Sophie: sbcl-1.0.31-1mdv2010.0 i586

sbcl-1.0.31-1mdv2010.0.i586.rpm

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<h4 class="subsection">5.5.2 Source Location Availability</h4>

<p><a name="index-Debug-optimization-quality-92"></a><a name="index-Block_002c-basic-93"></a><a name="index-Block_002c-start-location-94"></a>
Source location information is only available when the <code>debug</code>
optimization quality is at least <code>2</code>.  If source location
information is unavailable, the source commands will give an error
message.

   <p>If source location information is available, but the source location
is unknown because of an interrupt or unexpected hardware error
(see <a href="Unknown-Locations-and-Interrupts.html#Unknown-Locations-and-Interrupts">Unknown Locations and Interrupts</a>), then the command will
print:

<pre class="example">     Unknown location: using block start.
</pre>
   <p>and then proceed to print the source location for the start of the
<em>basic block</em> enclosing the code location.  It's a bit
complicated to explain exactly what a basic block is, but here are
some properties of the block start location:

     <ul>
<li>The block start location may be the same as the true location.

     <li>The block start location will never be later in the
program's flow of control than the true location.

     <li>No conditional control structures (such as <code>if</code>,
<code>cond</code>, <code>or</code>) will intervene between the block start and the
true location (but note that some conditionals present in the original
source could be optimized away.)  Function calls <em>do not</em> end
basic blocks.

     <li>The head of a loop will be the start of a block.

     <li>The programming language concept of &ldquo;block structure&rdquo; and the
Lisp <code>block</code> special form are totally unrelated to the compiler's
basic block.

   </ul>

   <p>In other words, the true location lies between the printed location and the
next conditional (but watch out because the compiler may have changed the
program on you.)

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