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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - 128 bit division generates __udivti3 and __umodti3 instead of calling __udivmodti4 once"
href="https://bugs.llvm.org/show_bug.cgi?id=47006">47006</a>
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<th>Summary</th>
<td>128 bit division generates __udivti3 and __umodti3 instead of calling __udivmodti4 once
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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>Linux
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>enhancement
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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>kutdanila@yandex.ru
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<th>CC</th>
<td>craig.topper@gmail.com, llvm-bugs@lists.llvm.org, llvm-dev@redking.me.uk, spatel+llvm@rotateright.com
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<p>
<div>
<pre>128 bit division generates __udivti3 and __umodti3 instead of calling
__udivmodti4 once
This happens because of DivRemPairs pass and lack of instrumentation in the
backend.
; Unsigned 128-bit division
define i128 @udiv128(i128 %a, i128 %b) {
%quot = udiv i128 %a, %b
%rem = urem i128 %a, %b
%sum = add i128 %quot, %rem
ret i128 %sum
}
=>
<a href="https://gcc.godbolt.org/z/PorhMz">https://gcc.godbolt.org/z/PorhMz</a>
Will call __udivti3 on LP64 but libgcc and compiler-rt have __udivmodti4 which
computes the quotient and the remainder at the same time. This particular hurts
x86 as divq instruction is presented. Other backends can also benefit from this
too</pre>
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