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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 - Missing optimization: SUBCARRY (SBB) not used"
href="https://bugs.llvm.org/show_bug.cgi?id=49225">49225</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Missing optimization: SUBCARRY (SBB) not used
</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>Linux
</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>
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<th>Component</th>
<td>Common Code Generator Code
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>chfast@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
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<pre>This reports an issue where a sequence of additions with carry propagation are
nicely optimized to ADDCARRY nodes / ADC instruction. While the symmetric code
for subtraction is not optimized to SUBCARRY / SBB.
The reason may be that in the DAG combiner the combineCarryDiamond() and/or the
combineADDCARRYDiamond() is not properly specified for SUB case. Or
canonicalization causes the SUB case not being recognized (you can see that the
last SUB in sequence is converted to ADD).
<a href="https://godbolt.org/z/7cr1o9">https://godbolt.org/z/7cr1o9</a>
using uint64_t = unsigned long;
bool uaddo(uint64_t al, uint64_t ah, uint64_t bl, uint64_t bh, uint64_t* ol,
uint64_t* oh) noexcept
{
uint64_t l = al + bl;
bool k0 = l < al;
uint64_t t = ah + bh;
bool k1 = t < ah;
uint64_t h = t + k0;
bool k2 = h < t;
bool k = k1 || k2;
*ol = l;
*oh = h;
return k;
}
bool usubo(uint64_t al, uint64_t ah, uint64_t bl, uint64_t bh, uint64_t* ol,
uint64_t* oh) noexcept
{
uint64_t l = al - bl;
bool k0 = l > al;
uint64_t t = ah - bh;
bool k1 = t > ah;
uint64_t h = t - k0;
bool k2 = h > t;
bool k = k1 || k2;
*ol = l;
*oh = h;
return k;
}
define dso_local zeroext i1 @_Z5uaddommmmPmS_(i64 %0, i64 %1, i64 %2, i64 %3,
i64* nocapture %4, i64* nocapture %5) local_unnamed_addr #0 {
%7 = add i64 %2, %0
%8 = icmp ult i64 %7, %0
%9 = add i64 %3, %1
%10 = icmp ult i64 %9, %1
%11 = zext i1 %8 to i64
%12 = add i64 %9, %11
%13 = icmp ult i64 %12, %9
%14 = or i1 %10, %13
store i64 %7, i64* %4, align 8, !tbaa !2
store i64 %12, i64* %5, align 8, !tbaa !2
ret i1 %14
}
define dso_local zeroext i1 @_Z5usubommmmPmS_(i64 %0, i64 %1, i64 %2, i64 %3,
i64* nocapture %4, i64* nocapture %5) local_unnamed_addr #0 {
%7 = sub i64 %0, %2
%8 = icmp ult i64 %0, %2
%9 = sub i64 %1, %3
%10 = icmp ult i64 %1, %3
%11 = sext i1 %8 to i64
%12 = add i64 %9, %11
%13 = icmp ugt i64 %12, %9
%14 = or i1 %10, %13
store i64 %7, i64* %4, align 8, !tbaa !2
store i64 %12, i64* %5, align 8, !tbaa !2
ret i1 %14
}
attributes #0 = { nofree norecurse nounwind uwtable willreturn writeonly
mustprogress "disable-tail-calls"="false" "frame-pointer"="none"
"less-precise-fpmad"="false" "min-legal-vector-width"="0"
"no-infs-fp-math"="false" "no-jump-tables"="false" "no-nans-fp-math"="false"
"no-signed-zeros-fp-math"="false" "no-trapping-math"="true"
"stack-protector-buffer-size"="8" "target-cpu"="x86-64"
"target-features"="+cx8,+fxsr,+mmx,+sse,+sse2,+x87" "tune-cpu"="generic"
"unsafe-fp-math"="false" "use-soft-float"="false" }
uaddo(unsigned long, unsigned long, unsigned long, unsigned long, unsigned
long*, unsigned long*): # @uaddo(unsigned long, unsigned
long, unsigned long, unsigned long, unsigned long*, unsigned long*)
add rdi, rdx
adc rsi, rcx
setb al
mov qword ptr [r8], rdi
mov qword ptr [r9], rsi
ret
usubo(unsigned long, unsigned long, unsigned long, unsigned long, unsigned
long*, unsigned long*): # @usubo(unsigned long, unsigned
long, unsigned long, unsigned long, unsigned long*, unsigned long*)
sub rsi, rcx
setb cl
sub rdi, rdx
mov rdx, rsi
sbb rdx, 0
cmp rdx, rsi
seta al
or al, cl
mov qword ptr [r8], rdi
mov qword ptr [r9], rdx
ret</pre>
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