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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 - Missed optimization to remove unnecessary branch from loop entry"
href="https://bugs.llvm.org/show_bug.cgi?id=50171">50171</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Missed optimization to remove unnecessary branch from loop entry
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>unspecified
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>Windows NT
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>C++
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>scovich@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>blitzrakete@gmail.com, dgregor@apple.com, erik.pilkington@gmail.com, llvm-bugs@lists.llvm.org, richard-llvm@metafoo.co.uk
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<p>
<div>
<pre>The following toy example:
void Loop(int len) {
int i = 0;
const int kUnrollFactor = 8;
for (int num_calls = 0; i <= len - kUnrollFactor; ) {
if (num_calls + kUnrollFactor > 100) {
extern void Foo(); Foo();
num_calls = 0;
}
for (int j = 0; j < kUnrollFactor; j++, i++, num_calls++) {
extern void Bar(int); Bar(i);
}
}
}
Compiles to the following x86 assembly code with clang-9:
Loop(int):
... prolog ...
mov r14d, edi
add r14d, -8
js .LBB0_5
xor r15d, r15d <=== num_calls = 0
xor ebx, ebx
cmp r15d, 93 <=== num_calls still zero here
jge .LBB0_3 <=== branch can NEVER be taken
.LBB0_4:
... unrolled loop body with 8 calls to Bar() ...
add r15d, 8
add ebx, 8
cmp ebp, r14d
jge .LBB0_5
cmp r15d, 93
jl .LBB0_4
.LBB0_3:
call Foo()
xor r15d, r15d
jmp .LBB0_4
.LBB0_5:
... epilogue ...
ret
Ideally, the compiler should elide the provably redundant cmp+jge pair, leaving
only the xor:
xor r15d, r15d
cmp r15d, 93
jge .LBB0_3
With clang-12, the result is arguably worse, because the extra branching masks
the missed opportunity altogether:
Loop(int):
... prologue ...
cmp edi, 8
jge .LBB0_1
.LBB0_5:
... epilogue ...
ret
.LBB0_1:
... loop initialization ...
xor r15d, r15d <=== num_calls = 0
xor ebx, ebx
jmp .LBB0_2
.LBB0_4:
... unrolled loop body with 8 calls to Bar() ...
add r15d, 8
add ebx, 8
cmp ebp, r14d
jge .LBB0_5
.LBB0_2:
cmp r15d, 93 <=== num_calls = 0 the first time
jl .LBB0_4 <=== branch never taken the first time
call Foo()
xor r15d, r15d
jmp .LBB0_4</pre>
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