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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Missed canonicalizations for losslessness checks of unsigned integer demotions"
href="https://bugs.llvm.org/show_bug.cgi?id=44100">44100</a>
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<th>Summary</th>
<td>Missed canonicalizations for losslessness checks of unsigned integer demotions
</td>
</tr>
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<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>Linux
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Severity</th>
<td>enhancement
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Scalar Optimizations
</td>
</tr>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>lebedev.ri@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
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<p>
<div>
<pre>Given
unsigned short t0(unsigned short x) {
x+=1;
return x;
}
unsigned short t1(unsigned short x) {
x-=1;
return x;
}
With -fsanitize=implicit-conversion we produce:
%3 = add nuw nsw i32 %2, 1, !dbg !15
%4 = trunc i32 %3 to i16, !dbg !15
%5 = icmp ult i32 %3, 65536, !dbg !15
and
%2 = zext i16 %0 to i32, !dbg !22
%3 = add nsw i32 %2, -1, !dbg !22
%4 = trunc i32 %3 to i16, !dbg !22
%5 = icmp ult i32 %3, 65536, !dbg !22
<a href="https://godbolt.org/z/Us33sA">https://godbolt.org/z/Us33sA</a>
But in these cases we can simply check the original %2:
Name: unsigned truncation check - increment
Pre: C2 u>= C1
%add = add nuw i32 %conv, C1 ; has extra uses
%cmp = icmp ult i32 %add, C2 ; is truncation NUW?
=>
%cmp = icmp ult i32 %conv, (C2-C1)
Name: unsigned truncation check - decrement
Pre: C1 <= 0 ; C1 is non-positive
%conv = zext i16 %x to i32
%add = add i32 %conv, C1 ; has extra uses
%cmp = icmp ult i32 %add, 65536 ; is truncation NUW?
=>
%cmp = icmp uge i32 %conv, -C1
<a href="https://rise4fun.com/Alive/x35G">https://rise4fun.com/Alive/x35G</a></pre>
</div>
</p>
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