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<base href="https://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - [InlineCost] CallAnalyzer::visitGetElementPtr can return true for GEPS which are not cost-free"
href="https://bugs.llvm.org/show_bug.cgi?id=33642">33642</a>
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<th>Summary</th>
<td>[InlineCost] CallAnalyzer::visitGetElementPtr can return true for GEPS which are not cost-free
</td>
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<th>Product</th>
<td>libraries
</td>
</tr>
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<th>Version</th>
<td>trunk
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Linux
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>enhancement
</td>
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>Interprocedural Optimizations
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
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<th>Reporter</th>
<td>eastig@yandex.ru
</td>
</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
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<p>
<div>
<pre>In the following IR produced from the code of a simple memcopy function GEPs
are not free:
while.cond: ; preds = %while.body, %entry
%dest.addr.0 = phi i8* [ %dest, %entry ], [ %incdec.ptr1, %while.body ]
%src.addr.0 = phi i8* [ %src, %entry ], [ %incdec.ptr, %while.body ]
%tobool = icmp eq i32 %size.addr.0, 0
br i1 %tobool, label %while.end, label %while.body
while.body: ; preds = %while.cond
%dec = add nsw i32 %size.addr.0, -1
%incdec.ptr = getelementptr inbounds i8, i8* %src.addr.0, i32 1
%0 = load i8, i8* %src.addr.0, align 1, !tbaa !12
%incdec.ptr1 = getelementptr inbounds i8, i8* %dest.addr.0, i32 1
store i8 %0, i8* %dest.addr.0, align 1, !tbaa !12
br label %while.cond
while.end: ; preds = %while.cond
For x86 and ARM they are lowered into ADD instructions.
The analysis of GEPs is done in CallAnalyzer::visitGetElementPtr.
There is the code:
if (IsGEPOffsetConstant(I)) {
if (SROACandidate)
SROAArgValues[&I] = SROAArg;
// Constant GEPs are modeled as free.
return true;
}
For all GEPs with constant offsets true is returned. The example above shows
when GEPs with constant offsets are not free. So we need to call isGEPFree to
check if GEP is actually free of cost.
Note: there is a patch (<a href="https://reviews.llvm.org/D33685">https://reviews.llvm.org/D33685</a>) of isGEPFree to use
TTI::getUserCost instead of TTI::getGEPCost. TTI::getUserCost will catch the
cases above.</pre>
</div>
</p>
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