Re: [PATCH v3 28/28] Add DW_OP_LLVM_select_bit_piece DWARF operation

[Lancelot SIX <lsix@lancelotsix.com>]

Hi,

I have included remarks below.

On Thu, Oct 14, 2021 at 10:32:35AM +0100, Zoran Zaric via Gdb-patches wrote:
> Second more complex DWARF expression operation, that is usefull for
> SIMD/SIMT like architectures is DW_OP_LLVM_select_bit_piece. This
> operation pops three stack entries, where the first must be an integral
> type value that represents a bit mask, the second must be a location
> description that represents the one-location description and the third
> must be a location description that represents the zero-location
> description.
> 
> Resulting composite location description contains a given number of
> pieces of a given bit size, created with parts from either of the two
> location description, based on the bit mask.
> 
> gdb/ChangeLog:
> 
>         * compile/compile-loc2c.c (compute_stack_depth_worker): Add
>         new DW_OP_LLVM_select_bit_piece operation support.
>         * dwarf2/expr.c (dwarf_expr_context::create_select_composite):
>         New method that creates the select bit piece composite.
>         (dwarf_location::slice): New method.
>         (dwarf_composite::slice): New method.
>         (dwarf_expr_context::execute_stack_op): Add new
>         DW_OP_LLVM_select_bit_piece operation support.
>         * dwarf2/loc.c (dwarf2_get_symbol_read_needs): Add new
>         DW_OP_LLVM_select_bit_piece operation support.
>         (disassemble_dwarf_expression): Add new
>         DW_OP_LLVM_select_bit_piece operation support.
> 
> include/ChangeLog:
> 
>         * dwarf2.def: Add new DW_OP_LLVM_select_bit_piece enumeration.
> 
> gdb/testsuite/ChangeLog:
> 
>         * gdb.dwarf2/dw2-llvm-select-bit-piece.exp: New test.
>         * lib/dwarf.exp: Add new DW_OP_LLVM_select_bit_piece operation
>         support.
> ---
>  gdb/compile/compile-loc2c.c                   |   1 +
>  gdb/dwarf2/expr.c                             | 160 ++++++++++++++++++
>  gdb/dwarf2/loc.c                              |  12 ++
>  .../gdb.dwarf2/dw2-llvm-select-bit-piece.exp  | 138 +++++++++++++++
>  gdb/testsuite/lib/dwarf.exp                   |   5 +
>  include/dwarf2.def                            |   1 +
>  6 files changed, 317 insertions(+)
>  create mode 100644 gdb/testsuite/gdb.dwarf2/dw2-llvm-select-bit-piece.exp
> 
> diff --git a/gdb/compile/compile-loc2c.c b/gdb/compile/compile-loc2c.c
> index 2c5e13cfb6c..5991760adbc 100644
> --- a/gdb/compile/compile-loc2c.c
> +++ b/gdb/compile/compile-loc2c.c
> @@ -367,6 +367,7 @@ compute_stack_depth_worker (int start, int *need_tempvar,
>  	  break;
>  
>  	case DW_OP_LLVM_extend:
> +	case DW_OP_LLVM_select_bit_piece:
>  	  stack_depth -= 2;
>  	  break;
>  
> diff --git a/gdb/dwarf2/expr.c b/gdb/dwarf2/expr.c
> index 711a5918946..874a5d1923b 100644
> --- a/gdb/dwarf2/expr.c
> +++ b/gdb/dwarf2/expr.c
> @@ -461,6 +461,15 @@ class dwarf_location : public dwarf_entry
>      ill_formed_expression ();
>    }
>  
> +  /* Make a slice of a location description with an added bit offset
> +     BIT_OFFSET and BIT_SIZE size in bits.
> +
> +     In the case of a composite location description, function returns
> +     a minimum subset of that location description that starts on a
> +     given offset of a given size.  */
> +  virtual std::unique_ptr<dwarf_location> slice (LONGEST bit_offset,
> +						 LONGEST bit_size) const;
> +
>    /* Read contents from the described location.
>  
>       The read operation is performed in the context of a FRAME.
> @@ -621,6 +630,17 @@ const dwarf_location &computed_closure::get_location () const
>    return *m_location;
>  }
>  
> +/* This is a default implementation used for
> +   non-composite location descriptions.  */
> +
> +std::unique_ptr<dwarf_location>
> +dwarf_location::slice (LONGEST bit_offset, LONGEST bit_size) const
> +{
> +  dwarf_location_up location_slice = this->clone_location ();
> +  location_slice->add_bit_offset (bit_offset);
> +  return location_slice;
> +}
> +
>  void
>  dwarf_location::read_from_gdb_value (frame_info *frame, struct value *value,
>  				     int value_bit_offset,
> @@ -1567,6 +1587,9 @@ class dwarf_composite final : public dwarf_location
>      return make_unique<dwarf_composite> (*this);
>    }
>  
> +  std::unique_ptr<dwarf_location> slice (LONGEST bit_offset,
> +					 LONGEST bit_size) const override;
> +
>    void add_piece (std::unique_ptr<dwarf_location> location, ULONGEST bit_size)
>    {
>      gdb_assert (location != nullptr);
> @@ -1653,6 +1676,64 @@ class dwarf_composite final : public dwarf_location
>    bool m_completed = false;
>  };
>  
> +std::unique_ptr<dwarf_location>
> +dwarf_composite::slice (LONGEST bit_offset, LONGEST bit_size) const
> +{
> +  /* Size 0 is never expected at this point.  */
> +  gdb_assert (bit_size != 0);
> +
> +  unsigned int pieces_num = m_pieces.size ();
> +  LONGEST total_bit_size = bit_size;
> +  LONGEST total_bits_to_skip = m_offset * HOST_CHAR_BIT
> +			       + m_bit_suboffset + bit_offset;
> +  std::vector<piece> piece_slices;
> +  unsigned int i;
> +
> +  for (i = 0; i < pieces_num; i++)
> +    {
> +      LONGEST piece_bit_size = m_pieces[i].size;
> +
> +      if (total_bits_to_skip < piece_bit_size)
> +	break;
> +
> +      total_bits_to_skip -= piece_bit_size;
> +    }
> +
> +  for (; i < pieces_num; i++)
> +    {
> +      if (total_bit_size == 0)
> +	break;
> +
> +      gdb_assert (total_bit_size > 0);
> +      LONGEST slice_bit_size = m_pieces[i].size - total_bits_to_skip;
> +
> +      if (total_bit_size < slice_bit_size)
> +	slice_bit_size = total_bit_size;
> +
> +      std::unique_ptr<dwarf_location> slice
> +	= m_pieces[i].location->slice (total_bits_to_skip, slice_bit_size);
> +      piece_slices.emplace_back (std::move (slice), slice_bit_size);
> +
> +      total_bit_size -= slice_bit_size;
> +      total_bits_to_skip = 0;
> +    }
> +
> +  unsigned int slices_num = piece_slices.size ();
> +
> +  /* Only one piece found, so there is no reason to
> +      make a composite location description.  */
> +  if (slices_num == 1)
> +    return std::move (piece_slices[0].location);
> +
> +  std::unique_ptr<dwarf_composite> composite_slice
> +    = make_unique<dwarf_composite> (m_arch, m_per_cu);
> +
> +  for (piece &piece : piece_slices)
> +    composite_slice->add_piece (std::move (piece.location), piece.size);
> +
> +  return composite_slice;
> +}
> +
>  void
>  dwarf_composite::read (frame_info *frame, gdb_byte *buf,
>  		       int buf_bit_offset, size_t bit_size,
> @@ -2507,6 +2588,20 @@ struct dwarf_expr_context
>    void create_extend_composite (ULONGEST piece_bit_size,
>  				ULONGEST pieces_count);
>  
> +  /* It pops three stack entries.  The first must be an integral type
> +     value that represents a bit mask.  The second must be a location
> +     description that represents the one-location description.  The
> +     third must be a location description that represents the
> +     zero-location description.
> +
> +     A complete composite location description created with parts from
> +     either of the two location description, based on the bit mask,
> +     is pushed on top of the DWARF stack.  PIECE_BIT_SIZE represent
> +     a size in bits of each piece and PIECES_COUNT represents a number
> +     of pieces required.  */
> +  void create_select_composite (ULONGEST piece_bit_size,
> +				ULONGEST pieces_count);
> +
>    /* The engine for the expression evaluator.  Using the context in this
>       object, evaluate the expression between OP_PTR and OP_END.  */
>    void execute_stack_op (const gdb_byte *op_ptr, const gdb_byte *op_end);
> @@ -2893,6 +2988,60 @@ dwarf_expr_context::create_extend_composite (ULONGEST piece_bit_size,
>    push (std::move (composite));
>  }
>  
> +void
> +dwarf_expr_context::create_select_composite (ULONGEST piece_bit_size,
> +					     ULONGEST pieces_count)
> +{
> +  gdb::byte_vector mask_buf;
> +  gdbarch *arch = this->m_per_objfile->objfile->arch ();
> +
> +  if (stack_empty_p () || piece_bit_size == 0 || pieces_count == 0)
> +    ill_formed_expression ();
> +
> +  dwarf_value_up mask = to_value (pop (), address_type ());
> +
> +  type *mask_type = mask->type ();
> +  ULONGEST mask_size = TYPE_LENGTH (mask_type);
> +  dwarf_require_integral (mask_type);
> +
> +  if (mask_size * HOST_CHAR_BIT < pieces_count)
> +    ill_formed_expression ();
> +
> +  mask_buf.resize (mask_size);
> +
> +  copy_bitwise (mask_buf.data (), 0, mask->contents ().data (),
> +		0, mask_size * HOST_CHAR_BIT,
> +		type_byte_order (mask_type) == BFD_ENDIAN_BIG);
> +
> +  if (stack_empty_p ())
> +    ill_formed_expression ();
> +
> +  dwarf_location_up one = to_location (pop (), arch);
> +
> +  if (stack_empty_p ())
> +    ill_formed_expression ();
> +
> +  dwarf_location_up zero = to_location (pop (), arch);
> +
> +  std::unique_ptr<dwarf_composite> composite
> +    = make_unique<dwarf_composite> (arch, this->m_per_cu);
> +
> +  for (ULONGEST i = 0; i < pieces_count; i++)
> +    {
> +      std::unique_ptr<dwarf_location> slice;
> +
> +      if ((mask_buf.data ()[i / HOST_CHAR_BIT] >> (i % HOST_CHAR_BIT)) & 1)
> +	slice = one->slice (i * piece_bit_size, piece_bit_size);
> +      else
> +	slice = zero->slice (i * piece_bit_size, piece_bit_size);
> +
> +      composite->add_piece (std::move (slice), piece_bit_size);
> +    }
> +
> +  composite->set_completed (true);
> +  push (std::move (composite));
> +}
> +
>  void
>  dwarf_expr_context::eval (const gdb_byte *addr, size_t len)
>  {
> @@ -4134,6 +4283,17 @@ dwarf_expr_context::execute_stack_op (const gdb_byte *op_ptr,
>  	    break;
>  	  }
>  
> +	case DW_OP_LLVM_select_bit_piece:
> +	  {
> +	    uint64_t piece_bit_size, pieces_count;
> +
> +	    /* Record the piece.  */
> +	    op_ptr = safe_read_uleb128 (op_ptr, op_end, &piece_bit_size);
> +	    op_ptr = safe_read_uleb128 (op_ptr, op_end, &pieces_count);
> +	    create_select_composite (piece_bit_size, pieces_count);
> +	    break;
> +	  }
> +
>  	default:
>  	  error (_("Unhandled dwarf expression opcode 0x%x"), op);
>  	}
> diff --git a/gdb/dwarf2/loc.c b/gdb/dwarf2/loc.c
> index de3a904695a..5ca9f525f39 100644
> --- a/gdb/dwarf2/loc.c
> +++ b/gdb/dwarf2/loc.c
> @@ -1954,6 +1954,7 @@ dwarf2_get_symbol_read_needs (gdb::array_view<const gdb_byte> expr,
>  
>  	case DW_OP_bit_piece:
>  	case DW_OP_LLVM_extend:
> +	case DW_OP_LLVM_select_bit_piece:
>  	  op_ptr = safe_skip_leb128 (op_ptr, expr_end);
>  	  op_ptr = safe_skip_leb128 (op_ptr, expr_end);
>  	  break;
> @@ -3678,6 +3679,17 @@ disassemble_dwarf_expression (struct ui_file *stream,
>  	                      pulongest (ul), pulongest (count));
>  	  }
>  	  break;
> +
> +	case DW_OP_LLVM_select_bit_piece:
> +	  {
> +	    uint64_t count;
> +
> +	    data = safe_read_uleb128 (data, end, &ul);
> +	    data = safe_read_uleb128 (data, end, &count);
> +	    fprintf_filtered (stream, " piece size %s (bits) pieces count %s",
> +			      pulongest (ul), pulongest (count));
> +	  }
> +	  break;
>  	}
>  
>        fprintf_filtered (stream, "\n");
> diff --git a/gdb/testsuite/gdb.dwarf2/dw2-llvm-select-bit-piece.exp b/gdb/testsuite/gdb.dwarf2/dw2-llvm-select-bit-piece.exp
> new file mode 100644
> index 00000000000..c5ed6efbd75
> --- /dev/null
> +++ b/gdb/testsuite/gdb.dwarf2/dw2-llvm-select-bit-piece.exp
> @@ -0,0 +1,138 @@
> +# Copyright (C) 2017-2021 Free Software Foundation, Inc.
> +# Copyright (C) 2020-2021 Advanced Micro Devices, Inc. All rights reserved.

Based on a comment in the following thread:
https://sourceware.org/pipermail/gdb-patches/2021-September/182104.html,
I guess that this second line of copyright should be removed.

I did not spot in the earlier patch, but it looks that the same line
remains in gdb.dwarf2/dw2-llvm-extend.exp as well.

> +
> +# This program is free software; you can redistribute it and/or modify
> +# it under the terms of the GNU General Public License as published by
> +# the Free Software Foundation; either version 3 of the License, or
> +# (at your option) any later version.
> +#
> +# This program is distributed in the hope that it will be useful,
> +# but WITHOUT ANY WARRANTY; without even the implied warranty of
> +# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> +# GNU General Public License for more details.
> +#
> +# You should have received a copy of the GNU General Public License
> +# along with this program.  If not, see <http://www.gnu.org/licenses/>.
> +
> +# Test the new DW_OP_LLVM_select_bit_piece operation.
> +#
> +# The test uses a composite location description, where all the pieces
> +# are selected from two registers in odd/even pattern using the new
> +# operation.
> +
> +load_lib dwarf.exp
> +
> +# This test can only be run on targets which support DWARF-2 and use gas.
> +if {![dwarf2_support]} {
> +    return 0
> +}
> +
> +# Choose suitable integer registers for the test.
> +
> +set dwarf_regnum {0 1}
> +
> +if { [is_aarch64_target] } {
> +    set regname {x0 x1}
> +} elseif { [is_aarch32_target]
> +	   || [istarget "s390*-*-*" ]
> +	   || [istarget "powerpc*-*-*"]
> +	   || [istarget "rs6000*-*-aix*"] } {
> +    set regname {r0 r1}
> +} elseif { [is_x86_like_target] } {
> +    set regname {eax ecx}
> +} elseif { [is_amd64_regs_target] } {
> +    set regname {rax rdx}
> +} else {
> +    verbose "Skipping $gdb_test_file_name."
> +    return
> +}
> +
> +standard_testfile var-access.c ${gdb_test_file_name}-dw.S
> +
> +# Make some DWARF for the test.
> +
> +set asm_file [standard_output_file $srcfile2]
> +Dwarf::assemble $asm_file {
> +    global dwarf_regnum regname srcdir subdir srcfile
> +    set buf_src [gdb_target_symbol buf]
> +
> +    set main_result [function_range main ${srcdir}/${subdir}/${srcfile}]
> +    set main_start [lindex $main_result 0]
> +    set main_length [lindex $main_result 1]
> +
> +    cu {} {
> +	DW_TAG_compile_unit {
> +	    {DW_AT_name var-access.c}
> +	    {DW_AT_comp_dir /tmp}
> +	} {
> +	    declare_labels int_type_label char_type_label array_type_label
> +
> +	    # define char type
> +	    char_type_label: DW_TAG_base_type {
> +		{DW_AT_name "char"}
> +		{DW_AT_encoding @DW_ATE_signed}
> +		{DW_AT_byte_size 1 DW_FORM_sdata}
> +	    }
> +
> +	    int_type_label: DW_TAG_base_type {
> +		{DW_AT_name "int"}
> +		{DW_AT_encoding @DW_ATE_signed}
> +		{DW_AT_byte_size 4 DW_FORM_sdata}
> +	    }
> +
> +	    array_type_label: DW_TAG_array_type {
> +		{DW_AT_type :$char_type_label}
> +	    } {
> +		DW_TAG_subrange_type {
> +		    {DW_AT_type :$int_type_label}
> +		    {DW_AT_upper_bound 7 DW_FORM_udata}
> +		}
> +	    }
> +
> +	    DW_TAG_subprogram {
> +		{DW_AT_name main}
> +		{DW_AT_low_pc $main_start addr}
> +		{DW_AT_high_pc $main_length data8}
> +	    } {
> +
> +		# Odd array elements are in first byte of REGNAME 0 register,
> +		# while even elements are in first byte of REGNAME 1 register.
> +		DW_TAG_variable {
> +		    {DW_AT_name var_array}
> +		    {DW_AT_type :$array_type_label}
> +		    {DW_AT_location {
> +			DW_OP_regx [lindex $dwarf_regnum 0]
> +			DW_OP_LLVM_extend 8 8
> +                        DW_OP_regx [lindex $dwarf_regnum 1]

Looks like you have spaces instead of tabs to indent this line.

Best,
Lancelot.

> +			DW_OP_LLVM_extend 8 8
> +			DW_OP_constu 0xaa
> +			DW_OP_LLVM_select_bit_piece 8 8
> +		    } SPECIAL_expr}
> +		}
> +	    }
> +	}
> +    }
> +}
> +
> +if { [prepare_for_testing ${testfile}.exp ${testfile} \
> +     [list $srcfile $asm_file] {nodebug}] } {
> +    return -1
> +}
> +
> +if ![runto_main] {
> +    return -1
> +}
> +
> +gdb_test_no_output "set var \$[lindex $regname 0] = 0x04030201" "init reg 0"
> +gdb_test_no_output "set var \$[lindex $regname 1] = 0x01020304" "init reg 1"
> +
> +# Determine byte order.
> +set endian [get_endianness]
> +
> +switch $endian {
> +	little {set val "0x1, 0x4, 0x1, 0x4, 0x1, 0x4, 0x1, 0x4"}
> +	big {set val "0x4, 0x1, 0x4, 0x1, 0x4, 0x1, 0x4, 0x1"}
> +}
> +
> +gdb_test "print/x var_array" " = \\{${val}\\}" "var_array print"
> +
> diff --git a/gdb/testsuite/lib/dwarf.exp b/gdb/testsuite/lib/dwarf.exp
> index 41415986e64..a8b9518a415 100644
> --- a/gdb/testsuite/lib/dwarf.exp
> +++ b/gdb/testsuite/lib/dwarf.exp
> @@ -1230,6 +1230,11 @@ namespace eval Dwarf {
>  		    _op .uleb128 [lindex $line 2]
>  		}
>  
> +		DW_OP_LLVM_select_bit_piece {
> +		    _op .uleb128 [lindex $line 1]
> +		    _op .uleb128 [lindex $line 2]
> +		}
> +
>  		default {
>  		    if {[llength $line] > 1} {
>  			error "Unimplemented: operands in location for $opcode"
> diff --git a/include/dwarf2.def b/include/dwarf2.def
> index 3d1d831d0fb..abc1125df2c 100644
> --- a/include/dwarf2.def
> +++ b/include/dwarf2.def
> @@ -711,6 +711,7 @@ DW_OP_DUP (DW_OP_LLVM_bit_offset, 0xe5)
>  DW_OP (DW_OP_LLVM_undefined, 0xe7)
>  DW_OP_DUP (DW_OP_LLVM_piece_end, 0xea)
>  DW_OP (DW_OP_LLVM_extend, 0xeb)
> +DW_OP (DW_OP_LLVM_select_bit_piece, 0xec)
>  DW_END_OP
>  
>  DW_FIRST_ATE (DW_ATE_void, 0x0)
> -- 
> 2.17.1
>