Re: [PATCH 2/2] btf: do not skip members of data type with type id BTF_VOID_TYPEID

[David Faust <david.faust@oracle.com>]

Hi Indu,

On 4/10/24 11:25, Indu Bhagat wrote:
> Testing the previous fix in gen_ctf_sou_type () reveals an issue in BTF
> generation, however: BTF emission was currently decrementing the vlen
> (indicating the number of members) to skip members of type CTF_K_UNKNOWN
> altogether, but still emitting the BTF for the corresponding member (in
> output_asm_btf_sou_fields ()).
> 
> One can see malformed BTF by executing the newly added CTF testcase
> (gcc.dg/debug/ctf/ctf-bitfields-5.c) with -gbtf instead or even existing
> btf-struct-2.c without this patch.
> 
> To fix the issue, it makes sense to rather _not_ skip members of data
> type of type id BTF_VOID_TYPEID.

Thank you for catching and fixing this.

FWIW, what to do in such cases for a struct with a member that has no
representation is undefined behavior in BTF.  I certainly agree it's
better not to emit something malformed, and using 'void' is a good
choice.  Better to know there was a member there that could not be
represented than to skip it altogether, and the total struct size
shall still be correct.

OK.
Thanks!

> 
> gcc/ChangeLog:
> 	* btfout.cc (btf_asm_type): Do not skip emitting members of
> 	unknown type.
> 
> gcc/testsuite/ChangeLog:
> 	* btf-bitfields-4.c: Update the vlen check.
> 	* btf-struct-2.c: Check that member named 'f' with void data
> 	type is emitted.
> ---
>  gcc/btfout.cc                                    | 5 -----
>  gcc/testsuite/gcc.dg/debug/btf/btf-bitfields-4.c | 6 +++---
>  gcc/testsuite/gcc.dg/debug/btf/btf-struct-2.c    | 9 +++++----
>  3 files changed, 8 insertions(+), 12 deletions(-)
> 
> diff --git a/gcc/btfout.cc b/gcc/btfout.cc
> index 4a8ec4d1ff0..ab491f0297f 100644
> --- a/gcc/btfout.cc
> +++ b/gcc/btfout.cc
> @@ -820,11 +820,6 @@ btf_asm_type (ctf_container_ref ctfc, ctf_dtdef_ref dtd)
>  	  /* Set kflag if this member is a representable bitfield.  */
>  	  if (btf_dmd_representable_bitfield_p (ctfc, dmd))
>  	    btf_kflag = 1;
> -
> -	  /* Struct members that refer to unsupported types or bitfield formats
> -	     shall be skipped. These are marked during preprocessing.  */
> -	  else if (!btf_emit_id_p (dmd->dmd_type))
> -	    btf_vlen -= 1;
>  	}
>      }
>  
> diff --git a/gcc/testsuite/gcc.dg/debug/btf/btf-bitfields-4.c b/gcc/testsuite/gcc.dg/debug/btf/btf-bitfields-4.c
> index c00c8b3d87f..d4a6ef6a1eb 100644
> --- a/gcc/testsuite/gcc.dg/debug/btf/btf-bitfields-4.c
> +++ b/gcc/testsuite/gcc.dg/debug/btf/btf-bitfields-4.c
> @@ -6,14 +6,14 @@
>     In this test, we construct a structure such that the bitfield will have
>     an offset so large as to be unrepresentable in BTF. We expect that the
>     resulting BTF will describe the rest of the structure, ignoring the
> -   non-representable bitfield.  */
> +   non-representable bitfield by simply using void data type for the same.  */
>  
>  /* { dg-do compile } */
>  /* { dg-options "-O0 -gbtf -dA" } */
>  /* { dg-require-effective-target size32plus } */
>  
> -/* Struct with 3 members and no bitfield (kind_flag not set).  */
> -/* { dg-final { scan-assembler-times "\[\t \]0x4000003\[\t \]+\[^\n\]*btt_info" 1 } } */
> +/* Struct with 4 members and no bitfield (kind_flag not set).  */
> +/* { dg-final { scan-assembler-times "\[\t \]0x4000004\[\t \]+\[^\n\]*btt_info" 1 } } */
>  
>  struct bigly
>  {
> diff --git a/gcc/testsuite/gcc.dg/debug/btf/btf-struct-2.c b/gcc/testsuite/gcc.dg/debug/btf/btf-struct-2.c
> index e9ff06883db..fa7231be75c 100644
> --- a/gcc/testsuite/gcc.dg/debug/btf/btf-struct-2.c
> +++ b/gcc/testsuite/gcc.dg/debug/btf/btf-struct-2.c
> @@ -2,14 +2,15 @@
>     unsupported type.
>  
>     BTF does not support vector types (among other things). When
> -   generating BTF for a struct (or union) type, members which refer to
> -   unsupported types should be skipped.  */
> +   generating BTF for a struct (or union) type.  Members which refer to
> +   unsupported types should not be skipped, however.  */
>  
>  /* { dg-do compile } */
>  /* { dg-options "-O0 -gbtf -dA" } */
>  
> -/* Expect a struct with only 2 members - 'f' should not be present.  */
> -/* { dg-final { scan-assembler-times "\[\t \]0x4000002\[\t \]+\[^\n\]*btt_info" 1 } } */
> +/* Expect a struct with 3 members - 'f' is present but is of data type void.  */
> +/* { dg-final { scan-assembler-times "\[\t \]0x4000003\[\t \]+\[^\n\]*btt_info" 1 } } */
> +/* { dg-final { scan-assembler-times " MEMBER 'f' idx=1\[\\r\\n\]+\[^\\r\\n\]*0\[\t \]+\[^\n\]*btm_type: void" 1 } } */
>  
>  struct with_float
>  {