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<base href="https://bugs.llvm.org/">
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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 - __builtin_ia32_addcarry_u64 produces better code then __builtin_addcll."
href="https://bugs.llvm.org/show_bug.cgi?id=39464">39464</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>__builtin_ia32_addcarry_u64 produces better code then __builtin_addcll.
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>Windows NT
</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>Backend: X86
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>ilya.lesokhin@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>craig.topper@gmail.com, llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>When __builtin_ia32_addcarry_u64 is used the compiler is able to take
advantage of the ADC command to do carry propagation.
But it is unable to do so with __builtin_addcll.
<a href="https://godbolt.org/z/UQTOJX">https://godbolt.org/z/UQTOJX</a>
compiling
#define N 3
auto add_ia32 (const std::array<uint64_t, N> &a, const std::array<uint64_t, N>
&b)
{
uint64_t carry = 0;
std::array<uint64_t, N> r;
for (int i = 0; i < N; ++i) {
carry = __builtin_ia32_addcarry_u64(carry, a[i], b[i], (unsigned long
long *)&r[i]);
}
return r;
}
results in:
movq %rdi, %rax
movq (%rsi), %rcx
xorl %edi, %edi
addb $-1, %dil
adcq (%rdx), %rcx
movq %rcx, (%rax)
movq 8(%rsi), %rcx
adcq 8(%rdx), %rcx
movq %rcx, 8(%rax)
movq 16(%rsi), %rcx
adcq 16(%rdx), %rcx
movq %rcx, 16(%rax)
retq
while
#define N 3
auto add_builtin (const std::array<uint64_t, N> &a, const std::array<uint64_t,
N> &b)
{
unsigned long long carry = 0;
std::array<uint64_t, N> r;
for (int i = 0; i < N; ++i) {
r[i] = __builtin_addcll(a[i], b[i], carry, &carry);
}
return r;
}
results in:
movq %rdi, %rax
movq (%rsi), %rcx
movq (%rdx), %r8
leaq (%rcx,%r8), %rdi
movq %rdi, (%rax)
movq 8(%rsi), %rdi
addq 8(%rdx), %rdi
setb %r9b
addq %r8, %rcx
adcq $0, %rdi
setb %cl
orb %r9b, %cl
movzbl %cl, %ecx
movq %rdi, 8(%rax)
movq 16(%rsi), %rsi
addq 16(%rdx), %rsi
addq %rcx, %rsi
movq %rsi, 16(%rax)</pre>
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