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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 - Suboptimal 128 bit integer division in loop"
href="https://bugs.llvm.org/show_bug.cgi?id=38717">38717</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Suboptimal 128 bit integer division in loop
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</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>Scalar Optimizations
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>david.bolvansky@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>int main(void)
{
const __int128 m = 3329589384618324948;
__int128 n = m * m;
enum { base = 10, max_width = 39 };
char a[max_width + 1] = { '\0' };
char *p = a + max_width;
for (; n != 0; n /= base)
*--p = (char)('0' + n % base);
printf("%s\n", p);
}
Clang trunk with O3:
.LBB0_1:
mov edx, 10
xor ecx, ecx
mov rdi, r15
mov rsi, r14
call __divti3
mov rbx, rax
mov r12, rdx
lea eax, [rax + rax]
lea eax, [rax + 4*rax]
mov ecx, r15d
sub ecx, eax
add cl, 48
mov byte ptr [r13], cl
mov edx, 10
xor ecx, ecx
mov rdi, rbx
mov rsi, r12
call __modti3
add al, 48
mov byte ptr [r13 - 1], al
mov edx, 100
xor ecx, ecx
mov rdi, r15
mov rsi, r14
call __divti3 <---- elide it like GCC did?
mov r15, rax
mov r14, rdx
add rbx, 9
adc r12, 0
add r13, -2
cmp rbp, rbx
mov eax, 0
sbb rax, r12
jb .LBB0_1
lea rdi, [rsp + 1]
call puts
xor eax, eax
add rsp, 40
pop rbx
pop r12
pop r13
pop r14
pop r15
pop rbp
ret
GCC:
.L2:
mov rbx, rdx
mov rdi, r13
xor ecx, ecx
mov edx, 10
mov rsi, rbx
sub rbp, 1
call __modti3
mov rdi, r13
mov edx, 10
xor ecx, ecx
add eax, 48
mov rsi, rbx
mov BYTE PTR [rbp+0], al
call __divti3
mov r13, rax
cmp rbp, r12
jne .L2
mov rdi, rbp
call puts
add rsp, 56
xor eax, eax
pop rbx
pop rbp
pop r12
pop r13
ret</pre>
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