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<base href="https://llvm.org/bugs/" />
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<body><table border="1" cellspacing="0" cellpadding="8">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Lost opportunity to fold op into phi"
href="https://llvm.org/bugs/show_bug.cgi?id=29039">29039</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Lost opportunity to fold op into phi
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>All
</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>Scalar Optimizations
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>wmi@google.com
</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>-------- testcase 1.c -----------
int hoo(int p);
int goo(int p) {
if (__builtin_expect(p, 1)) return 1;
return hoo(p);
}
long t;
volatile long cond;
void foo(int p) {
while (cond) {
t += goo(p);
if (cond > t)
t += 2;
}
xoo();
}
----------------------------------
llvm generated code for the while loop:
~/workarea/llvm-r278149/dbuild/bin/clang -O2 -S 1.c
.LBB1_1: # %entry
# =>This Inner Loop Header: Depth=1
cmpq $0, cond(%rip)
je .LBB1_6
# BB#2: # %while.body
# in Loop: Header=BB1_1 Depth=1
movl $1, %eax
testl %ebx, %ebx
jne .LBB1_4
# BB#3: # %if.end.i
# in Loop: Header=BB1_1 Depth=1
xorl %edi, %edi
callq hoo
.LBB1_4: # %goo.exit
# in Loop: Header=BB1_1 Depth=1
cltq ====> The cltq can be hoisted to BB#3.
addq t(%rip), %rax
movq %rax, t(%rip)
movq cond(%rip), %rcx
cmpq %rax, %rcx
jle .LBB1_1
It is better to generate cltq immediately behind "callq hoo" since the def of
eax in BB#2 is $1 and cltq is of no use.
This could have been done by InstCombiner::FoldOpIntoPhi. However, because
r221187 is too conservative, any such folding opportunity in a loop is lost.
I am thinking about how to replace r221187 with something less restricted. An
idea is to only fold op from more frequently executed BB to the less, so no
infinite combine will be introduced. However, we don't have BFI in instcombine.
I am not sure whether it is intentional. Maybe the job of instcombine is to do
canonicalization so it doesn't want to depend on BFI?</pre>
</div>
</p>
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