[PATCH] [gdb/tdep] Fix gdb.ada/O2_float_param.exp on s390x-linux

[Tom de Vries]

On 1/11/25 09:45, Tom de Vries wrote:
> With test-case gdb.ada/O2_float_param.exp on s390x-linux, I get:
> ...
>   (gdb) frame^M
>   #0  callee.increment (val=99.0, val@entry=<error reading variable: \
>     register has not been saved in frame>, msg=...) at callee.adb:19^M
>   19         procedure Increment (Val : in out Float; Msg: String) is^M
>   (gdb) FAIL: $exp: scenario=all: frame
> ...
> 
> The frame command calls read_frame_arg to get:
> - the current value of val, and
> - the value of val at function entry.
> 
> The first scenario succeeds, and the second scenario fails.
> 
> For context and contrast, let's also investigate the first scenario: getting
> the current value of val.
> 
> Function parameter val:
> ...
>   <2><b51>: Abbrev Number: 4 (DW_TAG_formal_parameter)
>      <b52>   DW_AT_name        : val
>      <b58>   DW_AT_type        : <0xb86>
>      <b5c>   DW_AT_location    : 0xab (location list)
> ...
> has location list:
> ...
>      000000ab 0000000001002928 0000000001002967
>        (DW_OP_reg16 (f0))
>      000000be 0000000001002967 0000000001002968
>        (DW_OP_reg24 (f8))
>      000000d1 0000000001002968 0000000001002974
>        (DW_OP_GNU_regval_type: 24 (f8) <0xb29>;
>         DW_OP_GNU_const_type: <0xb29>  4 byte block: 3f 80 0 0 ; DW_OP_plus;
>         DW_OP_stack_value)
>      000000ef 0000000001002974 0000000001002982
>        (DW_OP_GNU_entry_value: (DW_OP_GNU_regval_type: 16 (f0) <0xb29>);
>         DW_OP_GNU_const_type: <0xb29>  4 byte block: 3f 80 0 0 ; DW_OP_plus;
>         DW_OP_stack_value)
>      0000010f <End of list>
> ...
> and since we're stopped at address 0x1002928:
> ...
> (gdb) print $pc
> $1 = (access procedure) 0x1002928 <callee.increment>
> ...
> we get the value from dwarf register 16.
> 
> The s390x ABI specifies that dwarf register 16 maps onto 8-byte register f0 or
> 16-byte register v0 (where f0 is part of v0), and in this case (because the
> v0 register is available) s390_dwarf_reg_to_regnum maps it to v0.
> 
> Val is only 4 bytes:
> ...
> (gdb) ptype val
> type = <4-byte float>
> ...
> and s390_value_from_register takes care to get the value from the correct part
> of v0.
> 
> The value of v0 is found in the prologue cache, and the value of parameter val
> is printed.
> 
> Now the second scenario: getting the value of val at function entry.
> 
> FWIW, since we're stopped at function entry, we could simply return the same
> value, reading the same register, but that's currently not implemented [2].
> 
> Instead we start from the fact that val is in dwarf reg 16 at function entry,
> and then use call site information:
> ...
>   <4><cf7>: Abbrev Number: 13 (DW_TAG_GNU_call_site)
>      <cf8>   DW_AT_low_pc      : 0x1002a46
>      <d00>   DW_AT_abstract_origin: <0xdda>
>   <5><d04>: Abbrev Number: 12 (DW_TAG_GNU_call_site_parameter)
>      <d05>   DW_AT_location    : 1 byte block: 60        (DW_OP_reg16 (f0))
>      <d07>   DW_AT_GNU_call_site_value: 3 byte block: f5 18 2d   \
>                (DW_OP_GNU_regval_type: 24 (f8) <0xc42>)
>   <5><d0b>: Abbrev Number: 12 (DW_TAG_GNU_call_site_parameter)
> ...
> to conclude that the value we're looking for is in dwarf reg 24, which
> s390_dwarf_reg_to_regnum maps to v8.
> 
> As before, s390_value_from_register takes care to get the value from the
> correct part of v8.
> 
> However, v8 is not available in the prologue cache, and we take a different
> path and end up in s390_unwind_pseudo_register, where v8 and similar
> (regnum_is_vxr_full) is unhandled, and we get:
> ...
>     return value::allocate_optimized_out (type);
> ...
> which eventually causes the "error reading variable: register has not been
> saved in frame".
> 
> Fix this by handling the regnum_is_vxr_full case in
> s390_unwind_pseudo_register, similar to how that is done in
> s390_pseudo_register_read.
> 

I've pushed this.

Thanks,
- Tom

> Tested on s390x-linux.
> 
> This also fixes test-case gdb.base/savedregs.exp.
> 
> [1] https://github.com/IBM/s390x-abi
> [2] https://sourceware.org/pipermail/gdb-patches/2024-September/211589.html
> ---
>   gdb/s390-tdep.c | 16 ++++++++++++++++
>   1 file changed, 16 insertions(+)
> 
> diff --git a/gdb/s390-tdep.c b/gdb/s390-tdep.c
> index 36a70d8642c..c57b6119e63 100644
> --- a/gdb/s390-tdep.c
> +++ b/gdb/s390-tdep.c
> @@ -2310,6 +2310,22 @@ s390_unwind_pseudo_register (const frame_info_ptr &this_frame, int regnum)
>   	return value_cast (type, val);
>       }
>   
> +  if (regnum_is_vxr_full (tdep, regnum))
> +    {
> +      struct value *val = value::allocate_register (this_frame, regnum);
> +
> +      int reg = regnum - tdep->v0_full_regnum;
> +      struct value *val1
> +	= frame_unwind_register_value (this_frame, S390_F0_REGNUM + reg);
> +      struct value *val2
> +	= frame_unwind_register_value (this_frame, S390_V0_LOWER_REGNUM + reg);
> +
> +      val1->contents_copy (val, 0, 0, 8);
> +      val2->contents_copy (val, 8, 0, 8);
> +
> +      return value_cast (type, val);
> +    }
> +
>     return value::allocate_optimized_out (type);
>   }
>   
> 
> base-commit: e564115c8a0e8f9b993cf0add01fa34548005d20