[PATCH v3] x86: adjust which Dwarf2 register numbers to use

[PATCH v3] x86: adjust which Dwarf2 register numbers to use

[Jan Beulich]

Consumers can't know which execution mode is in effect for a certain
piece of code; they can only go from object file properties. Hence which
register numbers to encode ought to depend solely on object file type.

In tc_x86_frame_initial_instructions() do away with parsing a register
name: We have a symbolic constant already for the 64-bit case, and the
32-bit number isn't going to change either. Said constant's definition
needs moving, though, to be available also for non-ELF. While moving
also adjust the comment to clarify that it's applicable to 64-bit mode
only.
---
x86_cie_data_alignment, independent of this change, likely needs
adjusting as flag_code changes.

The COFF/PE setting of x86_dwarf2_return_column looks bogus as well: 32
is already in use for %xmm15 for 64-bit. Commit ca19b261ecc3 sadly has
no explanation at all. Nor did it adjust objdump accordingly. Given that
the author has moved on (couldn't find an applicable email address), I'm
inclined to simply revert that change. If anything a proper complete set
of Windows register number mappings (wherever those are formally
documented) would need putting in place.
---
v3: Also adjust tc_x86_frame_initial_instructions().
v2: Also adjust md_begin().

--- a/gas/config/tc-i386.c
+++ b/gas/config/tc-i386.c
@@ -3276,7 +3276,7 @@ md_begin (void)
       operand_chars[(unsigned char) *p] = *p;
   }
 
-  if (flag_code == CODE_64BIT)
+  if (object_64bit)
     {
 #if defined (OBJ_COFF) && defined (TE_PE)
       x86_dwarf2_return_column = (OUTPUT_FLAVOR == bfd_target_coff_flavour
@@ -5410,7 +5410,7 @@ ginsn_dw2_regnum (const reg_entry *ireg)
   if (ireg->reg_num == RegIP || ireg->reg_num == RegIZ)
     return GINSN_DW2_REGNUM_RSI_DUMMY;
 
-  dwarf_reg = ireg->dw2_regnum[flag_code >> 1];
+  dwarf_reg = ireg->dw2_regnum[object_64bit];
 
   if (dwarf_reg == Dw2Inval)
     {
@@ -17548,7 +17548,7 @@ tc_x86_parse_to_dw2regnum (expressionS *
       if ((addressT) exp->X_add_number < i386_regtab_size)
 	{
 	  exp->X_add_number = i386_regtab[exp->X_add_number]
-			      .dw2_regnum[flag_code >> 1];
+			      .dw2_regnum[object_64bit];
 	  if (exp->X_add_number != Dw2Inval)
 	    exp->X_op = O_constant;
 	}
@@ -17558,22 +17558,7 @@ tc_x86_parse_to_dw2regnum (expressionS *
 void
 tc_x86_frame_initial_instructions (void)
 {
-  static unsigned int sp_regno[2];
-
-  if (!sp_regno[flag_code >> 1])
-    {
-      char *saved_input = input_line_pointer;
-      char sp[][4] = {"esp", "rsp"};
-      expressionS exp;
-
-      input_line_pointer = sp[flag_code >> 1];
-      tc_x86_parse_to_dw2regnum (&exp);
-      gas_assert (exp.X_op == O_constant);
-      sp_regno[flag_code >> 1] = exp.X_add_number;
-      input_line_pointer = saved_input;
-    }
-
-  cfi_add_CFA_def_cfa (sp_regno[flag_code >> 1], -x86_cie_data_alignment);
+  cfi_add_CFA_def_cfa (object_64bit ? REG_SP : 4, -x86_cie_data_alignment);
   cfi_add_CFA_offset (x86_dwarf2_return_column, x86_cie_data_alignment);
 }
 
--- a/gas/config/tc-i386.h
+++ b/gas/config/tc-i386.h
@@ -396,6 +396,11 @@ extern void tc_x86_parse_to_dw2regnum (e
 #define tc_cfi_frame_initial_instructions tc_x86_frame_initial_instructions
 extern void tc_x86_frame_initial_instructions (void);
 
+/* DWARF register number of the frame-pointer register in 64-bit mode.  */
+#define REG_FP 6
+/* DWARF register number of the stack-pointer register in 64-bit mode.  */
+#define REG_SP 7
+
 #define md_elf_section_type(str,len) i386_elf_section_type (str, len)
 extern int i386_elf_section_type (const char *, size_t);
 
@@ -425,10 +430,6 @@ extern void x86_cleanup (void);
    R15 (15).  Use SCFI_CALLEE_SAVED_REG_P to identify which registers
    are callee-saved from this set.  */
 #define SCFI_MAX_REG_ID 15
-/* Identify the DWARF register number of the frame-pointer register.  */
-#define REG_FP 6
-/* Identify the DWARF register number of the stack-pointer register.  */
-#define REG_SP 7
 /* Some ABIs, like AMD64, use stack for call instruction.  For such an ABI,
    identify the initial (CFA) offset from RSP at the entry of function.  */
 #define SCFI_INIT_CFA_OFFSET 8

Re: [PATCH v3] x86: adjust which Dwarf2 register numbers to use

[Indu Bhagat]

On 2/23/24 03:07, Jan Beulich wrote:
> Consumers can't know which execution mode is in effect for a certain
> piece of code; they can only go from object file properties. Hence which
> register numbers to encode ought to depend solely on object file type.
> 
> In tc_x86_frame_initial_instructions() do away with parsing a register
> name: We have a symbolic constant already for the 64-bit case, and the
> 32-bit number isn't going to change either. Said constant's definition
> needs moving, though, to be available also for non-ELF. While moving
> also adjust the comment to clarify that it's applicable to 64-bit mode
> only.

I have no further comments.

Thanks for the patch,
Indu

> ---
> x86_cie_data_alignment, independent of this change, likely needs
> adjusting as flag_code changes.
> 
> The COFF/PE setting of x86_dwarf2_return_column looks bogus as well: 32
> is already in use for %xmm15 for 64-bit. Commit ca19b261ecc3 sadly has
> no explanation at all. Nor did it adjust objdump accordingly. Given that
> the author has moved on (couldn't find an applicable email address), I'm
> inclined to simply revert that change. If anything a proper complete set
> of Windows register number mappings (wherever those are formally
> documented) would need putting in place.
> ---
> v3: Also adjust tc_x86_frame_initial_instructions().
> v2: Also adjust md_begin().
> 
> --- a/gas/config/tc-i386.c
> +++ b/gas/config/tc-i386.c
> @@ -3276,7 +3276,7 @@ md_begin (void)
>         operand_chars[(unsigned char) *p] = *p;
>     }
>   
> -  if (flag_code == CODE_64BIT)
> +  if (object_64bit)
>       {
>   #if defined (OBJ_COFF) && defined (TE_PE)
>         x86_dwarf2_return_column = (OUTPUT_FLAVOR == bfd_target_coff_flavour
> @@ -5410,7 +5410,7 @@ ginsn_dw2_regnum (const reg_entry *ireg)
>     if (ireg->reg_num == RegIP || ireg->reg_num == RegIZ)
>       return GINSN_DW2_REGNUM_RSI_DUMMY;
>   
> -  dwarf_reg = ireg->dw2_regnum[flag_code >> 1];
> +  dwarf_reg = ireg->dw2_regnum[object_64bit];
>   
>     if (dwarf_reg == Dw2Inval)
>       {
> @@ -17548,7 +17548,7 @@ tc_x86_parse_to_dw2regnum (expressionS *
>         if ((addressT) exp->X_add_number < i386_regtab_size)
>   	{
>   	  exp->X_add_number = i386_regtab[exp->X_add_number]
> -			      .dw2_regnum[flag_code >> 1];
> +			      .dw2_regnum[object_64bit];
>   	  if (exp->X_add_number != Dw2Inval)
>   	    exp->X_op = O_constant;
>   	}
> @@ -17558,22 +17558,7 @@ tc_x86_parse_to_dw2regnum (expressionS *
>   void
>   tc_x86_frame_initial_instructions (void)
>   {
> -  static unsigned int sp_regno[2];
> -
> -  if (!sp_regno[flag_code >> 1])
> -    {
> -      char *saved_input = input_line_pointer;
> -      char sp[][4] = {"esp", "rsp"};
> -      expressionS exp;
> -
> -      input_line_pointer = sp[flag_code >> 1];
> -      tc_x86_parse_to_dw2regnum (&exp);
> -      gas_assert (exp.X_op == O_constant);
> -      sp_regno[flag_code >> 1] = exp.X_add_number;
> -      input_line_pointer = saved_input;
> -    }
> -
> -  cfi_add_CFA_def_cfa (sp_regno[flag_code >> 1], -x86_cie_data_alignment);
> +  cfi_add_CFA_def_cfa (object_64bit ? REG_SP : 4, -x86_cie_data_alignment);
>     cfi_add_CFA_offset (x86_dwarf2_return_column, x86_cie_data_alignment);
>   }
>   
> --- a/gas/config/tc-i386.h
> +++ b/gas/config/tc-i386.h
> @@ -396,6 +396,11 @@ extern void tc_x86_parse_to_dw2regnum (e
>   #define tc_cfi_frame_initial_instructions tc_x86_frame_initial_instructions
>   extern void tc_x86_frame_initial_instructions (void);
>   
> +/* DWARF register number of the frame-pointer register in 64-bit mode.  */
> +#define REG_FP 6
> +/* DWARF register number of the stack-pointer register in 64-bit mode.  */
> +#define REG_SP 7
> +
>   #define md_elf_section_type(str,len) i386_elf_section_type (str, len)
>   extern int i386_elf_section_type (const char *, size_t);
>   
> @@ -425,10 +430,6 @@ extern void x86_cleanup (void);
>      R15 (15).  Use SCFI_CALLEE_SAVED_REG_P to identify which registers
>      are callee-saved from this set.  */
>   #define SCFI_MAX_REG_ID 15
> -/* Identify the DWARF register number of the frame-pointer register.  */
> -#define REG_FP 6
> -/* Identify the DWARF register number of the stack-pointer register.  */
> -#define REG_SP 7
>   /* Some ABIs, like AMD64, use stack for call instruction.  For such an ABI,
>      identify the initial (CFA) offset from RSP at the entry of function.  */
>   #define SCFI_INIT_CFA_OFFSET 8