Re: [PATCH 1/2] gas: sframe: Fix non-SP/FP CFA base register if flexible FDE

On 2026-07-29 09:34, Jens Remus wrote:
> If a target supports SFrame V3 flexible FDE, a non-SP/FP CFA base
> register can be represented.  Do not warn and skip generating FDE, if
> .cfi_def_cfa_offset with a non-SP/FP register in effect, if target
> supports flexible FDE.
> 
> gas/
> 	* gen-sframe.c (sframe_xlate_do_def_cfa_offset): Allow non-SP/FP
> 	CFA base register if flexible FDE.
> 
> gas/testsuite/
> 	* gas/cfi-sframe/common-empty-2.d: Skip test on targets that
> 	support SFrame V3 flexible FDE.
> 
> Signed-off-by: Jens Remus <[email protected]>
> ---
> 
> Notes (jremus):
>      Why does sframe_xlate_do_def_cfa_offset care about the CFA base
>      register at all?  With SFrame V3 it could test cur_cfa_reg for
>      SFRAME_FRE_REG_INVALID.  But wouldn't the following be valid DWARF CFI:
>      
>      	.cfi_startproc simple
>      	.cfi_def_cfa_offset 0
>      	.cfi_def_cfa_register <SP>
>      	...
>      	.cfi_endproc
>      
>      to express the same as:
>      
>      	.cfi_startproc simple
>      	.cfi_def_cfa <SP>, 0
>      	...
>      	.cfi_endproc
> 

In theory yes, but SFrame code enforces a presence of 
DW_CFA_def_cfa_register (or others as applicable to basically setup the 
CFA register) to apply the following specified in the DWARF standard: 
For the DW_CFA_def_cfa_offset, it says: "This operation is valid only if 
the current CFA rule is defined to use a register and offset."

>   gas/gen-sframe.c                              | 3 ++-
>   gas/testsuite/gas/cfi-sframe/common-empty-2.d | 1 +
>   2 files changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/gas/gen-sframe.c b/gas/gen-sframe.c
> index b5c8f649beac..f0576869579a 100644
> --- a/gas/gen-sframe.c
> +++ b/gas/gen-sframe.c
> @@ -1360,7 +1360,8 @@ sframe_xlate_do_def_cfa_offset (struct sframe_xlate_ctx *xlate_ctx,
>     /*  Define the current CFA rule to use the provided offset (but to keep
>         the old register).  However, if the old register is not FP/SP,
>         skip creating SFrame stack trace info for the function.  */
> -  if (cur_cfa_reg == SFRAME_CFA_FP_REG || cur_cfa_reg == SFRAME_CFA_SP_REG)
> +  if (cur_cfa_reg == SFRAME_CFA_FP_REG || cur_cfa_reg == SFRAME_CFA_SP_REG
> +      || sframe_support_flex_fde_p ())
>       {
>         if (sframe_fre_stack_offset_bound_p (cfi_insn->u.i, true))
>   	{
> diff --git a/gas/testsuite/gas/cfi-sframe/common-empty-2.d b/gas/testsuite/gas/cfi-sframe/common-empty-2.d
> index c80ea50c75f9..ce13d5ef055a 100644
> --- a/gas/testsuite/gas/cfi-sframe/common-empty-2.d
> +++ b/gas/testsuite/gas/cfi-sframe/common-empty-2.d
> @@ -2,6 +2,7 @@
>   #warning: \.cfi_def_cfa_offset without CFA base register in effect
>   #objdump: --sframe=.sframe
>   #name: SFrame supports only FP/SP based CFA
> +#notarget: s390x-*-* x86_64-*-*
>   #...
>   Contents of the SFrame section .sframe:
>