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140425.1 Jakub Jelinek Typed DWARF stack Enhancement Accepted Jakub Jelinek


Section 2.5, pg 
Typed DWARF stack
=================

Overview
--------

The addition of DW_OP_stack_value operation in DWARF4 allowed the debugging
information to describe even values of variables partially or completely
optimized away, either in some ranges or everywhere.  But the values are
still computed on the DWARF stack which has only integral types with the
size of the target machine's address, so it is still very hard to represent
non-integral variables or variables with integral types larger than the size
of address.  Consider target with 32-bit addresses and 64-bit long long:

void
foo (unsigned long long x, unsigned long long y, double z, volatile int *p)
{
  unsigned long long a = x + y;
  double b = z * 2.5;
  (*p)++;
}

On 64-bit architecture e.g. a could be described as (x86_64)
DW_OP_breg5 0 DW_OP_breg4 0 DW_OP_plus DW_OP_stack_value
because the addresses are 64-bit, but when they are 32-bit, one would
have to resort to something like (i?86 little endian):
DW_OP_fbreg 0 DW_OP_deref DW_OP_fbreg 8 DW_OP_deref DW_OP_plus \
DW_OP_stack_value DW_OP_piece 4 \
DW_OP_fbreg 4 DW_OP_deref DW_OP_fbreg 12 DW_OP_deref DW_OP_plus \
DW_OP_fbreg 0 DW_OP_deref DW_OP_plus_uconst 0x80000000 DW_OP_dup \
DW_OP_fbreg 8 DW_OP_deref DW_OP_plus DW_OP_gt DW_OP_plus \
DW_OP_stack_value DW_OP_piece 4
(i.e. low 32 bits are (unsigned) x + (unsigned) y, the high 32 bits
are (unsigned) (x >> 32) + (unsigned (y >> 32) + ((unsigned) x > (unsigned) x + (unsigned) y)
), but for double address size multiplication this already would be much
larger, and for IEEE double, while in theory implementable, would basically
require writing a software IEEE floating point emulation library in DWARF
expressions that one would use with DW_OP_call*.  With the following
proposal, a can use:
DW_OP_fbreg 0 DW_OP_deref_type 8 <unsigned long long> \
DW_OP_fbreg 8 DW_OP_deref_type 8 <unsigned long long> \
DW_OP_plus DW_OP_stack_value
and for b:
DW_OP_fbreg 16 DW_OP_deref_type 8 <double> \
DW_OP_const_type <double> 8 <0x4004000000000000ULL> \
DW_OP_mul DW_OP_stack_value

The extension (with DW_OP_GNU_ rather than DW_OP_ and without
DW_OP_xderef_type) has been implemented for 3 years now in GCC/GDB.

The DWARF stack is enhanced, so that instead of a stack element being
just an address sized integer, the stack element is a pair of a type
identifier and union which contains the address sized integer, various other
integral and floating point (perhaps _Decimal/fixed point etc.) types.

So, something along the lines of:

struct DWARF_stack_element { 
  int type_id; 
  union { 
    intptr_t address; 
    long long llong; 
    unsigned long long ullong; 
    __int128_t i128t; 
    __uint128_t u128t; 
    float flt; 
    double dbl; 
    long double ldbl;
    ... 
  } 
};

For compatibility reasons and also because most of the operations performed
on the DWARF stack are still integral values with address sizes, plus in
order to give more freedom to debug information consumers, the way this
extension is proposed is that most operations (other than the newly added
ones) if they don't pop anything from the stack and just push a new value
push values with a special address type, DW_OP_{convert,reinterpret} refer
to this as type with offset 0, while operations that consume stack elements
as operands typically are overloaded on that type, if it has more than
one operand require that all operands have the same type and usually push
the same type of result value.

Debug info consumers must handle as minimum (as before) at least the special
address type, plus whatever other base types they choose to support.
When they see any of the new
DW_OP_{{regval,{,x}deref,const}_type,convert,reinterpret} operations
refering to a base type that they don't support, they should just give up
on trying to evaluate the whole expression and suggest to the user that
the value is optimized away/can't be computed.  So, debug info consumers
can as well choose not to support any of the typed DWARF stack at all,
as long as they are able to just parse the 6 new operations, anything above
that is a QoI issue.

Proposed changes to DWARF
-------------------------

2.5.1

Change:

Each element of the stack is the size of an address on the target machine.

to:

Each element of the stack has a type and a value, and can represent
a value of any supported base type of the target machine.  Instead of
a base type, elements can have a special address type, which is an integral
type that has the size of an address on the target machine and unspecified
signedness.

Add after the paragraph:

While the abstract definition of the stack calls for variable-size entries
able to hold any supported base type, in practice it is expected that each
element of the stack can be represented as a fixed-size element large enough
to hold a value of any type supported by the DWARF consumer for that target,
plus a small identifier sufficient to encode the type of that element.
Support for base types other than what is required to do address arithmetic
is intended only for debugging of optimized code, and the completeness of the
DWARF consumer's support for the full set of base types is a
quality-of-implementation issue. If a consumer encounters a DWARF expression
that uses a type it does not support, it should ignore the entire expression
and report its inability to provide the requested information.

It should also be noted that floating-point arithmetic is highly dependent
on the computational environment. It is not the intention of this expression
evaluation facility to produce identical results to those produced by the
program being debugged while executing on the target machine. Floating-point
computations in this stack machine will be done with precision control and
rounding modes as defined by the implementation.

2.5.1.1

Change:

If the value of a constant in one of these operations is larger than can be stored in a
single stack element, the value is truncated to the element size and the low-order bits
are pushed on the stack.

to:

Operations other than DW_OP_const_type push a value with the special address
type, and if the value of a constant in one of these operations is larger than can be stored
in a single stack element of the special address type, the value is truncated to the
element size and the low-order bits are pushed on the stack.

Add at the end of section:

9. DW_OP_const_type

The DW_OP_const_type operation takes three operands. The first operand is an
unsigned LEB128 integer that represents the offset of a debugging
information entry in the current compilation unit, which must be a
DW_TAG_base_type entry that provides the type of the constant provided. The
second operand is 1-byte unsigned integer that represents the size n of the
constant, which may not be larger than the size of the largest supported
base type of the target machine. The third operand is a block of n bytes to
be interpreted as a value of the referenced type.

While the size of the constant could be inferred from the base type
definition, it is encoded explicitly into the expression so that the
expression can be parsed easily without reference to the .debug_info
section.

2.5.1.2

Change subsection title to:

Register Values

Change first sentence to:

The following operations push a value onto the stack that is either the
contents of a register or the result of adding the contents of a register
to a given signed offset.  DW_OP_regval_type pushes just the content
of the register, with the given base type, while the other operations
push a value of the register with the special address type plus given
signed offset.

Add as a new operation at the end of the list:

4. DW_OP_regval_type

The DW_OP_regval_type operation takes two parameters. The first parameter is
an unsigned LEB128 number, which identifies a register whose contents is to
be pushed onto the stack. The second parameter is an unsigned LEB128 number
that represents the offset of a debugging information entry in the current
compilation unit, which must be a DW_TAG_base_type entry that provides the
type of the value contained in the specified register.

2.5.1.3

Add after the first two sentences:

The DW_OP_dup, DW_OP_drop, DW_OP_pick, DW_OP_over, DW_OP_swap
and DW_OP_rot operations manipulate the elements of the stack as full pairs
of type identifier and corresponding value.  The DW_OP_deref,
DW_OP_deref_size, DW_OP_xderef, DW_OP_xderef_size and DW_OP_form_tls_address
operations require the popped values to have integral type, either special
address type or some integral base type, and push a value with the special
address type.  DW_OP_deref_type and DW_OP_xderef_type operations have the
same requirement on the popped values, but push a value with the requested type.
All other operations push a value with the special address type.

After DW_OP_deref_size description add new operation (and renumber all the
following operations):

9. DW_OP_deref_type

The DW_OP_deref_type operation behaves like the DW_OP_deref_size operation:
it pops the top stack entry and treats it as an address. The value retrieved
from that address is pushed. In the DW_OP_deref_type operation, the size in
bytes of the data retrieved from the dereferenced address is specified by
the first operand. This operand is a 1-byte unsigned integral constant whose
value may not be larger than the size of the largest supported base type on
the target machine. The second operand is an unsigned LEB128 integer that
represents the offset of a debugging information entry in the current
compilation unit, which must be a DW_TAG_base_type entry that provides the
type of the data retrieved.

After DW_OP_xderef_size description add new operation (and again renumber
all the following operations):

12. DW_OP_xderef_type

The DW_OP_xderef_type operation behaves like the DW_OP_xderef_size
operation: it pops the top two stack entries, treats them as an address and
an address space identifier, and pushes the value retrieved. In the
DW_OP_xderef_type operation, the size in bytes of the data retrieved from
the dereferenced address is specified by the first operand. This operand is
a 1-byte unsigned integral constant whose value may not be larger than the
size of the largest supported base type on the target machine. The second
operand is an unsigned LEB128 integer that represents the offset of a
debugging information entry in the current compilation unit, which must be a
DW_TAG_base_type entry that provides the type of the data retrieved.

2.5.1.4

Rewrite first paragraph to:

The following provide arithmetic and logical operations.  If an operation
pops two values from the stack, both values should have the same type,
either the same base type or both should have the special address type.
The result of the operation which is pushed back should have the same type
as the type of the operands.  If the type of the operands is the special
address type, except as otherwise specified, the arithmetic operations
perform addressing arithmetic, that is, unsigned arithmetic that is performed
modulo one plus the largest representable address (for example, 0x100000000
when the size of an address is 32 bits).  Operations other than DW_OP_abs,
DW_OP_div, DW_OP_minus, DW_OP_mul, DW_OP_neg and DW_OP_plus require integral
types of the operand (either integral base type or the special address
type).  Operations do not cause an exception on overflow.

2.5.1.5

Change:

pop the top two stack values,

to:

pop the top two stack values, which should both have the same type,
either same base type or both the special address type.

After:

push the constant value 1 onto the stack if the result of the operation is
true or the constant value 0 if the result of the operation is false.

add:

The pushed constant value has the special address type.

Change:

Comparisons are performed as signed operations.

to:

If the operands have the special address type, the comparisons are performed
as signed operations.

2.5.1.6

Renumber to 2.5.1.7, insert before that:

2.5.1.6  Type Conversions

The following operation provides for explicit type conversion.

1. DW_OP_convert

The DW_OP_convert operation pops the top stack entry, converts it to a
different type, then pushes the result. It takes one operand, which is an
unsigned LEB128 integer that represents the offset of a debugging
information entry in the current compilation unit, or value 0 which
represents the special address type. If the operand is non-zero, the
referenced entry must be a DW_TAG_base_type entry that provides the type
to which the value is converted.

2. DW_OP_reinterpret

The DW_OP_reinterpret operation pops the top stack entry, reinterprets
the bits in its value as a value of a different type, then pushes the
result. It takes one operand, which is an unsigned LEB128 integer that
represents the offset of a debugging information entry in the current
compilation unit, or value 0 which represents the special address type.
If the operand is non-zero, the referenced entry must be a
DW_TAG_base_type entry that provides the type to which the value is converted.
The type of the operand and result type should have the same size in bits.

The semantics of the reinterpretation of a value is as if in C or C++
there are two variables, one with the type of the operand, into which
the popped value is stored, then copied using memcpy into the other variable
with the type of the result and the pushed result value is the value of the
other variable after memcpy.


--
2014-07-15:  Accepted.


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