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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 - LLVM misses loadpre opportunities"
href="https://bugs.llvm.org/show_bug.cgi?id=33234">33234</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>LLVM misses loadpre opportunities
</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>
<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>danielcdh@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>#cat t.cc
void foo(int *p, int t, int *a, int *b, int *c) {
for (int i = 0; i < t; i++) {
if (a[i]) {
*p += b[i];
} else {
*p *= c[i];
}
}
}
#clang -O2 -S t.cc -fno-unroll-loops
testl %esi, %esi
jle .LBB0_6
# BB#1:
movl %esi, %eax
.p2align 4, 0x90
.LBB0_2: # =>This Inner Loop Header: Depth=1
cmpl $0, (%rdx)
je .LBB0_4
# BB#3: # in Loop: Header=BB0_2 Depth=1
movl (%rdi), %esi
addl (%rcx), %esi
jmp .LBB0_5
.p2align 4, 0x90
.LBB0_4: # in Loop: Header=BB0_2 Depth=1
movl (%rdi), %esi
imull (%r8), %esi
.LBB0_5: # in Loop: Header=BB0_2 Depth=1
movl %esi, (%rdi)
addq $4, %rdx
addq $4, %rcx
addq $4, %r8
decq %rax
jne .LBB0_2
.LBB0_6:
retq
#gcc -O2 -S t.cc -fno-unroll-loops
testl %esi, %esi
jle .L1
movl (%rdi), %r9d
xorl %eax, %eax
jmp .L5
.p2align 4,,10
.p2align 3
.L9:
addl (%rcx,%rax,4), %r9d
addq $1, %rax
movl %r9d, (%rdi)
cmpl %eax, %esi
jle .L1
.L5:
movl (%rdx,%rax,4), %r10d
testl %r10d, %r10d
jne .L9
imull (%r8,%rax,4), %r9d
addq $1, %rax
movl %r9d, (%rdi)
cmpl %eax, %esi
jg .L5
.L1:
rep
ret
For the gcc version, there are only 2 loads + 1 store each iteration. For llvm
version, there are 3 loads + 1 store each iteration.
The problem is that llvm failed to move the load of *p outside of the loop.</pre>
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