Re: [PATCH v3 00/28] Allow location description on the DWARF stack

["Zaric, Zoran (Zare)" <Zoran.Zaric@amd.com>]

[AMD Official Use Only]

A new version of this patch series has been sent.

Thanks to everybody that reviewed the previous version,
Zoran
________________________________
From: Zaric, Zoran (Zare) <Zoran.Zaric@amd.com>
Sent: 14 October 2021 10:32
To: gdb-patches@sourceware.org <gdb-patches@sourceware.org>
Cc: Zaric, Zoran (Zare) <Zoran.Zaric@amd.com>
Subject: [PATCH v3 00/28] Allow location description on the DWARF stack

Based on gdb master: f19c3684a6db145f57048bff5485fec6e3dd0f76

The idea of this patch series is to allow a future extensions of the
DWARF expression evaluator needed for the upcoming vendor specific
DWARF extensions.

Main motivation behind this series of patches is AMD’s effort to
improve DWARF support for heavily optimized code, but more
specifically, optimized code for SIMD and SIMT architectures.

These patches are part of the AMD’s DWARF standard extensions that
can be found at:

https://llvm.org/docs/AMDGPUDwarfExtensionsForHeterogeneousDebugging.html

While trying to support optimized code for these architectures, we
found couple of restriction imposed by the version 5 of the standard:

- CFI describes restoring callee saved registers that are spilled.
  Currently CFI only allows a location description that is a register,
  memory address, or implicit location description. AMDGPU optimized
  code may spill scalar registers into portions of vector registers.
  This requires extending CFI to allow any location description.

- Optimized code may need to describe a variable that resides in pieces
  that are in different kinds of storage which may include parts that
  are in different kinds of memory address spaces. DWARF has the
  concept of segment addresses. However, the segment cannot be
  specified within a DWARF expression, which is only able to specify
  the offset portion of a segment address. The segment index is only
  provided by the entity that specifies the DWARF expression.
  Therefore, the segment index is a property that can only be put on
  complete objects, such as a variable. Another problem is with DWARF
  DW_OP_xderef* operations, which allow a value to be converted into
  an address of a specified address space which is then read. But it
  provides no way to create a memory location description for an
  address in the non-default address space.

- DW_OP_breg* treats the register as containing an address in the
  default address space. It is required to be able to specify the
  address space of the register value.

- There is really limited support for bit offsets of location
  description. This issue was already found by others who worked
  on packed array support for ADA language. The array could be packed
  in such a way that their start is not byte alligned. There is no way
  to describe this in DWARF, so gdb implementation in required for the
  packed array to be copied to the byte alligned addresses and then
  passed that buffer in the expression evaluator. This approach is
  restrictive for more complex scenarios.

All these restriction are caused by the fact that DWARF stack can only
hold values which size is not bigger then the size of the DWARF generic
type. This means that intermediate result of any DWARF operation needs
to fit in such a small representation.

DWARF Version 5 does not allow location descriptions to be entries on
the DWARF stack. They can only be the final result of the evaluation of
a DWARF expression. However, by allowing a location description to be a
first-class entry on the DWARF stack it becomes possible to compose
expressions containing both values and location descriptions naturally.
It allows objects to be located in any kind of memory address space,
in registers, be implicit values, be undefined, or a composite of any
of these.

This approach unifies the location description operations with general
operations. This in turn allows addition of new DWARF operations which
can push a memory location description with any offset and in any kind
of memory address space.

The number of registers and the cost of memory operations is much
higher for some architectures than a typical CPU. The compiler attempts
to optimize whole variables and arrays into registers. Currently DWARF
only allows DW_OP_push_object_address and related operations to work
with a global memory location. With this change, push object address
mechanism can be modified to work with any location description. This
would also removed the need for the passed in buffer mechanism that ADA
and GO languages currently use.

The proposed implementation in this patch series is completely backward
compatible with the DWARF 5 standard.

Although the patch series is designed as a whole, the first 23
patches could be viewed as a standalone subset that introduces the idea
of allowing location descriptions on the DWARF expression stack, while
last 5 take advantage of that functionality to implement new set of
DWARF operation which would not be possible without it.

Zoran Zaric (28):
  Add new register access interface to expr.c
  Add new memory access interface to expr.c
  Add new classes that model DWARF stack element
  Add to_location method to DWARF entry classes
  Add to_value method to DWARF entry classes
  Add read method to location description classes
  Add write method to location description classes
  Add deref method to location description classes
  Add read_from_gdb_value method to dwarf_location
  Add write_to_gdb_value method to dwarf_location
  Add is_implicit_ptr_at method to dwarf_location
  Add indirect_implicit_ptr to dwarf_location class
  Add is_optimized_out to dwarf_location class
  Add new computed struct value callback interface
  Add to_gdb_value method to DWARF entry class
  Change DWARF stack to use new dwarf_entry classes
  Remove old computed struct value callbacks
  Comments cleanup between expr.h and expr.c
  Remove dwarf_expr_context from expr.h interface
  Move read_addr_from_reg function to frame.c
  Add frame info check to DW_OP_reg operations
  Remove DWARF expression composition check
  Add support for any location description in CFI
  Add DWARF operations for byte and bit offset
  Add support for DW_OP_LLVM_undefined operation
  Add support for nested composite locations
  Add DW_OP_LLVM_extend DWARF operation
  Add DW_OP_LLVM_select_bit_piece DWARF operation

 gdb/ada-lang.c                                |    2 +-
 gdb/compile/compile-loc2c.c                   |   16 +
 gdb/dwarf2/expr.c                             | 4229 ++++++++++++-----
 gdb/dwarf2/expr.h                             |  272 +-
 gdb/dwarf2/frame.c                            |   74 +-
 gdb/dwarf2/loc.c                              |   57 +-
 gdb/f-lang.c                                  |    6 +-
 gdb/findvar.c                                 |    4 +-
 gdb/frame.c                                   |   48 +-
 gdb/testsuite/gdb.dwarf2/dw2-llvm-extend.exp  |  148 +
 gdb/testsuite/gdb.dwarf2/dw2-llvm-offset.exp  |  328 ++
 .../gdb.dwarf2/dw2-llvm-piece-end.exp         |  191 +
 .../gdb.dwarf2/dw2-llvm-select-bit-piece.exp  |  138 +
 .../gdb.dwarf2/dw2-llvm-undefined.exp         |  144 +
 gdb/testsuite/gdb.dwarf2/dw2-op-call.exp      |    2 +-
 gdb/testsuite/gdb.dwarf2/dw2-param-error.exp  |    2 +-
 .../gdb.dwarf2/dw2-stack-boundary.exp         |    2 +-
 gdb/testsuite/lib/dwarf.exp                   |   14 +
 gdb/valops.c                                  |  129 +-
 gdb/value.c                                   |   68 +-
 gdb/value.h                                   |    4 +-
 include/dwarf2.def                            |    8 +
 22 files changed, 4381 insertions(+), 1505 deletions(-)
 create mode 100644 gdb/testsuite/gdb.dwarf2/dw2-llvm-extend.exp
 create mode 100644 gdb/testsuite/gdb.dwarf2/dw2-llvm-offset.exp
 create mode 100644 gdb/testsuite/gdb.dwarf2/dw2-llvm-piece-end.exp
 create mode 100644 gdb/testsuite/gdb.dwarf2/dw2-llvm-select-bit-piece.exp
 create mode 100644 gdb/testsuite/gdb.dwarf2/dw2-llvm-undefined.exp

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2.17.1