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<base href="https://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - LiveDebugVariables propagates constant values without joining at bb boundaries"
href="https://llvm.org/bugs/show_bug.cgi?id=24563">24563</a>
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<th>Summary</th>
<td>LiveDebugVariables propagates constant values without joining at bb boundaries
</td>
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<th>Product</th>
<td>libraries
</td>
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<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>All
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>normal
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>DebugInfo
</td>
</tr>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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</tr>
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<th>Reporter</th>
<td>aprantl@apple.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
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</tr>
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<th>Classification</th>
<td>Unclassified
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</tr></table>
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<pre>Found while reviewing <a href="http://reviews.llvm.org/D11933">http://reviews.llvm.org/D11933</a>
Compiling the following testcase at -O1
void g(int *);
int f() {
int x = 23;
g(&x);
if (x == 42)
++x;
return x;
}
will cause the constant value 23 to be propagated into the last basic block:
BB#2: derived from LLVM BB %if.end
Predecessors according to CFG: BB#0 BB#1
DBG_VALUE 23, 0, !"x", <!14>; line no:3
%EAX<def> = MOV32rm %RSP, 1, %noreg, 4, %noreg; mem:LD4[%x](tbaa=!17)
dbg:r.c:7:10
%RDX<def> = POP64r %RSP<imp-def>, %RSP<imp-use>; dbg:r.c:7:3
RETQ %EAX; dbg:r.c:7:3
Looking into this I found that LiveDebugVariables is the culprit here:
void UserValue::extendDef(SlotIndex Idx, unsigned LocNo,
LiveRange *LR, const VNInfo *VNI,
SmallVectorImpl<SlotIndex> *Kills,
LiveIntervals &LIS, MachineDominatorTree &MDT,
UserValueScopes &UVS) {
...
for (unsigned i = 0, e = Children.size(); i != e; ++i) {
MachineBasicBlock *MBB = Children[i]->getBlock();
if (UVS.dominates(MBB))
Todo.push_back(LIS.getMBBStartIdx(MBB));
}
}
It look like it just unconditionally propagates all DBG_VALUEs down the
dominator tree, which happens to work fine if there already is another
DBG_VALUE but is wrong if the DBG_VALUE is coming from only one of the
predecessors like in this example here.
The DBG_VALUES should only end up in a subsequent basic block if they appear at
the end of all predecessors.</pre>
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