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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 - Unnecessary memory read in a loop"
href="https://bugs.llvm.org/show_bug.cgi?id=43962">43962</a>
</td>
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<tr>
<th>Summary</th>
<td>Unnecessary memory read in a loop
</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>Linux
</td>
</tr>
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<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>antoshkka@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org, neeilans@live.com, richard-llvm@metafoo.co.uk
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</tr></table>
<p>
<div>
<pre>Consider the example:
int* f2(int** x) {
int** max = x;
for (int i =0 ; i < 5; ++ i) {
++ x;
if (**max < **x) {
max = x;
}
}
return *max;
}
Clang with '-O2 -fno-unroll-loops' generates the following assembly:
f2(int**): # @f2(int**)
mov eax, 8
mov rcx, rdi
jmp .LBB0_1
.LBB0_3: # in Loop: Header=BB0_1 Depth=1
add rax, 8
cmp eax, 48
je .LBB0_4
.LBB0_1: # =>This Inner Loop Header: Depth=1
mov rdx, qword ptr [rcx]
mov edx, dword ptr [rdx]
mov rsi, qword ptr [rdi + rax]
cmp edx, dword ptr [rsi]
jge .LBB0_3
lea rcx, [rdi + rax]
jmp .LBB0_3
.LBB0_4:
mov rax, qword ptr [rcx]
ret
That assembly has 4 memory reads in a loop.
However other compilers generate loops with only 3 reads, for example:
f2(int**):
mov rax, QWORD PTR [rdi]
lea rcx, [rdi+40]
.L3:
mov rdx, QWORD PTR [rdi+8]
add rdi, 8
mov esi, DWORD PTR [rdx]
cmp DWORD PTR [rax], esi
cmovl rax, rdx
cmp rcx, rdi
jne .L3
ret
Please improve the memory accesses.
Godbolt playground: <a href="https://godbolt.org/z/F-dJSX">https://godbolt.org/z/F-dJSX</a></pre>
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