[PATCH v3 10/15] gas: Skip SFrame FDE if FP without RA on stack

[Indu Bhagat]

On 4/12/24 07:47, Jens Remus wrote:
> The SFrame format cannot represent the frame pointer (FP) being saved
> on the stack without the return address (RA) also being saved on the
> stack, if RA tracking is used.
> 
> A SFrame FDE is followed by 1-3 offsets with the following information:
> 
> Without RA tracking:
> 1. Offset from base pointer (SP or FP) to locate the CFA
> 2. Optional: Offset to CFA to restore the frame pointer (FP)
> 
> With RA tracking:
> 1. Offset from base pointer (SP or FP) to locate the CFA
> 2. Optional: Offset to CFA to restore the return address (RA)
> 3. Optional: Offset to CFA to restore the frame pointer (FP)
> 
> When RA tracking is used and a FDE is followed by two offsets the
> SFrame format does not provide any information to distinguish whether
> the second offset is the RA or FP offset. SFrame assumes the offset to
> be the RA offset, which may be wrong.
> 
> Therefore skip generation of SFrame FDE information and print the
> following warning, if RA tracking is used and the FP is saved on the
> stack without the RA being saved as well:
> 
>    skipping SFrame FDE due to FP without RA on stack
> 

OK.

One comment below, otherwise LGTM.

Thanks

> gas/
> 	* gen-sframe.c (sframe_do_fde): Skip SFrame FDE if FP without RA
> 	on stack, as the SFrame format cannot represent this case.
> 
> Signed-off-by: Jens Remus <jremus@linux.ibm.com>
> ---
> 
> Notes (jremus):
>      Changes v2 -> v3:
>      - New patch.
>      
>      Without this patch the assembler would generate incorrect SFrame
>      information without warning for the s390-specific SFrame error test
>      case 5, that gets introduced by patch "s390: Initial support to
>      generate .sframe from CFI directives in assembler". The FRE would
>      be followed by two offsets for the CFA and FP. SFrame would
>      erroneously interpret them as CFA and RA offsets, as it cannot
>      represent FP without RA on stack.
> 
>   gas/gen-sframe.c | 19 +++++++++++++++++++
>   1 file changed, 19 insertions(+)
> 
> diff --git a/gas/gen-sframe.c b/gas/gen-sframe.c
> index a3b6f75cfe85..87be3eb05ad2 100644
> --- a/gas/gen-sframe.c
> +++ b/gas/gen-sframe.c
> @@ -1439,6 +1439,25 @@ sframe_do_fde (struct sframe_xlate_ctx *xlate_ctx,
>   	= get_dw_fde_end_addrS (xlate_ctx->dw_fde);
>       }
>   
> +#ifdef SFRAME_FRE_RA_TRACKING
> +  if (sframe_ra_tracking_p ())
> +    {
> +      struct sframe_row_entry *fre;
> +
> +      /* Iterate over the scratchpad FREs and validate them.  */
> +      for (fre = xlate_ctx->first_fre; fre; fre = fre->next)
> +	{
> +	  /* SFrame format cannot represent FP on stack without RA on stack.  */
> +	  if (fre->ra_loc != SFRAME_FRE_ELEM_LOC_STACK
> +	      && fre->bp_loc == SFRAME_FRE_ELEM_LOC_STACK)
> +	    {
> +	      as_warn (_("skipping SFrame FDE due to FP without RA on stack"));
> +	      return SFRAME_XLATE_ERR_NOTREPRESENTED;
> +	    }
> +	}
> +    }
> +#endif /* SFRAME_FRE_RA_TRACKING  */
> +

There is a comment "/* No errors encountered.  */" at line 1452 (few 
lines above the proposed diff after applying).  I suggest we also remove 
/ move that comment too.

>     return SFRAME_XLATE_OK;
>   }
>