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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 --- - Failing to hoist all loads out of a loop"
href="https://llvm.org/bugs/show_bug.cgi?id=30692">30692</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Failing to hoist all loads out of a loop
</td>
</tr>
<tr>
<th>Product</th>
<td>tools
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</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>opt
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>charles_li@playstation.sony.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>Hi Everyone,
Found a optimization issue.
Here is the test case.
$ cat t.cpp
/*************************/
int aaa = 10;
int bbb = 20;
int ccc = 30;
void foo(const int *pData)
{
while (1)
{
int x = *pData++;
if (x > ccc)
bbb += x;
if (x > bbb)
aaa += x;
if (x > aaa)
ccc += x;
if (x < 0)
break;
}
}
/*************************/
Compile this test with optimization.
$ clang t.cpp -S -O2
Here is the assembly.
$ cat t.s
.text
.file "t.cpp"
.globl _Z3fooPKi
.p2align 4, 0x90
.type _Z3fooPKi,@function
_Z3fooPKi: # @_Z3fooPKi
.cfi_startproc
# BB#0: # %entry
movl ccc(%rip), %eax
.p2align 4, 0x90
.LBB0_1: # %while.body
# =>This Inner Loop Header: Depth=1
movl (%rdi), %ecx
movl bbb(%rip), %edx
cmpl %eax, %ecx
jle .LBB0_3
# BB#2: # %if.then
# in Loop: Header=BB0_1 Depth=1
addl %ecx, %edx
movl %edx, bbb(%rip)
.LBB0_3: # %if.end
# in Loop: Header=BB0_1 Depth=1
movl aaa(%rip), %esi
cmpl %edx, %ecx
jle .LBB0_5
# BB#4: # %if.then2
# in Loop: Header=BB0_1 Depth=1
addl %ecx, %esi
movl %esi, aaa(%rip)
.LBB0_5: # %if.end4
# in Loop: Header=BB0_1 Depth=1
cmpl %esi, %ecx
jle .LBB0_7
# BB#6: # %if.then6
# in Loop: Header=BB0_1 Depth=1
addl %ecx, %eax
movl %eax, ccc(%rip)
.LBB0_7: # %if.end8
# in Loop: Header=BB0_1 Depth=1
addq $4, %rdi
testl %ecx, %ecx
jns .LBB0_1
# BB#8: # %while.end
retq
.Lfunc_end0:
.size _Z3fooPKi, .Lfunc_end0-_Z3fooPKi
.cfi_endproc
.type aaa,@object # @aaa
.data
.globl aaa
.p2align 2
aaa:
.long 10 # 0xa
.size aaa, 4
.type bbb,@object # @bbb
.globl bbb
.p2align 2
bbb:
.long 20 # 0x14
.size bbb, 4
.type ccc,@object # @ccc
.globl ccc
.p2align 2
ccc:
.long 30 # 0x1e
.size ccc, 4
Looking at the location of these 3 instructions:
movl ccc(%rip), %eax
movl bbb(%rip), %edx
movl aaa(%rip), %esi
we can see that,
only the load of “ccc” got hoisted into the %entry block,
while the load of “bbb” and “aaa” are still inside the %while.body.</pre>
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