The GDB Agent Expression Mechanism
In some applications, it is not feasable for the debugger to interrupt the program's execution long enough for the developer to learn anything helpful about its behavior. If the program's correctness depends on its real-time behavior, delays introduced by a debugger might cause the program to fail, even when the code itself is correct. It is useful to be able to observe the program's behavior without interrupting it.
Using GDB's trace
and collect
commands, the user can
specify locations in the program, and arbitrary expressions to evaluate
when those locations are reached. Later, using the tfind
command, she can examine the values those expressions had when the
program hit the trace points. The expressions may also denote objects
in memory -- structures or arrays, for example -- whose values GDB
should record; while visiting a particular tracepoint, the user may
inspect those objects as if they were in memory at that moment.
However, because GDB records these values without interacting with the
user, it can do so quickly and unobtrusively, hopefully not disturbing
the program's behavior.
When GDB is debugging a remote target, the GDB agent code running on the target computes the values of the expressions itself. To avoid having a full symbolic expression evaluator on the agent, GDB translates expressions in the source language into a simpler bytecode language, and then sends the bytecode to the agent; the agent then executes the bytecode, and records the values for GDB to retrieve later.
The bytecode language is simple; there are forty-odd opcodes, the bulk of which are the usual vocabulary of C operands (addition, subtraction, shifts, and so on) and various sizes of literals and memory reference operations. The bytecode interpreter operates strictly on machine-level values -- various sizes of integers and floating point numbers -- and requires no information about types or symbols; thus, the interpreter's internal data structures are simple, and each bytecode requires only a few native machine instructions to implement it. The interpreter is small, and strict limits on the memory and time required to evaluate an expression are easy to determine, making it suitable for use by the debugging agent in real-time applications.
General Bytecode Design
The agent represents bytecode expressions as an array of bytes. Each
instruction is one byte long (thus the term bytecode). Some
instructions are followed by operand bytes; for example, the goto
instruction is followed by a destination for the jump.
The bytecode interpreter is a stack-based machine; most instructions pop their operands off the stack, perform some operation, and push the result back on the stack for the next instruction to consume. Each element of the stack may contain either a integer or a floating point value; these values are as many bits wide as the largest integer that can be directly manipulated in the source language. Stack elements carry no record of their type; bytecode could push a value as an integer, then pop it as a floating point value. However, GDB will not generate code which does this. In C, one might define the type of a stack element as follows:
union agent_val { LONGEST l; DOUBLEST d; };
where LONGEST
and DOUBLEST
are typedef
names for
the largest integer and floating point types on the machine.
By the time the bytecode interpreter reaches the end of the expression,
the value of the expression should be the only value left on the stack.
For tracing applications, trace
bytecodes in the expression will
have recorded the necessary data, and the value on the stack may be
discarded. For other applications, like conditional breakpoints, the
value may be useful.
Separate from the stack, the interpreter has two registers:
pc
- The address of the next bytecode to execute.
start
-
The address of the start of the bytecode expression, necessary for
interpreting the
goto
andif_goto
instructions.
Neither of these registers is directly visible to the bytecode language itself, but they are useful for defining the meanings of the bytecode operations.
There are no instructions to perform side effects on the running program, or call the program's functions; we assume that these expressions are only used for unobtrusive debugging, not for patching the running code.
Most bytecode instructions do not distinguish between the various sizes of values, and operate on full-width values; the upper bits of the values are simply ignored, since they do not usually make a difference to the value computed. The exceptions to this rule are:
- memory reference instructions (
ref
n) -
There are distinct instructions to fetch different word sizes from
memory. Once on the stack, however, the values are treated as full-size
integers. They may need to be sign-extended; the
ext
instruction exists for this purpose. - the sign-extension instruction (
ext
n) - These clearly need to know which portion of their operand is to be extended to occupy the full length of the word.
If the interpreter is unable to evaluate an expression completely for some reason (a memory location is inaccessible, or a divisor is zero, for example), we say that interpretation "terminates with an error". This means that the problem is reported back to the interpreter's caller in some helpful way. In general, code using agent expressions should assume that they may attempt to divide by zero, fetch arbitrary memory locations, and misbehave in other ways.
Even complicated C expressions compile to a few bytecode instructions;
for example, the expression x + y * z
would typically produce
code like the following, assuming that x
and y
live in
registers, and z
is a global variable holding a 32-bit
int
:
reg 1 reg 2 const32 address of z ref32 ext 32 mul add end
In detail, these mean:
reg 1
-
Push the value of register 1 (presumably holding
x
) onto the stack. reg 2
-
Push the value of register 2 (holding
y
). const32 address of z
-
Push the address of
z
onto the stack. ref32
-
Fetch a 32-bit word from the address at the top of the stack; replace
the address on the stack with the value. Thus, we replace the address
of
z
withz
's value. ext 32
-
Sign-extend the value on the top of the stack from 32 bits to full
length. This is necessary because
z
is a signed integer. mul
-
Pop the top two numbers on the stack, multiply them, and push their
product. Now the top of the stack contains the value of the expression
y * z
. add
-
Pop the top two numbers, add them, and push the sum. Now the top of the
stack contains the value of
x + y * z
. end
- Stop executing; the value left on the stack top is the value to be recorded.
Bytecode Descriptions
Each bytecode description has the following form:
add
(0x02): a b => a+b- Pop the top two stack items, a and b, as integers; push their sum, as an integer.
In this example, add
is the name of the bytecode, and
(0x02)
is the one-byte value used to encode the bytecode, in
hexidecimal. The phrase "a b => a+b" shows
the stack before and after the bytecode executes. Beforehand, the stack
must contain at least two values, a and b; since the top of
the stack is to the right, b is on the top of the stack, and
a is underneath it. After execution, the bytecode will have
popped a and b from the stack, and replaced them with a
single value, a+b. There may be other values on the stack below
those shown, but the bytecode affects only those shown.
Here is another example:
const8
(0x22) n: => n- Push the 8-bit integer constant n on the stack, without sign extension.
In this example, the bytecode const8
takes an operand n
directly from the bytecode stream; the operand follows the const8
bytecode itself. We write any such operands immediately after the name
of the bytecode, before the colon, and describe the exact encoding of
the operand in the bytecode stream in the body of the bytecode
description.
For the const8
bytecode, there are no stack items given before
the =>; this simply means that the bytecode consumes no values
from the stack. If a bytecode consumes no values, or produces no
values, the list on either side of the => may be empty.
If a value is written as a, b, or n, then the bytecode treats it as an integer. If a value is written is addr, then the bytecode treats it as an address.
We do not fully describe the floating point operations here; although this design can be extended in a clean way to handle floating point values, they are not of immediate interest to the customer, so we avoid describing them, to save time.
float
(0x01): =>- Prefix for floating-point bytecodes. Not implemented yet.
add
(0x02): a b => a+b- Pop two integers from the stack, and push their sum, as an integer.
sub
(0x03): a b => a-b- Pop two integers from the stack, subtract the top value from the next-to-top value, and push the difference.
mul
(0x04): a b => a*b- Pop two integers from the stack, multiply them, and push the product on the stack. Note that, when one multiplies two n-bit numbers yielding another n-bit number, it is irrelevant whether the numbers are signed or not; the results are the same.
div_signed
(0x05): a b => a/b- Pop two signed integers from the stack; divide the next-to-top value by the top value, and push the quotient. If the divisor is zero, terminate with an error.
div_unsigned
(0x06): a b => a/b- Pop two unsigned integers from the stack; divide the next-to-top value by the top value, and push the quotient. If the divisor is zero, terminate with an error.
rem_signed
(0x07): a b => a modulo b- Pop two signed integers from the stack; divide the next-to-top value by the top value, and push the remainder. If the divisor is zero, terminate with an error.
rem_unsigned
(0x08): a b => a modulo b- Pop two unsigned integers from the stack; divide the next-to-top value by the top value, and push the remainder. If the divisor is zero, terminate with an error.
lsh
(0x09): a b => a<<b- Pop two integers from the stack; let a be the next-to-top value, and b be the top value. Shift a left by b bits, and push the result.
rsh_signed
(0x0a): a b =>(signed)
a>>b- Pop two integers from the stack; let a be the next-to-top value, and b be the top value. Shift a right by b bits, inserting copies of the top bit at the high end, and push the result.
rsh_unsigned
(0x0b): a b => a>>b- Pop two integers from the stack; let a be the next-to-top value, and b be the top value. Shift a right by b bits, inserting zero bits at the high end, and push the result.
log_not
(0x0e): a => !a- Pop an integer from the stack; if it is zero, push the value one; otherwise, push the value zero.
bit_and
(0x0f): a b => a&b-
Pop two integers from the stack, and push their bitwise
and
. bit_or
(0x10): a b => a|b-
Pop two integers from the stack, and push their bitwise
or
. bit_xor
(0x11): a b => a^b-
Pop two integers from the stack, and push their bitwise
exclusive-
or
. bit_not
(0x12): a => ~a- Pop an integer from the stack, and push its bitwise complement.
equal
(0x13): a b => a=b- Pop two integers from the stack; if they are equal, push the value one; otherwise, push the value zero.
less_signed
(0x14): a b => a<b- Pop two signed integers from the stack; if the next-to-top value is less than the top value, push the value one; otherwise, push the value zero.
less_unsigned
(0x15): a b => a<b- Pop two unsigned integers from the stack; if the next-to-top value is less than the top value, push the value one; otherwise, push the value zero.
ext
(0x16) n: a => a, sign-extended from n bits-
Pop an unsigned value from the stack; treating it as an n-bit
twos-complement value, extend it to full length. This means that all
bits to the left of bit n-1 (where the least significant bit is bit
0) are set to the value of bit n-1. Note that n may be
larger than or equal to the width of the stack elements of the bytecode
engine; in this case, the bytecode should have no effect.
The number of source bits to preserve, n, is encoded as a single
byte unsigned integer following the
ext
bytecode. zero_ext
(0x2a) n: a => a, zero-extended from n bits-
Pop an unsigned value from the stack; zero all but the bottom n
bits. This means that all bits to the left of bit n-1 (where the
least significant bit is bit 0) are set to the value of bit n-1.
The number of source bits to preserve, n, is encoded as a single
byte unsigned integer following the
zero_ext
bytecode. ref8
(0x17): addr => aref16
(0x18): addr => aref32
(0x19): addr => aref64
(0x1a): addr => a-
Pop an address addr from the stack. For bytecode
ref
n, fetch an n-bit value from addr, using the natural target endianness. Push the fetched value as an unsigned integer. Note that addr may not be aligned in any particular way; therefn
bytecodes should operate correctly for any address. If attempting to access memory at addr would cause a processor exception of some sort, terminate with an error. ref_float
(0x1b): addr => dref_double
(0x1c): addr => dref_long_double
(0x1d): addr => dl_to_d
(0x1e): a => dd_to_l
(0x1f): d => a- Not implemented yet.
dup
(0x28): a => a a- Push another copy of the stack's top element.
swap
(0x2b): a b => b a- Exchange the top two items on the stack.
pop
(0x29): a =>- Discard the top value on the stack.
if_goto
(0x20) offset: a =>-
Pop an integer off the stack; if it is non-zero, branch to the given
offset in the bytecode string. Otherwise, continue to the next
instruction in the bytecode stream. In other words, if a is
non-zero, set the
pc
register tostart
+ offset. Thus, an offset of zero denotes the beginning of the expression. The offset is stored as a sixteen-bit unsigned value, stored immediately following theif_goto
bytecode. It is always stored most significant byte first, regardless of the target's normal endianness. The offset is not guaranteed to fall at any particular alignment within the bytecode stream; thus, on machines where fetching a 16-bit on an unaligned address raises an exception, you should fetch the offset one byte at a time. goto
(0x21) offset: =>-
Branch unconditionally to offset; in other words, set the
pc
register tostart
+ offset. The offset is stored in the same way as for theif_goto
bytecode. const8
(0x22) n: => nconst16
(0x23) n: => nconst32
(0x24) n: => nconst64
(0x25) n: => n-
Push the integer constant n on the stack, without sign extension.
To produce a small negative value, push a small twos-complement value,
and then sign-extend it using the
ext
bytecode. The constant n is stored in the appropriate number of bytes following theconst
b bytecode. The constant n is always stored most significant byte first, regardless of the target's normal endianness. The constant is not guaranteed to fall at any particular alignment within the bytecode stream; thus, on machines where fetching a 16-bit on an unaligned address raises an exception, you should fetch n one byte at a time. reg
(0x26) n: => a-
Push the value of register number n, without sign extension. The
registers are numbered following GDB's conventions.
The register number n is encoded as a 16-bit unsigned integer
immediately following the
reg
bytecode. It is always stored most significant byte first, regardless of the target's normal endianness. The register number is not guaranteed to fall at any particular alignment within the bytecode stream; thus, on machines where fetching a 16-bit on an unaligned address raises an exception, you should fetch the register number one byte at a time. trace
(0x0c): addr size =>- Record the contents of the size bytes at addr in a trace buffer, for later retrieval by GDB.
trace_quick
(0x0d) size: addr => addr-
Record the contents of the size bytes at addr in a trace
buffer, for later retrieval by GDB. size is a single byte
unsigned integer following the
trace
opcode. This bytecode is equivalent to the sequencedup const8 size trace
, but we provide it anyway to save space in bytecode strings. trace16
(0x30) size: addr => addr-
Identical to trace_quick, except that size is a 16-bit big-endian
unsigned integer, not a single byte. This should probably have been
named
trace_quick16
, for consistency. end
(0x27): =>- Stop executing bytecode; the result should be the top element of the stack. If the purpose of the expression was to compute an lvalue or a range of memory, then the next-to-top of the stack is the lvalue's address, and the top of the stack is the lvalue's size, in bytes.
Using Agent Expressions
Here is a sketch of a full non-stop debugging cycle, showing how agent expressions fit into the process.
- The user selects trace points in the program's code at which GDB should collect data.
- The user specifies expressions to evaluate at each trace point. These expressions may denote objects in memory, in which case those objects' contents are recorded as the program runs, or computed values, in which case the values themselves are recorded.
- GDB transmits the tracepoints and their associated expressions to the GDB agent, running on the debugging target.
- The agent arranges to be notified when a trace point is hit. Note that, on some systems, the target operating system is completely responsible for collecting the data; see section Tracing on Symmetrix.
- When execution on the target reaches a trace point, the agent evaluates the expressions associated with that trace point, and records the resulting values and memory ranges.
- Later, when the user selects a given trace event and inspects the objects and expression values recorded, GDB talks to the agent to retrieve recorded data as necessary to meet the user's requests. If the user asks to see an object whose contents have not been recorded, GDB reports an error.
Varying Target Capabilities
Some targets don't support floating-point, and some would rather not
have to deal with long long
operations. Also, different targets
will have different stack sizes, and different bytecode buffer lengths.
Thus, GDB needs a way to ask the target about itself. We haven't worked out the details yet, but in general, GDB should be able to send the target a packet asking it to describe itself. The reply should be a packet whose length is explicit, so we can add new information to the packet in future revisions of the agent, without confusing old versions of GDB, and it should contain a version number. It should contain at least the following information:
- whether floating point is supported
-
whether
long long
is supported - maximum acceptable size of bytecode stack
- maximum acceptable length of bytecode expressions
- which registers are actually available for collection
- whether the target supports disabled tracepoints
Tracing on Symmetrix
This section documents the API used by the GDB agent to collect data on Symmetrix systems.
Cygnus originally implemented these tracing features to help EMC Corporation debug their Symmetrix high-availability disk drives. The Symmetrix application code already includes substantial tracing facilities; the GDB agent for the Symmetrix system uses those facilities for its own data collection, via the API described here.
- Function: DTC_RESPONSE adbg_find_memory_in_frame (FRAME_DEF *frame, char *address, char **buffer, unsigned int *size)
-
Search the trace frame frame for memory saved from address.
If the memory is available, provide the address of the buffer holding
it; otherwise, provide the address of the next saved area.
-
If the memory at address was saved in frame, set
*buffer
to point to the buffer in which that memory was saved, set*size
to the number of bytes from address that are saved at*buffer
, and returnOK_TARGET_RESPONSE
. (Clearly, in this case, the function will always set*size
to a value greater than zero.) -
If frame does not record any memory at address, set
*size
to the distance from address to the start of the saved region with the lowest address higher than address. If there is no memory saved from any higher address, set*size
to zero. ReturnNOT_FOUND_TARGET_RESPONSE
.
These two possibilities allow the caller to either retrieve the data, or walk the address space to the next saved area.
-
If the memory at address was saved in frame, set
This function allows the GDB agent to map the regions of memory saved in a particular frame, and retrieve their contents efficiently.
This function also provides a clean interface between the GDB agent and the Symmetrix tracing structures, making it easier to adapt the GDB agent to future versions of the Symmetrix system, and vice versa. This function searches all data saved in frame, whether the data is there at the request of a bytecode expression, or because it falls in one of the format's memory ranges, or because it was saved from the top of the stack. EMC can arbitrarily change and enhance the tracing mechanism, but as long as this function works properly, all collected memory is visible to GDB.
The function itself is straightforward to implement. A single pass over the trace frame's stack area, memory ranges, and expression blocks can yield the address of the buffer (if the requested address was saved), and also note the address of the next higher range of memory, to be returned when the search fails.
As an example, suppose the trace frame f
has saved sixteen bytes
from address 0x8000
in a buffer at 0x1000
, and thirty-two
bytes from address 0xc000
in a buffer at 0x1010
. Here are
some sample calls, and the effect each would have:
adbg_find_memory_in_frame (f, (char*) 0x8000, &buffer, &size)
-
This would set
buffer
to0x1000
, setsize
to sixteen, and returnOK_TARGET_RESPONSE
, sincef
saves sixteen bytes from0x8000
at0x1000
. adbg_find_memory_in_frame (f, (char *) 0x8004, &buffer, &size)
-
This would set
buffer
to0x1004
, setsize
to twelve, and returnOK_TARGET_RESPONSE
, since `f' saves the twelve bytes from0x8004
starting four bytes into the buffer at0x1000
. This shows that request addresses may fall in the middle of saved areas; the function should return the address and size of the remainder of the buffer. adbg_find_memory_in_frame (f, (char *) 0x8100, &buffer, &size)
-
This would set
size
to0x3f00
and returnNOT_FOUND_TARGET_RESPONSE
, since there is no memory saved inf
from the address0x8100
, and the next memory available is at0x8100 + 0x3f00
, or0xc000
. This shows that request addresses may fall outside of all saved memory ranges; the function should indicate the next saved area, if any. adbg_find_memory_in_frame (f, (char *) 0x7000, &buffer, &size)
-
This would set
size
to0x1000
and returnNOT_FOUND_TARGET_RESPONSE
, since the next saved memory is at0x7000 + 0x1000
, or0x8000
. adbg_find_memory_in_frame (f, (char *) 0xf000, &buffer, &size)
-
This would set
size
to zero, and returnNOT_FOUND_TARGET_RESPONSE
. This shows how the function tells the caller that no further memory ranges have been saved.
As another example, here is a function which will print out the
addresses of all memory saved in the trace frame frame
on the
Symmetrix INLINES console:
void print_frame_addresses (FRAME_DEF *frame) { char *addr; char *buffer; unsigned long size; addr = 0; for (;;) { /* Either find out how much memory we have here, or discover where the next saved region is. */ if (adbg_find_memory_in_frame (frame, addr, &buffer, &size) == OK_TARGET_RESPONSE) printp ("saved %x to %x\n", addr, addr + size); if (size == 0) break; addr += size; } }
Note that there is not necessarily any connection between the order in
which the data is saved in the trace frame, and the order in which
adbg_find_memory_in_frame
will return those memory ranges. The
code above will always print the saved memory regions in order of
increasing address, while the underlying frame structure might store the
data in a random order.
[[This section should cover the rest of the Symmetrix functions the stub relies upon, too.]]
Rationale
Some of the design decisions apparent above are arguable.
- What about stack overflow/underflow?
- GDB should be able to query the target to discover its stack size. Given that information, GDB can determine at translation time whether a given expression will overflow the stack. But this spec isn't about what kinds of error-checking GDB ought to do.
- Why are you doing everything in LONGEST?
- Speed isn't important, but agent code size is; using LONGEST brings in a bunch of support code to do things like division, etc. So this is a serious concern. First, note that you don't need different bytecodes for different operand sizes. You can generate code without knowing how big the stack elements actually are on the target. If the target only supports 32-bit ints, and you don't send any 64-bit bytecodes, everything just works. The observation here is that the MIPS and the Alpha have only fixed-size registers, and you can still get C's semantics even though most instructions only operate on full-sized words. You just need to make sure everything is properly sign-extended at the right times. So there is no need for 32- and 64-bit variants of the bytecodes. Just implement everything using the largest size you support. GDB should certainly check to see what sizes the target supports, so the user can get an error earlier, rather than later. But this information is not necessary for correctness.
- Why don't you have
>
or<=
operators? -
I want to keep the interpreter small, and we don't need them. We can
combine the
less_
opcodes withlog_not
, and swap the order of the operands, yielding all four asymmetrical comparison operators. For example,(x <= y)
is! (x > y)
, which is! (y < x)
. - Why do you have
log_not
? - Why do you have
ext
? - Why do you have
zero_ext
? -
These are all easily synthesized from other instructions, but I expect
them to be used frequently, and they're simple, so I include them to
keep bytecode strings short.
log_not
is equivalent toconst8 0 equal
; it's used in half the relational operators.ext n
is equivalent toconst8 s-n lsh const8 s-n rsh_signed
, where s is the size of the stack elements; it followsrefm
and reg bytecodes when the value should be signed. See the next bulleted item.zero_ext n
is equivalent toconstm mask log_and
; it's used whenever we push the value of a register, because we can't assume the upper bits of the register aren't garbage. - Why not have sign-extending variants of the
ref
operators? -
Because that would double the number of
ref
operators, and we need theext
bytecode anyway for accessing bitfields. - Why not have constant-address variants of the
ref
operators? -
Because that would double the number of
ref
operators again, andconst32 address ref32
is only one byte longer. - Why do the
refn
operators have to support unaligned fetches? - GDB will generate bytecode that fetches multi-byte values at unaligned addresses whenever the executable's debugging information tells it to. Furthermore, GDB does not know the value the pointer will have when GDB generates the bytecode, so it cannot determine whether a particular fetch will be aligned or not. In particular, structure bitfields may be several bytes long, but follow no alignment rules; members of packed structures are not necessarily aligned either. In general, there are many cases where unaligned references occur in correct C code, either at the programmer's explicit request, or at the compiler's discretion. Thus, it is simpler to make the GDB agent bytecodes work correctly in all circumstances than to make GDB guess in each case whether the compiler did the usual thing.
- Why are there no side-effecting operators?
- Because our current client doesn't want them? That's a cheap answer. I think the real answer is that I'm afraid of implementing function calls. We should re-visit this issue after the present contract is delivered.
- Why aren't the
goto
ops PC-relative? - The interpreter has the base address around anyway for PC bounds checking, and it seemed simpler.
- Why is there only one offset size for the
goto
ops? - Offsets are currently sixteen bits. I'm not happy with this situation either: Suppose we have multiple branch ops with different offset sizes. As I generate code left-to-right, all my jumps are forward jumps (there are no loops in expressions), so I never know the target when I emit the jump opcode. Thus, I have to either always assume the largest offset size, or do jump relaxation on the code after I generate it, which seems like a big waste of time. I can imagine a reasonable expression being longer than 256 bytes. I can't imagine one being longer than 64k. Thus, we need 16-bit offsets. This kind of reasoning is so bogus, but relaxation is pathetic. The other approach would be to generate code right-to-left. Then I'd always know my offset size. That might be fun.
- Where is the function call bytecode?
- When we add side-effects, we should add this.
- Why does the
reg
bytecode take a 16-bit register number? - Intel's IA-64 architecture has 128 general-purpose registers, and 128 floating-point registers, and I'm sure it has some random control registers.
- Why do we need
trace
andtrace_quick
? -
Because GDB needs to record all the memory contents and registers an
expression touches. If the user wants to evaluate an expression
x->y->z
, the agent must record the values ofx
andx->y
as well as the value ofx->y->z
. - Don't the
trace
bytecodes make the interpreter less general? -
They do mean that the interpreter contains special-purpose code, but
that doesn't mean the interpreter can only be used for that purpose. If
an expression doesn't use the
trace
bytecodes, they don't get in its way. - Why doesn't
trace_quick
consume its arguments the way everything else does? -
In general, you do want your operators to consume their arguments; it's
consistent, and generally reduces the amount of stack rearrangement
necessary. However,
trace_quick
is a kludge to save space; it only exists so we needn't writedup const8 SIZE trace
before every memory reference. Therefore, it's okay for it not to consume its arguments; it's meant for a specific context in which we know exactly what it should do with the stack. If we're going to have a kludge, it should be an effective kludge. - Why does
trace16
exist? -
That opcode was added by the customer that contracted Cygnus for the
data tracing work. I personally think it is unnecessary; objects that
large will be quite rare, so it is okay to use
dup const16 size trace
in those cases. Whatever we decide to do withtrace16
, we should at least leave opcode 0x30 reserved, to remain compatible with the customer who added it.
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