +   19 |       return abs (i);
> +      |              ^~~
> +  +++ |+#include <stdlib.h>
> +    1 |
> +     { dg-end-multiline-output "" } */
> +    }
> +  else
> +    putchar (m[0]); /* { dg-warning "implicit declaration of function" } */
> +  /* { dg-begin-multiline-output "" }
> +   28 |     putchar (m[0]);
> +      |     ^~~~~~~
> +  +++ |+#include <stdio.h>
> +    1 |
> +     { dg-end-multiline-output "" } */
> +  return i;
> +}
> 



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