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| <title>Debugging with GDB: Rationale</title> |
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| <a name="Rationale"></a> |
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| Previous: <a href="Varying-Target-Capabilities.html#Varying-Target-Capabilities" accesskey="p" rel="prev">Varying Target Capabilities</a>, Up: <a href="Agent-Expressions.html#Agent-Expressions" accesskey="u" rel="up">Agent Expressions</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p> |
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| <a name="Rationale-1"></a> |
| <h3 class="section">F.5 Rationale</h3> |
| |
| <p>Some of the design decisions apparent above are arguable. |
| </p> |
| <dl compact="compact"> |
| <dt><b>What about stack overflow/underflow?</b></dt> |
| <dd><p>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. |
| </p> |
| </dd> |
| <dt><b>Why are you doing everything in LONGEST?</b></dt> |
| <dd> |
| <p>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. |
| </p> |
| <p>First, note that you don’t need different bytecodes for different |
| operand sizes. You can generate code without <em>knowing</em> 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. |
| </p> |
| <p>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. |
| </p> |
| |
| </dd> |
| <dt><b>Why don’t you have <code>></code> or <code><=</code> operators?</b></dt> |
| <dd><p>I want to keep the interpreter small, and we don’t need them. We can |
| combine the <code>less_</code> opcodes with <code>log_not</code>, and swap the order |
| of the operands, yielding all four asymmetrical comparison operators. |
| For example, <code>(x <= y)</code> is <code>! (x > y)</code>, which is <code>! (y < |
| x)</code>. |
| </p> |
| </dd> |
| <dt><b>Why do you have <code>log_not</code>?</b></dt> |
| <dt><b>Why do you have <code>ext</code>?</b></dt> |
| <dt><b>Why do you have <code>zero_ext</code>?</b></dt> |
| <dd><p>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. |
| </p> |
| <p><code>log_not</code> is equivalent to <code>const8 0 equal</code>; it’s used in half |
| the relational operators. |
| </p> |
| <p><code>ext <var>n</var></code> is equivalent to <code>const8 <var>s-n</var> lsh const8 |
| <var>s-n</var> rsh_signed</code>, where <var>s</var> is the size of the stack elements; |
| it follows <code>ref<var>m</var></code> and <var>reg</var> bytecodes when the value |
| should be signed. See the next bulleted item. |
| </p> |
| <p><code>zero_ext <var>n</var></code> is equivalent to <code>const<var>m</var> <var>mask</var> |
| log_and</code>; 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. |
| </p> |
| </dd> |
| <dt><b>Why not have sign-extending variants of the <code>ref</code> operators?</b></dt> |
| <dd><p>Because that would double the number of <code>ref</code> operators, and we |
| need the <code>ext</code> bytecode anyway for accessing bitfields. |
| </p> |
| </dd> |
| <dt><b>Why not have constant-address variants of the <code>ref</code> operators?</b></dt> |
| <dd><p>Because that would double the number of <code>ref</code> operators again, and |
| <code>const32 <var>address</var> ref32</code> is only one byte longer. |
| </p> |
| </dd> |
| <dt><b>Why do the <code>ref<var>n</var></code> operators have to support unaligned fetches?</b></dt> |
| <dd><p>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. |
| </p> |
| <p>In particular, structure bitfields may be several bytes long, but follow |
| no alignment rules; members of packed structures are not necessarily |
| aligned either. |
| </p> |
| <p>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. |
| </p> |
| </dd> |
| <dt><b>Why are there no side-effecting operators?</b></dt> |
| <dd><p>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. |
| </p> |
| </dd> |
| <dt><b>Why aren’t the <code>goto</code> ops PC-relative?</b></dt> |
| <dd><p>The interpreter has the base address around anyway for PC bounds |
| checking, and it seemed simpler. |
| </p> |
| </dd> |
| <dt><b>Why is there only one offset size for the <code>goto</code> ops?</b></dt> |
| <dd><p>Offsets are currently sixteen bits. I’m not happy with this situation |
| either: |
| </p> |
| <p>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. |
| </p> |
| <p>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. |
| </p> |
| <p>The other approach would be to generate code right-to-left. Then I’d |
| always know my offset size. That might be fun. |
| </p> |
| </dd> |
| <dt><b>Where is the function call bytecode?</b></dt> |
| <dd> |
| <p>When we add side-effects, we should add this. |
| </p> |
| </dd> |
| <dt><b>Why does the <code>reg</code> bytecode take a 16-bit register number?</b></dt> |
| <dd> |
| <p>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. |
| </p> |
| </dd> |
| <dt><b>Why do we need <code>trace</code> and <code>trace_quick</code>?</b></dt> |
| <dd><p>Because GDB needs to record all the memory contents and registers an |
| expression touches. If the user wants to evaluate an expression |
| <code>x->y->z</code>, the agent must record the values of <code>x</code> and |
| <code>x->y</code> as well as the value of <code>x->y->z</code>. |
| </p> |
| </dd> |
| <dt><b>Don’t the <code>trace</code> bytecodes make the interpreter less general?</b></dt> |
| <dd><p>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 <code>trace</code> bytecodes, they don’t get in |
| its way. |
| </p> |
| </dd> |
| <dt><b>Why doesn’t <code>trace_quick</code> consume its arguments the way everything else does?</b></dt> |
| <dd><p>In general, you do want your operators to consume their arguments; it’s |
| consistent, and generally reduces the amount of stack rearrangement |
| necessary. However, <code>trace_quick</code> is a kludge to save space; it |
| only exists so we needn’t write <code>dup const8 <var>SIZE</var> trace</code> |
| 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. |
| </p> |
| </dd> |
| <dt><b>Why does <code>trace16</code> exist?</b></dt> |
| <dd><p>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 <code>dup const16 |
| <var>size</var> trace</code> in those cases. |
| </p> |
| <p>Whatever we decide to do with <code>trace16</code>, we should at least leave |
| opcode 0x30 reserved, to remain compatible with the customer who added |
| it. |
| </p> |
| </dd> |
| </dl> |
| |
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