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<base href="http://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Minor code difference building with/without -g"
href="http://llvm.org/bugs/show_bug.cgi?id=21807">21807</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Minor code difference building with/without -g
</td>
</tr>
<tr>
<th>Product</th>
<td>tools
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>Windows NT
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>llc
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>russell_gallop@sn.scee.net
</td>
</tr>
<tr>
<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
</td>
</tr>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=13451" name="attach_13451" title="Original source and IR to reproduce">attachment 13451</a> <a href="attachment.cgi?id=13451&action=edit" title="Original source and IR to reproduce">[details]</a></span>
Original source and IR to reproduce
I've observed a small difference in the code generated from the attached cpp
differs depending on the -g flag. This is a non-functional change. I am using
revision 223924.
e.g.
# Without -g
clang -O2 -c test.cpp -o test.o
objdump -d test.o > test.od
# With -g
clang -O2 -c test.cpp -g -o testg.o
objdump -d testg.o > testg.od
# Compare
diff -I "file format" test.od testg.od
17,18c17,18
< 1b: 89 da mov %ebx,%edx
< 1d: 41 89 d8 mov %ebx,%r8d
---
<span class="quote">> 1b: 41 89 d8 mov %ebx,%r8d
> 1e: 89 da mov %ebx,%edx</span >
We've determined that with -g the machine scheduler uses a different scheduling
strategy based on the number of instructions in a schedule region.
The difference is likely because it does not exclude debug instructions from
the count. With the following patch we get the same instruction sequence
with/without -g.
diff --git a/lib/CodeGen/MachineScheduler.cpp
b/lib/CodeGen/MachineScheduler.cpp
index 261942f..cc8f3a0 100644
--- a/lib/CodeGen/MachineScheduler.cpp
+++ b/lib/CodeGen/MachineScheduler.cpp
@@ -430,9 +430,11 @@ void
MachineSchedulerBase::scheduleRegions(ScheduleDAGInstrs &Scheduler) {
// instruction stream until we find the nearest boundary.
unsigned NumRegionInstrs = 0;
MachineBasicBlock::iterator I = RegionEnd;
- for(;I != MBB->begin(); --I, --RemainingInstrs, ++NumRegionInstrs) {
+ for(;I != MBB->begin(); --I, --RemainingInstrs) {
if (isSchedBoundary(std::prev(I), MBB, MF, TII, IsPostRA))
break;
+ if (!I->isDebugValue())
+ ++NumRegionInstrs;
}
// Notify the scheduler of the region, even if we may skip scheduling
// it. Perhaps it still needs to be bundled.
The difference can also be seen with the attached IR:
llc test.ll
llc testg.ll</pre>
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