[PATCH 00/43 V2] Allow location description on the DWARF stack

Zoran Zaric via Gdb-patches
2021-03-01T14:46:46Z

Based on gdb master: 8255cf421c513747b7e189f932ca6e5b450e0d46

The idea of this patch series is to cleanup the design of the DWARF
expression evaluator (dwarf_expr_context class) and allow a future
extensions of that 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 sixteen
patches could be viewed as a standalone subset that introduces a
welcome cleanup of the DWARF expression evaluator module.

Zoran Zaric (43):
Replace the symbol needs evaluator with a parser
Cleanup of the dwarf_expr_context constructor
Move frame context info to dwarf_expr_context
Remove get_frame_cfa from dwarf_expr_context
Move compilation unit info to dwarf_expr_context
Move dwarf_call to dwarf_expr_context
Move get_object_address to dwarf_expr_context
Move read_mem to dwarf_expr_context
Move push_dwarf_reg_entry_value to expr.c
Inline get_reg_value method of dwarf_expr_context
Remove empty frame and full evaluators
Merge evaluate_for_locexpr_baton evaluator
Move piece_closure and its support to expr.c
Make value_copy also copy the stack data member
Make DWARF evaluator return a single struct value
Simplify dwarf_expr_context class interface
Add as_lval argument to expression evaluator
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 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
Change back the symbol needs to use the evaluator
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

gdb/ada-lang.c | 2 +-
gdb/compile/compile-loc2c.c | 11 +
gdb/dwarf2/expr.c | 3419 +++++++++++++++--
gdb/dwarf2/expr.h | 275 +-
gdb/dwarf2/frame.c | 163 +-
gdb/dwarf2/loc.c | 1271 +-----
gdb/dwarf2/loc.h | 34 +-
gdb/f-lang.c | 4 +-
gdb/findvar.c | 4 +-
gdb/frame.c | 37 +-
gdb/testsuite/gdb.dwarf2/dw2-llvm-offset.exp | 328 ++
.../gdb.dwarf2/dw2-llvm-piece-end.exp | 192 +
.../gdb.dwarf2/dw2-llvm-undefined.exp | 144 +
gdb/testsuite/gdb.dwarf2/symbol_needs_eval.c | 25 +
.../gdb.dwarf2/symbol_needs_eval_fail.exp | 108 +
.../gdb.dwarf2/symbol_needs_eval_timeout.exp | 127 +
.../amd64-py-framefilter-invalidarg.S | 1 -
gdb/testsuite/lib/dwarf.exp | 8 +
gdb/valops.c | 129 +-
gdb/value.c | 69 +-
gdb/value.h | 4 +-
include/dwarf2.def | 6 +
22 files changed, 4329 insertions(+), 2032 deletions(-)
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-undefined.exp
create mode 100644 gdb/testsuite/gdb.dwarf2/symbol_needs_eval.c
create mode 100644 gdb/testsuite/gdb.dwarf2/symbol_needs_eval_fail.exp
create mode 100644 gdb/testsuite/gdb.dwarf2/symbol_needs_eval_timeout.exp

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2.17.1