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<base href="https://llvm.org/bugs/" />
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<body><table border="1" cellspacing="0" cellpadding="8">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - carry detection missed optimization in chained add-with-carry"
href="https://llvm.org/bugs/show_bug.cgi?id=31755">31755</a>
</td>
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<tr>
<th>Summary</th>
<td>carry detection missed optimization in chained add-with-carry
</td>
</tr>
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<th>Product</th>
<td>new-bugs
</td>
</tr>
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<th>Version</th>
<td>3.9
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>Linux
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Severity</th>
<td>enhancement
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>new bugs
</td>
</tr>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>vincent-llvm@vinc17.net
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>A carry can be detected so that an adcq instruction can be generated on x86_64.
But this does not work when the carry-out of this second addition is used for a
third addition. I have tested only on x86_64 (see below), but other similar
targets by be affected too.
Example:
typedef unsigned long T;
void f (T, T, T);
void add0 (T a, T b, T c, T w)
{
T u = a, v = b;
T carry1;
a = u + w;
carry1 = a < u;
b = v + carry1;
f (a, b, c);
}
void add1 (T a, T b, T c, T w)
{
T u = a, v = b;
T carry1, carry2;
a = u + w;
carry1 = a < u;
b = v + carry1;
carry2 = b < v;
c += carry2;
f (a, b, c);
}
Under Debian/unstable with Clang 3.9.1, using -O3, I get:
add0: # @add0
.cfi_startproc
# BB#0:
addq %rcx, %rdi
adcq $0, %rsi
jmp f # TAILCALL
Here, one can see that an adcq instruction is generated for the second
addition.
add1: # @add1
.cfi_startproc
# BB#0:
addq %rcx, %rdi
sbbq %rax, %rax
andl $1, %eax
addq %rax, %rsi
adcq $0, %rdx
jmp f # TAILCALL
The adcq instruction is generated for the third addition, but not for the
second one. The sbbq / andl / addq sequence could have been replaced by a
single adcq.</pre>
</div>
</p>
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