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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - [X86] Failure to merge ISD::SUB(x,y) and X86ISD::SUB(x.y)"
href="https://bugs.llvm.org/show_bug.cgi?id=40483">40483</a>
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<th>Summary</th>
<td>[X86] Failure to merge ISD::SUB(x,y) and X86ISD::SUB(x.y)
</td>
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<th>Product</th>
<td>libraries
</td>
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<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Windows NT
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Severity</th>
<td>enhancement
</td>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Backend: X86
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>llvm-dev@redking.me.uk
</td>
</tr>
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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>We sometimes fail to recognise that ISD::SUB(x,y) and X86ISD::SUB(x,y) can be
represented by the X86ISD::SUB(x,y).
<a href="https://godbolt.org/z/-RFU9J">https://godbolt.org/z/-RFU9J</a>
#include <x86intrin.h>
using u64 = unsigned long long;
u64 test_sub1(u64 *p, u64 x) {
u64 r = *p - x;
_subborrow_u64(0, *p, x, p);
return *p - r; // zero
}
u64 test_sub2(u64 *p, u64 x) {
u64 r = *p - x;
_subborrow_u64(0, *p, x, p);
return r - *p; // zero
}
_Z9test_sub1Pyy:
movq (%rdi), %rcx
movq %rcx, %rax
subq %rsi, %rax
movq %rax, (%rdi)
subq %rcx, %rsi
addq %rsi, %rax
retq
_Z9test_sub2Pyy:
subq %rsi, (%rdi)
xorl %eax, %eax
retq</pre>
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