On Sun, Jun 13, 2021 at 02:27:38PM +0100, Hafiz Abid Qadeer wrote:
> *** with this patch (edited for brevity)***
>
> 00000000 00000024 ffffffff CIE
>
> DW_CFA_def_cfa_expression: DW_OP_bregx SGPR49+0, DW_OP_const1u 0x20,
> DW_OP_shl, DW_OP_bregx SGPR48+0, DW_OP_plus
> DW_CFA_expression: reg16 DW_OP_bregx SGPR51+0, DW_OP_const1u 0x20,
> DW_OP_shl, DW_OP_bregx SGPR50+0, DW_OP_plus
>
> 00000028 0000003c 00000000 FDE cie=00000000 pc=00000000...000001ac
> DW_CFA_advance_loc4: 96
> DW_CFA_offset: reg46 0
> DW_CFA_offset: reg47 4
> DW_CFA_offset: reg50 8
> DW_CFA_offset: reg51 12
> DW_CFA_offset: reg16 8
> DW_CFA_advance_loc4: 4
> DW_CFA_def_cfa_expression: DW_OP_bregx SGPR47+0, DW_OP_const1u 0x20,
> DW_OP_shl, DW_OP_bregx SGPR46+0, DW_OP_plus, DW_OP_lit16, DW_OP_minus
I guess as a temporary solution until DWARF6 comes with something more
compact for cases like that it can be fine, but is there a DWARF issue
filed for it? Is AMDGCN a DWARF2_ADDR_SIZE == 8 target?
> +/* This represents a register, in DWARF_FRAME_REGNUM space, for use in CFA
> + definitions and expressions.
> + Most architectures only need a single register number, but some (amdgcn)
> + have pointers that span multiple registers. DWARF permits arbitrary
> + register sets but existing use-cases only require contiguous register
> + sets, as represented here. */
> +struct GTY(()) cfa_reg {
> + unsigned int reg;
> + unsigned int span;
> + poly_uint16_pod span_width; /* A.K.A. register mode size. */
If this is only used for span > 1, wouldn't it be better to
make it
unsigned int reg;
unsigned short span;
unsigned short span_width;
and keep span_width 0 for the span == 1 cases and only set span_width
to ....to_constant () if span > 1 is needed? If at least for now
the only target that needs this is AMDGCN and the only target that has
NUM_POLY_INT_COEFFS != 1 is aarch64 (maybe eventually riscv?), then I don't
see why we should represent it in poly_uint16...
Of course we can change it later if a target which needs both
NUM_POLY_INT_COEFFS > 1 and span > 1 registers with non-constant span_width,
we can change it, but doing it just in case seems unnecessary
complication...
Jakub
