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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 - [X86] Instruction reordering prevents EFLAGS folding"
href="https://bugs.llvm.org/show_bug.cgi?id=33362">33362</a>
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<th>Summary</th>
<td>[X86] Instruction reordering prevents EFLAGS folding
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<th>Product</th>
<td>libraries
</td>
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<th>Version</th>
<td>trunk
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Windows NT
</td>
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<th>Status</th>
<td>NEW
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<th>Severity</th>
<td>enhancement
</td>
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>Backend: X86
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>llvm-dev@redking.me.uk
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<th>CC</th>
<td>andrea.dibiagio@gmail.com, davide@freebsd.org, filcab@gmail.com, llvm-bugs@lists.llvm.org, spatel+llvm@rotateright.com
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<pre><a href="https://godbolt.org/g/l1T1da">https://godbolt.org/g/l1T1da</a>
Swapping 2 instructions over prevents the folding of an EFLAGS result,
requiring an additional comparison to regenerate them. Ideally we'd recognise
which instructions are actually using the EFLAGS result:
define i32 @clz_i128(i64, i64) {
%3 = tail call i64 @llvm.ctlz.i64(i64 %1, i1 false)
%4 = tail call i64 @llvm.ctlz.i64(i64 %0, i1 false)
%5 = icmp ne i64 %0, 0
%6 = select i1 %5, i64 0, i64 %3
%7 = add nuw nsw i64 %6, %4
%8 = trunc i64 %7 to i32
ret i32 %8
}
define i32 @clz_i128_swap(i64, i64) {
%3 = tail call i64 @llvm.ctlz.i64(i64 %0, i1 false) <-- SWAP
%4 = tail call i64 @llvm.ctlz.i64(i64 %1, i1 false) <-- SWAP
%5 = icmp ne i64 %0, 0
%6 = select i1 %5, i64 0, i64 %4
%7 = add nuw nsw i64 %6, %3
%8 = trunc i64 %7 to i32
ret i32 %8
}
declare i64 @llvm.ctlz.i64(i64, i1)
clz_i128:
lzcntq %rsi, %rcx
xorl %edx, %edx
lzcntq %rdi, %rax
cmovael %edx, %ecx
addl %ecx, %eax
retq
clz_i128_swap:
lzcntq %rdi, %rax
lzcntq %rsi, %rcx
xorl %edx, %edx
testq %rdi, %rdi <-- EXTRA TEST
cmovnel %edx, %ecx
addl %ecx, %eax
retq</pre>
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