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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 signed integer demotions"
href="https://bugs.llvm.org/show_bug.cgi?id=44102">44102</a>
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<th>Summary</th>
<td>Missed canonicalizations for losslessness checks of signed integer demotions
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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>Linux
</td>
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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
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<th>Component</th>
<td>Scalar Optimizations
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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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</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
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<pre>Much like <a class="bz_bug_link
bz_status_NEW "
title="NEW - Missed canonicalizations for losslessness checks of unsigned integer demotions"
href="show_bug.cgi?id=44100">https://bugs.llvm.org/show_bug.cgi?id=44100</a>
we have a missed canonicalization for signed pattern:
Given
signed short t4(signed short x) {
x+=1;
return x;
}
signed short t5(signed short x) {
x-=1;
return x;
}
With -fsanitize=implicit-conversion we produce:
%2 = sext i16 %0 to i32, !dbg !15
%5 = add nsw i32 %2, 32769, !dbg !15
%6 = icmp ult i32 %5, 65536, !dbg !15
and
%2 = sext i16 %0 to i32, !dbg !22
%5 = add nsw i32 %2, 32767, !dbg !22
%6 = icmp ult i32 %5, 65536, !dbg !22
<a href="https://godbolt.org/z/eyc6qN">https://godbolt.org/z/eyc6qN</a>
But in these cases we can simply check the original %0:
Name: sext-signed-trunc-check - increment
Pre: C2 == 65536 && C1 == (C2/2)+C3 && C3 > 0 && C3 u<= C2
%ZZ_IGNOREME = C3
%conv = sext i16 %x to i32
%zz = add i32 %conv, C1
%cmp = icmp ult i32 %zz, C2
=>
%ZZ_IGNOREME = C3
%cmp = icmp slt i16 %x, (trunc (C2/2) - trunc (C3))
Name: sext-signed-trunc-check - decrement
Pre: C2 == 65536 && C1 == (C2/2)+C3 && C3 < 0 && C3 > -C2
%ZZ_IGNOREME = C3
%conv = sext i16 %x to i32
%zz = add i32 %conv, C1
%cmp = icmp ult i32 %zz, C2
=>
%ZZ_IGNOREME = C3
%cmp = icmp sge i16 %x, (-(trunc (C2/2)) - (trunc (C3)))
<a href="https://rise4fun.com/Alive/GC57">https://rise4fun.com/Alive/GC57</a></pre>
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