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<base href="http://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - LLVM pessimizes simple array comparison loop"
href="http://llvm.org/bugs/show_bug.cgi?id=19710">19710</a>
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</tr>
<tr>
<th>Summary</th>
<td>LLVM pessimizes simple array comparison loop
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>All
</td>
</tr>
<tr>
<th>OS</th>
<td>All
</td>
</tr>
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<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>Loop Optimizer
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>st@quanttec.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
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<div>
<pre>The current trunk version of clang/LLVM pessimizes the loop in the following
example (by unnecessarily introducing a new loop induction variable):
bool arrayLess(const int* p1, long length1,
const int* p2, long length2)
{
const long minLength = length1 <= length2 ? length1 : length2;
if (minLength > 0) {
const int* const end = p1 + minLength;
do {
if (*p1 != *p2) {
return *p1 < *p2;
}
++p1;
++p2;
} while (p1 != end);
}
return length1 < length2;
}
clang++ -O3 -S test.cpp yields the following assembly:
_Z9arrayLessPKilS0_l: # @_Z9arrayLessPKilS0_l
.cfi_startproc
# BB#0: # %entry
cmpq %rcx, %rsi
movq %rcx, %rax
cmovleq %rsi, %rax
testq %rax, %rax
jle .LBB0_4
# BB#1: # %if.then
movq %rcx, %rax
notq %rax
movq %rsi, %r8
notq %r8
cmpq %r8, %rax
cmovgeq %rax, %r8
shlq $2, %r8
movq $-4, %r9
subq %r8, %r9
.align 16, 0x90
.LBB0_2: # %do.body
# =>This Inner Loop Header: Depth=1
movl (%rdx), %eax
cmpl %eax, (%rdi)
jne .LBB0_5
# BB#3: # %if.end
# in Loop: Header=BB0_2 Depth=1
addq $4, %rdi
addq $4, %rdx
addq $-4, %r9
jne .LBB0_2
.LBB0_4: # %if.end7
cmpq %rcx, %rsi
.LBB0_5: # %return
setl %al
retq
.Ltmp0:
.size _Z9arrayLessPKilS0_l, .Ltmp0-_Z9arrayLessPKilS0_l
.cfi_endproc
For comparison, GCC generates the following assembly:
_Z9arrayLessPKilS0_l:
.LFB0:
.cfi_startproc
cmpq %rcx, %rsi
movq %rcx, %rax
cmovle %rsi, %rax
testq %rax, %rax
jle .L2
leaq (%rdi,%rax,4), %r9
jmp .L5
.p2align 4,,10
.p2align 3
.L3:
addq $4, %rdi
addq $4, %rdx
cmpq %r9, %rdi
je .L2
.L5:
movl (%rdi), %eax
movl (%rdx), %r8d
cmpl %r8d, %eax
je .L3
cmpl %eax, %r8d
setg %al
ret
.p2align 4,,10
.p2align 3
.L2:
cmpq %rcx, %rsi
setl %al
ret
.cfi_endproc</pre>
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