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<base href="https://llvm.org/bugs/" />
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<body><table border="1" cellspacing="0" cellpadding="8">
<tr>
<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Performance regression in simple linear search"
href="https://llvm.org/bugs/show_bug.cgi?id=25268">25268</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Performance regression in simple linear search
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>3.7
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>Linux
</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>
<tr>
<th>Component</th>
<td>LLVM Codegen
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>chris@detrino.org
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>struct Ret {
std::size_t index;
std::size_t pos;
};
Ret Test1(std::size_t const *array, std::size_t pos) {
std::size_t index = 0;
while (true) {
auto next = array[index];
if (pos < next) break;
pos -= next;
++index;
}
return {index, pos};
}
This generates the following IR with clang 3.7 -O2 -S -c -std=c++11 -emit-llvm:
; Function Attrs: nounwind readonly uwtable
define { i64, i64 } @_Z5Test1PKmm(i64* nocapture readonly %array, i64 %pos) #3
{
br label %1
; <label>:1 ; preds = %1, %0
%index.0 = phi i64 [ 0, %0 ], [ %index.1, %1 ]
%.01 = phi i64 [ %pos, %0 ], [ %.1, %1 ]
%2 = getelementptr inbounds i64, i64* %array, i64 %index.0
%3 = load i64, i64* %2, align 8, !tbaa !1
%4 = icmp ult i64 %.01, %3
%5 = zext i1 %4 to i64
%6 = xor i64 %5, 1
%index.1 = add i64 %6, %index.0
%7 = select i1 %4, i64 0, i64 %3
%.1 = sub i64 %.01, %7
br i1 %4, label %8, label %1
; <label>:8 ; preds = %1
%.1.lcssa = phi i64 [ %.1, %1 ]
%index.1.lcssa = phi i64 [ %index.1, %1 ]
%9 = insertvalue { i64, i64 } undef, i64 %index.1.lcssa, 0
%10 = insertvalue { i64, i64 } %9, i64 %.1.lcssa, 1
ret { i64, i64 } %10
}
And using Clang 3.5 -O2 -S -c -std=c++11 -emit-llvm:
; Function Attrs: nounwind readonly uwtable
define { i64, i64 } @_Z5Test1PKmm(i64* nocapture readonly %array, i64 %pos) #3
{
%1 = load i64* %array, align 8, !tbaa !1
%2 = icmp ugt i64 %1, %pos
br i1 %2, label %._crit_edge, label %.lr.ph.preheader
.lr.ph.preheader: ; preds = %0
br label %.lr.ph
.lr.ph: ; preds = %.lr.ph.preheader,
%.lr.ph
%3 = phi i64 [ %7, %.lr.ph ], [ %1, %.lr.ph.preheader ]
%.02 = phi i64 [ %4, %.lr.ph ], [ %pos, %.lr.ph.preheader ]
%index.01 = phi i64 [ %5, %.lr.ph ], [ 0, %.lr.ph.preheader ]
%4 = sub i64 %.02, %3
%5 = add i64 %index.01, 1
%6 = getelementptr inbounds i64* %array, i64 %5
%7 = load i64* %6, align 8, !tbaa !1
%8 = icmp ult i64 %4, %7
br i1 %8, label %._crit_edge.loopexit, label %.lr.ph
._crit_edge.loopexit: ; preds = %.lr.ph
%.lcssa6 = phi i64 [ %5, %.lr.ph ]
%.lcssa = phi i64 [ %4, %.lr.ph ]
br label %._crit_edge
._crit_edge: ; preds =
%._crit_edge.loopexit, %0
%.0.lcssa = phi i64 [ %pos, %0 ], [ %.lcssa, %._crit_edge.loopexit ]
%index.0.lcssa = phi i64 [ 0, %0 ], [ %.lcssa6, %._crit_edge.loopexit ]
%9 = insertvalue { i64, i64 } undef, i64 %index.0.lcssa, 0
%10 = insertvalue { i64, i64 } %9, i64 %.0.lcssa, 1
ret { i64, i64 } %10
}
I have measured 3.7 to be 7x slower than 3.5 on my computer for this function.
This is a reduced testcase from a B+Tree library that caused the overall
implementation to be 2x slower.
I can recover the same assembly using 3.7 that I get in 3.5 by writing it this
way instead:
Ret Test2(std::size_t const *array, std::size_t pos) {
std::size_t index = 0;
auto next = array[index];
while (pos >= next) {
pos -= next;
++index;
next = array[index];
}
return {index, pos};
}
The problem still exists in trunk.</pre>
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