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<base href="http://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 --- - the clang 3.5 loop optimizer seems to jump in unintentional for simple loops"
href="http://llvm.org/bugs/show_bug.cgi?id=20409">20409</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>the clang 3.5 loop optimizer seems to jump in unintentional for simple loops
</td>
</tr>
<tr>
<th>Product</th>
<td>new-bugs
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</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>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>new bugs
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>dl.soluz@gmx.net
</td>
</tr>
<tr>
<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>----
const int SIZE = 3;
int the_func(int* p_array)
{
int dummy = 0;
#if defined(ITER)
for(int* p = &p_array[0]; p < &p_array[SIZE]; ++p) dummy += *p;
#else
for(int i = 0; i < SIZE; ++i) dummy += p_array[i];
#endif
return dummy;
}
int main(int argc, char** argv)
{
int* array = new int[SIZE];
for(int i = 0; i < SIZE; ++i){ array[i] = *argv[i]; }
int dummy = the_func(array);
delete[] array;
return dummy;
}
----
compiled with gcc 4.9.1 and clang 3.5
with clang3.5 + #define ITER the_func contains masses of code
the code in main is also sometimes different to an inline the_func
(the loop vectorizer seems to be more active in the_func then main)
clang -DITER -O2
clang -DITER -O3
gives:
the_func:
leaq 12(%rdi), %rcx
leaq 4(%rdi), %rax
cmpq %rax, %rcx
cmovaq %rcx, %rax
movq %rdi, %rsi
notq %rsi
addq %rax, %rsi
shrq $2, %rsi
incq %rsi
xorl %edx, %edx
movabsq $9223372036854775800, %rax # imm = 0x7FFFFFFFFFFFFFF8
andq %rsi, %rax
pxor %xmm0, %xmm0
je .LBB0_1
# BB#2: # %vector.body.preheader
leaq (%rdi,%rax,4), %r8
addq $16, %rdi
movq %rsi, %rdx
andq $-8, %rdx
pxor %xmm0, %xmm0
pxor %xmm1, %xmm1
.align 16, 0x90
.LBB0_3: # %vector.body
# =>This Inner Loop Header: Depth=1
movdqa %xmm1, %xmm2
movdqa %xmm0, %xmm3
movdqu -16(%rdi), %xmm0
movdqu (%rdi), %xmm1
paddd %xmm3, %xmm0
paddd %xmm2, %xmm1
addq $32, %rdi
addq $-8, %rdx
jne .LBB0_3
# BB#4:
movq %r8, %rdi
movq %rax, %rdx
jmp .LBB0_5
.LBB0_1:
pxor %xmm1, %xmm1
.LBB0_5: # %middle.block
paddd %xmm1, %xmm0
movdqa %xmm0, %xmm1
movhlps %xmm1, %xmm1 # xmm1 = xmm1[1,1]
paddd %xmm0, %xmm1
pshufd $1, %xmm1, %xmm0 # xmm0 = xmm1[1,0,0,0]
paddd %xmm1, %xmm0
movd %xmm0, %eax
cmpq %rdx, %rsi
je .LBB0_7
.align 16, 0x90
.LBB0_6: # %scalar.ph
# =>This Inner Loop Header: Depth=1
addl (%rdi), %eax
addq $4, %rdi
cmpq %rcx, %rdi
jb .LBB0_6
.LBB0_7: # %._crit_edge
retq
isn't that a little bit too long?
other better looking results:
clang -O2
clang -O3
gcc -O3
gcc -DITER -O3
gives:
the_func:
movl 4(%rdi), %eax
addl (%rdi), %eax
addl 8(%rdi), %eax
ret(q)
looks good
gcc -DITER -O2
gives:
the_func:
leaq 12(%rdi), %rdx
xorl %eax, %eax
.L2:
addl (%rdi), %eax
addq $4, %rdi
cmpq %rdx, %rdi
jne .L2
rep ret
looks good
gcc4.9.1 seems to be more "stable" in its optimization for the_func and main</pre>
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