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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Ashr does not alter sign bit - could look past ashr, backend fails to undo transform"
href="https://bugs.llvm.org/show_bug.cgi?id=44025">44025</a>
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<th>Summary</th>
<td>Ashr does not alter sign bit - could look past ashr, backend fails to undo transform
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<th>Product</th>
<td>libraries
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<th>Version</th>
<td>trunk
</td>
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<th>Hardware</th>
<td>PC
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<th>OS</th>
<td>Linux
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<th>Status</th>
<td>NEW
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<th>Severity</th>
<td>enhancement
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>Backend: X86
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>lebedev.ri@gmail.com
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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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<pre>I'm not sure how relevant this is, but i noticed this while looking at some
code.
We could have something like:
#include <algorithm>
struct S {
int a, b;
};
unsigned long zz(S s) {
return std::abs(static_cast<signed long>(s.a)); // or s.b
}
Currently it ends as
define dso_local i64 @_Z2zz1S(i64 %0) local_unnamed_addr #0 {
%2 = shl i64 %0, 32
%3 = ashr exact i64 %2, 32
%4 = sub nsw i64 0, %3
%5 = icmp slt i64 %3, 0
%6 = select i1 %5, i64 %4, i64 %3
ret i64 %6
}
movsxd rcx, edi
mov rax, rcx
neg rax
cmovl rax, rcx
But we are checking sign bit of %3, which is ashr, and it preserves sign bit.
We could look past it: <a href="https://rise4fun.com/Alive/iANsl">https://rise4fun.com/Alive/iANsl</a>
But that exposes a weakness in backend part - it doesn't know that it could
undo such transform, and not perform the test:
movsxd rcx, edi
shl rdi, 32
mov rax, rcx
neg rax
test rdi, rdi
cmovns rax, rcx
<a href="https://godbolt.org/z/Hr4ceT">https://godbolt.org/z/Hr4ceT</a></pre>
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