c: Extend the -Wpadded message with actual padding size

  When the compiler warns about padding struct to alignment boundary, it
now also informs the user about the size of the alignment that needs to
be added to get rid of the warning.

This removes the need of using pahole or similar tools, or manually
determining the padding size.

Tested on x86_64-pc-linux-gnu.

gcc/ChangeLog:

	* stor-layout.cc (finalize_record_size): Improve warning message

Signed-off-by: Vit Kabele <vit.kabele@sysgo.com>
---
 gcc/stor-layout.cc | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)
  

On Thu, Jun 16, 2022 at 3:37 PM Vit Kabele <vit.kabele@sysgo.com> wrote:
>
> When the compiler warns about padding struct to alignment boundary, it
> now also informs the user about the size of the alignment that needs to
> be added to get rid of the warning.

Hi, thanks for taking the time to improve -Wpadded; I have been
wishing that GCC's implementation of -Wpadded would print this
information for a while now and thought there was a bug open for it,
but can't seem to find it now...

>
> This removes the need of using pahole or similar tools, or manually
> determining the padding size.
>
> Tested on x86_64-pc-linux-gnu.
>
> gcc/ChangeLog:
>
>         * stor-layout.cc (finalize_record_size): Improve warning message
>
> Signed-off-by: Vit Kabele <vit.kabele@sysgo.com>
> ---
>  gcc/stor-layout.cc | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/gcc/stor-layout.cc b/gcc/stor-layout.cc
> index 765f22f68b9..57ddb001780 100644
> --- a/gcc/stor-layout.cc
> +++ b/gcc/stor-layout.cc
> @@ -1781,7 +1781,14 @@ finalize_record_size (record_layout_info rli)
>        && simple_cst_equal (unpadded_size, TYPE_SIZE (rli->t)) == 0
>        && input_location != BUILTINS_LOCATION
>        && !TYPE_ARTIFICIAL (rli->t))
> -    warning (OPT_Wpadded, "padding struct size to alignment boundary");
> +  {
> +      tree padding_size
> +               = size_binop (MINUS_EXPR,
> +                       TYPE_SIZE_UNIT (rli->t), unpadded_size_unit);
> +      warning (OPT_Wpadded,
> +                  "padding struct size to alignment boundary with %E bytes",
> +                  padding_size);
> +  }

Style nit: indentation seems off; check your tabs vs. spaces etc.

>
>    if (warn_packed && TREE_CODE (rli->t) == RECORD_TYPE
>        && TYPE_PACKED (rli->t) && ! rli->packed_maybe_necessary
> --
> 2.30.2
  

On Thu, Jun 16, 2022 at 09:37:32PM +0200, Vit Kabele wrote:
> When the compiler warns about padding struct to alignment boundary, it
> now also informs the user about the size of the alignment that needs to
> be added to get rid of the warning.
> 
> This removes the need of using pahole or similar tools, or manually
> determining the padding size.

Thanks for the patch, it looks reasonable, with the formatting fixed.
It would be nice to have a testcase, at least something like

struct S {
  __UINT64_TYPE__ i;
  char c;
};

The problem is what value to check for, on 32-bit arches the padding is
probably 3 bytes large and on 64-bit arches probably 7 bytes.  So I think
you could use __attribute__((aligned (8))) and then it's always 7.

> Tested on x86_64-pc-linux-gnu.
> 
> gcc/ChangeLog:
> 
> 	* stor-layout.cc (finalize_record_size): Improve warning message

Missing '.' at the end.

> 
> Signed-off-by: Vit Kabele <vit.kabele@sysgo.com>
> ---
>  gcc/stor-layout.cc | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/gcc/stor-layout.cc b/gcc/stor-layout.cc
> index 765f22f68b9..57ddb001780 100644
> --- a/gcc/stor-layout.cc
> +++ b/gcc/stor-layout.cc
> @@ -1781,7 +1781,14 @@ finalize_record_size (record_layout_info rli)
>        && simple_cst_equal (unpadded_size, TYPE_SIZE (rli->t)) == 0
>        && input_location != BUILTINS_LOCATION
>        && !TYPE_ARTIFICIAL (rli->t))
> -    warning (OPT_Wpadded, "padding struct size to alignment boundary");
> +  {
> +      tree padding_size
> +		= size_binop (MINUS_EXPR,
> +			TYPE_SIZE_UNIT (rli->t), unpadded_size_unit);
> +      warning (OPT_Wpadded,
> +		   "padding struct size to alignment boundary with %E bytes",
> +		   padding_size);
> +  }
>  
>    if (warn_packed && TREE_CODE (rli->t) == RECORD_TYPE
>        && TYPE_PACKED (rli->t) && ! rli->packed_maybe_necessary
> -- 
> 2.30.2
> 

Marek
  

I fixed the formatting and added the test.

The test has first element 32bit so that it should work on both 32 and
64bit architectures, even without the aligned attribute.

If there is some better way how to write the test properly formatted
(i.e. not on a single line), please let me know.

-- >8 --
Subject: [PATCH] c: Extend the -Wpadded message with actual padding size

When the compiler warns about padding struct to alignment boundary, it
now also informs the user about the size of the alignment that needs to
be added to get rid of the warning.

This removes the need of using pahole or similar tools, or manually
determining the padding size.

Tested on x86_64-pc-linux-gnu.

gcc/ChangeLog:

	* stor-layout.cc (finalize_record_size): Extend warning message.

gcc/testsuite/ChangeLog:

	* c-c++-common/Wpadded.c: New test.

Signed-off-by: Vit Kabele <vit.kabele@sysgo.com>
---
 gcc/stor-layout.cc                   |  7 ++++++-
 gcc/testsuite/c-c++-common/Wpadded.c | 10 ++++++++++
 2 files changed, 16 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/c-c++-common/Wpadded.c

diff --git a/gcc/stor-layout.cc b/gcc/stor-layout.cc
index 765f22f68b9..88923c4136b 100644
--- a/gcc/stor-layout.cc
+++ b/gcc/stor-layout.cc
@@ -1781,7 +1781,12 @@ finalize_record_size (record_layout_info rli)
       && simple_cst_equal (unpadded_size, TYPE_SIZE (rli->t)) == 0
       && input_location != BUILTINS_LOCATION
       && !TYPE_ARTIFICIAL (rli->t))
-    warning (OPT_Wpadded, "padding struct size to alignment boundary");
+  {
+	tree pad_size
+	  = size_binop (MINUS_EXPR, TYPE_SIZE_UNIT (rli->t), unpadded_size_unit);
+	  warning (OPT_Wpadded,
+		"padding struct size to alignment boundary with %E bytes", pad_size);
+  }
 
   if (warn_packed && TREE_CODE (rli->t) == RECORD_TYPE
       && TYPE_PACKED (rli->t) && ! rli->packed_maybe_necessary
diff --git a/gcc/testsuite/c-c++-common/Wpadded.c b/gcc/testsuite/c-c++-common/Wpadded.c
new file mode 100644
index 00000000000..e8f1044a36b
--- /dev/null
+++ b/gcc/testsuite/c-c++-common/Wpadded.c
@@ -0,0 +1,10 @@
+/* { dg-do compile } */
+/* { dg-options "-Wpadded" } */
+
+/*
+ * The struct is on single line, because C++ compiler emits the -Wpadded
+ * warning at the first line of the struct, while the C compiler at the last
+ * line of the struct definition. This way the test passes on both
+ */
+struct S { __UINT32_TYPE__ i; char c; }; /* { dg-warning "padding struct size to alignment boundary with 3 bytes" } */
+
  

On Mon, Jun 20, 2022 at 6:50 AM Vit Kabele <vit.kabele@sysgo.com> wrote:
>
> I fixed the formatting and added the test.
>
> The test has first element 32bit so that it should work on both 32 and
> 64bit architectures, even without the aligned attribute.
>
> If there is some better way how to write the test properly formatted
> (i.e. not on a single line), please let me know.
>
> -- >8 --
> Subject: [PATCH] c: Extend the -Wpadded message with actual padding size
>
> When the compiler warns about padding struct to alignment boundary, it
> now also informs the user about the size of the alignment that needs to
> be added to get rid of the warning.
>
> This removes the need of using pahole or similar tools, or manually
> determining the padding size.
>
> Tested on x86_64-pc-linux-gnu.
>
> gcc/ChangeLog:
>
>         * stor-layout.cc (finalize_record_size): Extend warning message.
>
> gcc/testsuite/ChangeLog:
>
>         * c-c++-common/Wpadded.c: New test.
>
> Signed-off-by: Vit Kabele <vit.kabele@sysgo.com>
> ---
>  gcc/stor-layout.cc                   |  7 ++++++-
>  gcc/testsuite/c-c++-common/Wpadded.c | 10 ++++++++++
>  2 files changed, 16 insertions(+), 1 deletion(-)
>  create mode 100644 gcc/testsuite/c-c++-common/Wpadded.c
>
> diff --git a/gcc/stor-layout.cc b/gcc/stor-layout.cc
> index 765f22f68b9..88923c4136b 100644
> --- a/gcc/stor-layout.cc
> +++ b/gcc/stor-layout.cc
> @@ -1781,7 +1781,12 @@ finalize_record_size (record_layout_info rli)
>        && simple_cst_equal (unpadded_size, TYPE_SIZE (rli->t)) == 0
>        && input_location != BUILTINS_LOCATION
>        && !TYPE_ARTIFICIAL (rli->t))
> -    warning (OPT_Wpadded, "padding struct size to alignment boundary");
> +  {
> +       tree pad_size
> +         = size_binop (MINUS_EXPR, TYPE_SIZE_UNIT (rli->t), unpadded_size_unit);
> +         warning (OPT_Wpadded,
> +               "padding struct size to alignment boundary with %E bytes", pad_size);
> +  }
>
>    if (warn_packed && TREE_CODE (rli->t) == RECORD_TYPE
>        && TYPE_PACKED (rli->t) && ! rli->packed_maybe_necessary
> diff --git a/gcc/testsuite/c-c++-common/Wpadded.c b/gcc/testsuite/c-c++-common/Wpadded.c
> new file mode 100644
> index 00000000000..e8f1044a36b
> --- /dev/null
> +++ b/gcc/testsuite/c-c++-common/Wpadded.c
> @@ -0,0 +1,10 @@
> +/* { dg-do compile } */
> +/* { dg-options "-Wpadded" } */
> +
> +/*
> + * The struct is on single line, because C++ compiler emits the -Wpadded
> + * warning at the first line of the struct, while the C compiler at the last
> + * line of the struct definition. This way the test passes on both
> + */
> +struct S { __UINT32_TYPE__ i; char c; }; /* { dg-warning "padding struct size to alignment boundary with 3 bytes" } */
> +
Note the testcase will fail on some targets where alignment is 1 for everything.
You most likely want the dg-warning to be like it is in gcc.dg/Wpadded.c:
/* { dg-warning "padding struct size to alignment boundary with 3
bytes" ""  { target { ! default_packed } } } */

You might want the following from the same file too:
/* -fpack-struct is necessary because the warning expected requires the initial
   packing to be larger than 1, which cannot be guaranteed for all targets.
   We won't get a warning anyway if the target has "packed" structure
   layout.  */
/* { dg-options "-Wpadded -fpack-struct=8" } */
/* { dg-additional-options "-mno-ms-bitfields" { target *-*-mingw* } } */


Thanks,
Andrew Pinski

> --
> 2.30.2
  

Hello,

On Mon, Jun 20, 2022 at 04:05:17PM -0700, Andrew Pinski wrote:
> > new file mode 100644
> > index 00000000000..e8f1044a36b
> > --- /dev/null
> > +++ b/gcc/testsuite/c-c++-common/Wpadded.c
> > @@ -0,0 +1,10 @@
> > +/* { dg-do compile } */
> > +/* { dg-options "-Wpadded" } */
> > +
> > +/*
> > + * The struct is on single line, because C++ compiler emits the -Wpadded
> > + * warning at the first line of the struct, while the C compiler at the last
> > + * line of the struct definition. This way the test passes on both
> > + */
> > +struct S { __UINT32_TYPE__ i; char c; }; /* { dg-warning "padding struct size to alignment boundary with 3 bytes" } */
> > +
> Note the testcase will fail on some targets where alignment is 1 for everything.
> You most likely want the dg-warning to be like it is in gcc.dg/Wpadded.c:
> /* { dg-warning "padding struct size to alignment boundary with 3
> bytes" ""  { target { ! default_packed } } } */
> 
> You might want the following from the same file too:
> /* -fpack-struct is necessary because the warning expected requires the initial
>    packing to be larger than 1, which cannot be guaranteed for all targets.
>    We won't get a warning anyway if the target has "packed" structure
>    layout.  */
> /* { dg-options "-Wpadded -fpack-struct=8" } */
> /* { dg-additional-options "-mno-ms-bitfields" { target *-*-mingw* } } */
I added the ! default_packed directive, but I am not sure whether the
-fpack-struct is needed. Could you please provide a name of the particular target
with such alignment constraints so I can test it?
  

On Wed, Jun 22, 2022 at 1:34 AM Vit Kabele <vit.kabele@sysgo.com> wrote:
>
> Hello,
>
> On Mon, Jun 20, 2022 at 04:05:17PM -0700, Andrew Pinski wrote:
> > > new file mode 100644
> > > index 00000000000..e8f1044a36b
> > > --- /dev/null
> > > +++ b/gcc/testsuite/c-c++-common/Wpadded.c
> > > @@ -0,0 +1,10 @@
> > > +/* { dg-do compile } */
> > > +/* { dg-options "-Wpadded" } */
> > > +
> > > +/*
> > > + * The struct is on single line, because C++ compiler emits the -Wpadded
> > > + * warning at the first line of the struct, while the C compiler at the last
> > > + * line of the struct definition. This way the test passes on both
> > > + */
> > > +struct S { __UINT32_TYPE__ i; char c; }; /* { dg-warning "padding struct size to alignment boundary with 3 bytes" } */
> > > +
> > Note the testcase will fail on some targets where alignment is 1 for everything.
> > You most likely want the dg-warning to be like it is in gcc.dg/Wpadded.c:
> > /* { dg-warning "padding struct size to alignment boundary with 3
> > bytes" ""  { target { ! default_packed } } } */
> >
> > You might want the following from the same file too:
> > /* -fpack-struct is necessary because the warning expected requires the initial
> >    packing to be larger than 1, which cannot be guaranteed for all targets.
> >    We won't get a warning anyway if the target has "packed" structure
> >    layout.  */
> > /* { dg-options "-Wpadded -fpack-struct=8" } */
> > /* { dg-additional-options "-mno-ms-bitfields" { target *-*-mingw* } } */
> I added the ! default_packed directive, but I am not sure whether the
> -fpack-struct is needed. Could you please provide a name of the particular target
> with such alignment constraints so I can test it?

cris is one example. See
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=23304 (which is why
default_packed was added).

Thanks,
Andrew


>
> --
> Thank you,
> Vit Kabele