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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 - Inneficient code without constexpr"
href="https://bugs.llvm.org/show_bug.cgi?id=35712">35712</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Inneficient code without constexpr
</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>All
</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>zamazan4ik@tut.by
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>clang(trunk) with '-O3 -std=c++17' for this code:
unsigned test()
{
int arr[] = {0,1,2,3,4,5,6,7,8,9};
int sum = 0;
for(int i = 0;i < 10;++i)
{
for(int j = 0; j < 10; ++j)
{
int t = arr[i];
arr[i] = arr[j];
arr[j] = t;
}
}
for(int i = 0; i < 10; ++i)
{
sum += arr[i];
}
return sum;
}
generates this:
test(): # @test()
mov rax, qword ptr [rip + .L_ZZ4testvE3arr+32]
mov qword ptr [rsp - 8], rax
movaps xmm0, xmmword ptr [rip + .L_ZZ4testvE3arr+16]
movaps xmmword ptr [rsp - 24], xmm0
movaps xmm0, xmmword ptr [rip + .L_ZZ4testvE3arr]
movaps xmmword ptr [rsp - 40], xmm0
xor ecx, ecx
mov rax, -9
xor edx, edx
jmp .LBB0_1
.LBB0_2: # in Loop: Header=BB0_1 Depth=1
mov edx, dword ptr [rsp + 4*rax]
mov ecx, dword ptr [rsp - 40]
add rax, 1
.LBB0_1: # =>This Inner Loop Header: Depth=1
mov dword ptr [rsp + 4*rax - 4], ecx
mov dword ptr [rsp - 40], edx
mov ecx, dword ptr [rsp + 4*rax - 4]
mov edx, dword ptr [rsp - 36]
mov dword ptr [rsp + 4*rax - 4], edx
mov dword ptr [rsp - 36], ecx
mov ecx, dword ptr [rsp + 4*rax - 4]
mov edx, dword ptr [rsp - 32]
mov dword ptr [rsp + 4*rax - 4], edx
mov dword ptr [rsp - 32], ecx
mov ecx, dword ptr [rsp + 4*rax - 4]
mov edx, dword ptr [rsp - 28]
mov dword ptr [rsp + 4*rax - 4], edx
mov dword ptr [rsp - 28], ecx
mov ecx, dword ptr [rsp + 4*rax - 4]
mov edx, dword ptr [rsp - 24]
mov dword ptr [rsp + 4*rax - 4], edx
mov dword ptr [rsp - 24], ecx
mov ecx, dword ptr [rsp + 4*rax - 4]
mov edx, dword ptr [rsp - 20]
mov dword ptr [rsp + 4*rax - 4], edx
mov dword ptr [rsp - 20], ecx
mov ecx, dword ptr [rsp + 4*rax - 4]
mov edx, dword ptr [rsp - 16]
mov dword ptr [rsp + 4*rax - 4], edx
mov dword ptr [rsp - 16], ecx
mov ecx, dword ptr [rsp + 4*rax - 4]
mov edx, dword ptr [rsp - 12]
mov dword ptr [rsp + 4*rax - 4], edx
mov dword ptr [rsp - 12], ecx
mov ecx, dword ptr [rsp + 4*rax - 4]
mov edx, dword ptr [rsp - 8]
mov dword ptr [rsp + 4*rax - 4], edx
mov dword ptr [rsp - 8], ecx
mov ecx, dword ptr [rsp + 4*rax - 4]
mov edx, dword ptr [rsp - 4]
mov dword ptr [rsp + 4*rax - 4], edx
mov dword ptr [rsp - 4], ecx
test rax, rax
jne .LBB0_2
movdqu xmm0, xmmword ptr [rsp - 20]
movdqu xmm1, xmmword ptr [rsp - 36]
paddd xmm1, xmm0
pshufd xmm0, xmm1, 78 # xmm0 = xmm1[2,3,0,1]
paddd xmm0, xmm1
pshufd xmm1, xmm0, 229 # xmm1 = xmm0[1,1,2,3]
paddd xmm1, xmm0
movd eax, xmm1
add eax, dword ptr [rsp - 40]
add eax, ecx
ret
.L_ZZ4testvE3arr:
.long 0 # 0x0
.long 1 # 0x1
.long 2 # 0x2
.long 3 # 0x3
.long 4 # 0x4
.long 5 # 0x5
.long 6 # 0x6
.long 7 # 0x7
.long 8 # 0x8
.long 9 # 0x9
But as you see we can simply return just sum of digits from the array.</pre>
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