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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 - Missed optimization: inefficient codegen for __builtin_addc"
href="https://bugs.llvm.org/show_bug.cgi?id=36243">36243</a>
</td>
</tr>
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<th>Summary</th>
<td>Missed optimization: inefficient codegen for __builtin_addc
</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>
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<th>OS</th>
<td>All
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>Scalar Optimizations
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>koriakin@0x04.net
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>clang has __builtin_addc* functions, which are supposed to emit hardware
add-with-carry instructions. However, there is no corresponding intrinsic on
LLVM side, so clang emits a sequence of instructions that is only recognized
and folded to a single hw instruction in two cases:
- carry input is 0, or
- carry output is unused
This means that any carry chains longer than 2 result in inefficient code:
void add3(
unsigned long long *restrict a,
unsigned long long *restrict b,
unsigned long long *restrict c
) {
unsigned long long cf = 0;
c[0] = __builtin_addcll(a[0], b[0], cf, &cf);
c[1] = __builtin_addcll(a[1], b[1], cf, &cf);
c[2] = __builtin_addcll(a[2], b[2], cf, &cf);
}
Compiles to:
add3: # @add3
.cfi_startproc
# BB#0:
movq (%rdi), %rax
movq (%rsi), %r8
leaq (%rax,%r8), %rcx
movq %rcx, (%rdx)
movq 8(%rdi), %rcx
addq 8(%rsi), %rcx
setb %r9b
addq %r8, %rax
adcq $0, %rcx
setb %al
orb %r9b, %al
movzbl %al, %eax
movq %rcx, 8(%rdx)
movq 16(%rsi), %rcx
addq 16(%rdi), %rcx
addq %rax, %rcx
movq %rcx, 16(%rdx)
retq
I suppose we're going to need a new target-independent generic intrinsic,
say { iX, i1 } @llvm.uadd.with.overflow.carry.iX(iX, iX, i1) (and a
corresponding one for subtraction as well) and map it to ISD::ADDE /
ISD::ADDCARRY.</pre>
</div>
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