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<base href="https://bugs.llvm.org/">
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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 - tail duplication failed to happen when BB's pred (within the same loop) has >1 succ"
href="https://bugs.llvm.org/show_bug.cgi?id=33270">33270</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>tail duplication failed to happen when BB's pred (within the same loop) has >1 succ
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</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>enhancement
</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>danielcdh@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>For the following testcase, gcc managed to tail-duplicate the loop header to
make the loop more efficient (when the call pattern is foo-bar-foo-bar, the
gcc-generated-code saves 2 taken branches every foo-bar call pair.
The issue is that llvm will not duplicate a BB if any of BB's predecessor
(within the same loop) has multiple successors. As a result, the loop header
will not be duplicated if it contains a "continue" in the loop body.
$cat test.cc
void foo();
void bar();
bool test1();
bool test2();
void func() {
while (1) {
if (__builtin_expect(test1(), 0)) {
foo();
} else {
if (test2())
continue;
bar();
}
}
}
$clang -O2 -fno-unroll-loops -S test.cc -o - -mllvm
-tail-dup-placement-threshold=50
.text
.file "test.cc"
.globl _Z4funcv # -- Begin function _Z4funcv
.p2align 4, 0x90
.type _Z4funcv,@function
_Z4funcv: # @_Z4funcv
.cfi_startproc
# BB#0:
pushq %rax
.Lcfi0:
.cfi_def_cfa_offset 16
jmp .LBB0_1
.p2align 4, 0x90
.LBB0_4: # in Loop: Header=BB0_1 Depth=1
callq _Z3barv
.LBB0_1: # =>This Inner Loop Header: Depth=1
callq _Z5test1v
testb %al, %al
jne .LBB0_2
# BB#3: # in Loop: Header=BB0_1 Depth=1
callq _Z5test2v
testb %al, %al
jne .LBB0_1
jmp .LBB0_4
.LBB0_2: # in Loop: Header=BB0_1 Depth=1
callq _Z3foov
jmp .LBB0_1
.Lfunc_end0:
.size _Z4funcv, .Lfunc_end0-_Z4funcv
.cfi_endproc
# -- End function
.ident "clang version 5.0.0 (trunk 304371)"
.section ".note.GNU-stack","",@progbits
$g++ -O2 -fno-unroll-loops -S test.cc -o -
.file "test.cc"
.text
.p2align 4,,15
.globl _Z4funcv
.type _Z4funcv, @function
_Z4funcv:
.LFB0:
.cfi_startproc
subq $8, %rsp
.cfi_def_cfa_offset 16
.p2align 5,,20
.p2align 3
.L2:
call _Z5test1v
testb %al, %al
jne .L7
.L3:
call _Z5test2v
testb %al, %al
jne .L2
call _Z3barv
call _Z5test1v
testb %al, %al
je .L3
.L7:
call _Z3foov
jmp .L2
.cfi_endproc
.LFE0:
.size _Z4funcv, .-_Z4funcv
.ident "GCC: (GNU) 4.9.x-google 20150123 (prerelease)"
.section .note.GNU-stack,"",@progbits</pre>
</div>
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