Re: [PATCH] c-family: Honor -Wno-init-self for cv-qual vars [PR102633]

[Richard Biener <rguenther@suse.de>]

On Tue, 26 Jul 2022, Marek Polacek wrote:

> Since r11-5188-g32934a4f45a721, we drop qualifiers during l-to-r
> conversion by creating a NOP_EXPR.  For e.g.
> 
>   const int i = i;
> 
> that means that the DECL_INITIAL is '(int) i' and not 'i' anymore.
> Consequently, we don't suppress_warning here:
> 
> 711     case DECL_EXPR:
> 715       if (VAR_P (DECL_EXPR_DECL (*expr_p))
> 716           && !DECL_EXTERNAL (DECL_EXPR_DECL (*expr_p))
> 717           && !TREE_STATIC (DECL_EXPR_DECL (*expr_p))
> 718           && (DECL_INITIAL (DECL_EXPR_DECL (*expr_p)) == DECL_EXPR_DECL (*expr_p))
> 719           && !warn_init_self)
> 720         suppress_warning (DECL_EXPR_DECL (*expr_p), OPT_Winit_self);
> 
> because of the check on line 718 -- (int) i is not i.  So -Wno-init-self
> doesn't disable the warning as it's supposed to.
> 
> The following patch fixes it...except it doesn't, for volatile variables
> in C++.  The problem is that for
> 
>   volatile int k = k;
> 
> we see that the initializer has TREE_SIDE_EFFECTS, so we perform dynamic
> initialization.  So there's no DECL_INITIAL and the suppress_warning
> call above is never done.  I suppose we could amend get_no_uninit_warning
> to return true for volatile-qualified expressions.  I mean, can we ever
> say for a fact that a volatile variable is uninitialized?

As I said in the bug the bug is probably that we emit uninitialized
diagnostics for volatiles at all?  OTOH 'volatile' is recommended
for vars live around setjmp/longjmp and there diagnostics would be
welcome.  It's probably the difference between "compiler-hands-off"
and "hardware-controlled" :/

> 
> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
> 
> 	PR middle-end/102633
> 
> gcc/c-family/ChangeLog:
> 
> 	* c-gimplify.cc (c_gimplify_expr): Strip NOPs of DECL_INITIAL.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* c-c++-common/Winit-self1.c: New test.
> 	* c-c++-common/Winit-self2.c: New test.
> ---
>  gcc/c-family/c-gimplify.cc               | 18 +++++++------
>  gcc/testsuite/c-c++-common/Winit-self1.c | 32 ++++++++++++++++++++++++
>  gcc/testsuite/c-c++-common/Winit-self2.c | 31 +++++++++++++++++++++++
>  3 files changed, 74 insertions(+), 7 deletions(-)
>  create mode 100644 gcc/testsuite/c-c++-common/Winit-self1.c
>  create mode 100644 gcc/testsuite/c-c++-common/Winit-self2.c
> 
> diff --git a/gcc/c-family/c-gimplify.cc b/gcc/c-family/c-gimplify.cc
> index a6f26c9b0d3..2e011830846 100644
> --- a/gcc/c-family/c-gimplify.cc
> +++ b/gcc/c-family/c-gimplify.cc
> @@ -712,13 +712,17 @@ c_gimplify_expr (tree *expr_p, gimple_seq *pre_p ATTRIBUTE_UNUSED,
>        /* This is handled mostly by gimplify.cc, but we have to deal with
>  	 not warning about int x = x; as it is a GCC extension to turn off
>  	 this warning but only if warn_init_self is zero.  */
> -      if (VAR_P (DECL_EXPR_DECL (*expr_p))
> -	  && !DECL_EXTERNAL (DECL_EXPR_DECL (*expr_p))
> -	  && !TREE_STATIC (DECL_EXPR_DECL (*expr_p))
> -	  && (DECL_INITIAL (DECL_EXPR_DECL (*expr_p)) == DECL_EXPR_DECL (*expr_p))
> -	  && !warn_init_self)
> -	suppress_warning (DECL_EXPR_DECL (*expr_p), OPT_Winit_self);
> -      break;
> +      {
> +	tree &decl = DECL_EXPR_DECL (*expr_p);
> +	if (VAR_P (decl)
> +	    && !DECL_EXTERNAL (decl)
> +	    && !TREE_STATIC (decl)
> +	    && (DECL_INITIAL (decl)
> +		&& tree_strip_nop_conversions (DECL_INITIAL (decl)) == decl)
> +	    && !warn_init_self)
> +	  suppress_warning (decl, OPT_Winit_self);
> +	break;
> +      }
>  
>      case PREINCREMENT_EXPR:
>      case PREDECREMENT_EXPR:
> diff --git a/gcc/testsuite/c-c++-common/Winit-self1.c b/gcc/testsuite/c-c++-common/Winit-self1.c
> new file mode 100644
> index 00000000000..2a1a755fc71
> --- /dev/null
> +++ b/gcc/testsuite/c-c++-common/Winit-self1.c
> @@ -0,0 +1,32 @@
> +/* PR middle-end/102633 */
> +/* { dg-do compile } */
> +/* { dg-options "-Wuninitialized -Wno-init-self" } */
> +
> +int
> +fn1 (void)
> +{
> +  int i = i;
> +  return i;
> +}
> +
> +int
> +fn2 ()
> +{
> +  const int j = j;
> +  return j;
> +}
> +
> +int
> +fn3 ()
> +{
> +  /* ??? Do we want this warning in C++?  Probably not with -Wno-init-self.  */
> +  volatile int k = k; /* { dg-warning "used uninitialized" "" { target c++ } } */
> +  return k;
> +}
> +
> +int
> +fn4 ()
> +{
> +  const volatile int l = l; /* { dg-warning "used uninitialized" "" { target c++ } } */
> +  return l;
> +}
> diff --git a/gcc/testsuite/c-c++-common/Winit-self2.c b/gcc/testsuite/c-c++-common/Winit-self2.c
> new file mode 100644
> index 00000000000..13aa9efdf26
> --- /dev/null
> +++ b/gcc/testsuite/c-c++-common/Winit-self2.c
> @@ -0,0 +1,31 @@
> +/* PR middle-end/102633 */
> +/* { dg-do compile } */
> +/* { dg-options "-Wuninitialized -Winit-self" } */
> +
> +int
> +fn1 (void)
> +{
> +  int i = i; /* { dg-warning "used uninitialized" } */
> +  return i;
> +}
> +
> +int
> +fn2 ()
> +{
> +  const int j = j; /* { dg-warning "used uninitialized" } */
> +  return j;
> +}
> +
> +int
> +fn3 ()
> +{
> +  volatile int k = k; /* { dg-warning "used uninitialized" } */
> +  return k;
> +}
> +
> +int
> +fn4 ()
> +{
> +  const volatile int l = l; /* { dg-warning "used uninitialized" } */
> +  return l;
> +}
> 
> base-commit: 600956c81c784f4a0cc9d10f6e03e01847afd961
> 

-- 
Richard Biener <rguenther@suse.de>
SUSE Software Solutions Germany GmbH, Frankenstrasse 146, 90461 Nuernberg,
Germany; GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman;
HRB 36809 (AG Nuernberg)