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<base href="https://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Missed pattern 0 modulo/div/mul N in phi"
href="https://bugs.llvm.org/show_bug.cgi?id=38479">38479</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Missed pattern 0 modulo/div/mul N in phi
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
</tr>
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<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>
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<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>unsigned int hash_function(const char *str, unsigned hash_tab_size) {
unsigned int h = 0;
const unsigned char *p;
for (p = (const unsigned char *) str; *p != '\0'; p++)
h = 65599 * h + *p;
return h % hash_tab_size;
// return h / hash_tab_size;
// return h * hash_tab_size;
// similar code generation
}
Lets say p[0] is \0 -> .LBB0_1
Latest trunk, O3:
hash_function(char const*, unsigned int): #
@hash_function(char const*, unsigned int)
mov cl, byte ptr [rdi]
test cl, cl
je .LBB0_1
add rdi, 1
xor eax, eax
.LBB0_3: # =>This Inner Loop Header: Depth=1
movzx ecx, cl
imul eax, eax, 65599
add eax, ecx
movzx ecx, byte ptr [rdi]
add rdi, 1
test cl, cl
jne .LBB0_3
xor edx, edx
div esi
mov eax, edx
ret
.LBB0_1:
xor eax, eax
xor edx, edx
div esi // useless
mov
I believe these are important lines in the IR:
%16 = phi i32 [ 0, %2 ], [ %11, %5 ]
%17 = urem i32 %16, %1
ret i32 %17
In this case we should expand/simplify phi and remove "0 urem %1"</pre>
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