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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 - [ppc] Highly predictable ISEL should be changed to cmp/br"
href="https://bugs.llvm.org/show_bug.cgi?id=32971">32971</a>
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<th>Summary</th>
<td>[ppc] Highly predictable ISEL should be changed to cmp/br
</td>
</tr>
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<th>Product</th>
<td>libraries
</td>
</tr>
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<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>Linux
</td>
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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>Backend: PowerPC
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
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<th>Reporter</th>
<td>carrot@google.com
</td>
</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
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<pre>On power, use ISEL or cmp/br is a general hard problem. But if we can point out
the branch is highly predictable (either through runtime profiling or static
estimate), we should use cmp/br.
Following is a test case simplified from real world code.
#include <vector>
typedef std::vector<int> MyVec;
int foo(int iters, MyVec* vec) {
MyVec::const_iterator it = vec->begin();
for (int i = 0; i < iters; ++i, ++it) {
if (it == vec->end()) {
it = vec->begin();
}
if (*it >= iters)
return 1;
}
return 0;
}
The iterator "it" is incremented in each iteration, and it is compared to the
same value vec->end(), so we can expect the condition is false for most times,
and then it is true, and then false again ...
So this branch is highly predictable. In our code, cmp/br is 2x faster than
isel.
LLVM generates following code, which uses ISEL
# BB#0: # %entry
cmpwi 3, 1
blt 0, .LBB0_4
# BB#1: # %for.body.lr.ph
addi 6, 3, -1
ld 5, 0(4)
ld 4, 8(4)
clrldi 6, 6, 32
addi 6, 6, 1
mr 7, 5
mtctr 6
li 6, 0
.p2align 5
.LBB0_2: # %for.body
# =>This Inner Loop Header: Depth=1
cmpld 4, 7
isel 7, 5, 7, 2
lwz 8, 0(7)
cmpw 8, 3
bge 0, .LBB0_5
# BB#3: # %for.inc
# in Loop: Header=BB0_2 Depth=1
addi 6, 6, 1
addi 7, 7, 4
bdnz .LBB0_2
.LBB0_4:
li 3, 0
clrldi 3, 3, 32
blr
.LBB0_5:
li 3, 1
clrldi 3, 3, 32
blr</pre>
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