Re: [PATCH] x86: change type of Dwarf2 register numbers in register table

[Indu Bhagat <indu.bhagat@oracle.com>]

Hi Jan,

On 2/2/24 02:25, Jan Beulich wrote:
> Already the %bnd<N> registers used numbers beyond 127, and eGPR ones are
> all out of reach for "signed char", at least when CHAR_BITS=8. Switch to
> "unsigned char", covering appropriately in places where the value
> returned for "none" actually matters (in tc_x86_parse_to_dw2regnum()
> this is actually achieved by altering how X_op is set).
> ---
> I question the use of flag_code here, btw: Imo the choice ought to be
> tied to object format, not present assembly mode. Thoughts?
> 

I dont claim to have a full grasp of the matter yet.

IIUC, we need to use flag_code as DWARF register number will differ for 
32-bit reg vs 64-bit reg?  We do the other set of conditional 
assignments (to x86_dwarf2_return_column, x86_cie_data_alignment) in 
md_begin (). What I do not understand is why are the assignments 
(x86_dwarf2_return_column, x86_cie_data_alignment) not re-done in 
set_code_flag () as well ? (Consider user/compiler may pass a 
.code16/.code16gcc..)

(..And orthogonal to this, I see that SCFI needs to fail elegantly if 
.code16/.code32 appear ,i.e., when flag_code != CODE_64BIT.)

> --- a/gas/config/tc-i386.c
> +++ b/gas/config/tc-i386.c
> @@ -5395,10 +5395,8 @@ ginsn_opsize_prefix_p (void)
>   static unsigned int
>   ginsn_dw2_regnum (const reg_entry *ireg)
>   {
> -  /* PS: Note the data type here as int32_t, because of Dw2Inval (-1).  */
> -  int32_t dwarf_reg = Dw2Inval;
>     const reg_entry *temp = ireg;
> -  unsigned int idx = 0;
> +  unsigned int dwarf_reg = Dw2Inval, idx = 0;
>   
>     /* ginsn creation is available for AMD64 abi only ATM.  Other flag_code
>        are not expected.  */
> @@ -5441,7 +5439,7 @@ ginsn_dw2_regnum (const reg_entry *ireg)
>   
>     /* Sanity check - failure may indicate state corruption, bad ginsn or
>        perhaps the i386-reg table and the current function got out of sync.  */
> -  gas_assert (dwarf_reg >= 0);
> +  gas_assert (dwarf_reg < Dw2Inval);
>   
>     return (unsigned int) dwarf_reg;

This cast can be removed.

Thanks for the patch.

>   }
> @@ -17459,14 +17457,14 @@ tc_x86_parse_to_dw2regnum (expressionS *
>   
>     if (exp->X_op == O_register && exp->X_add_number >= 0)
>       {
> +      exp->X_op = O_illegal;
>         if ((addressT) exp->X_add_number < i386_regtab_size)
>   	{
> -	  exp->X_op = O_constant;
>   	  exp->X_add_number = i386_regtab[exp->X_add_number]
>   			      .dw2_regnum[flag_code >> 1];
> +	  if (exp->X_add_number != Dw2Inval)
> +	    exp->X_op = O_constant;
>   	}
> -      else
> -	exp->X_op = O_illegal;
>       }
>   }
>   
> --- a/opcodes/i386-opc.h
> +++ b/opcodes/i386-opc.h
> @@ -1051,7 +1051,7 @@ typedef struct
>   #define RegIZ	(RegIP - 1)
>   /* FLAT is a fake segment register (Intel mode).  */
>   #define RegFlat     ((unsigned char) ~0)
> -  signed char dw2_regnum[2];
> -#define Dw2Inval (-1)
> +  unsigned char dw2_regnum[2];
> +#define Dw2Inval 0xff
>   }
>   reg_entry;