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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 - InstCombine incorrectly optimizes bit mask operations"
href="https://bugs.llvm.org/show_bug.cgi?id=51732">51732</a>
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</tr>
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<th>Summary</th>
<td>InstCombine incorrectly optimizes bit mask operations
</td>
</tr>
<tr>
<th>Product</th>
<td>new-bugs
</td>
</tr>
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<th>Version</th>
<td>10.0
</td>
</tr>
<tr>
<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>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>new bugs
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>pfcittolin@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>htmldeveloper@gmail.com, llvm-bugs@lists.llvm.org
</td>
</tr></table>
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<pre>Hello everyone,
I noticed a certain pattern of incorrectly optimizing bit mask operations on
integers, specifically at bits located at the end of "byte chunks" (7, 15,
31...).
For example, giving the following
define dso_local i32 @main() {
%rlo.1 = alloca i1
store i1 1, i1* %rlo.1
%1 = load i64, i64* @byte ; @byte = dso_local global i64 30
%2 = shl i64 1, 30
%3 = and i64 %1, %2
%4 = icmp ne i64 %3, 0
%5 = load i1, i1* %rlo.1
%6 = and i1 %4, %5
store i1 %6, i1* %rlo.1
%7 = load i1, i1* %rlo.1
%8 = call i32 (i8*, ...) @printf(i8* getelementptr inbounds ([4 x i8], [4 x
i8]* @.str.newline, i64 0, i64 0), i1 %7)
ret i32 0
}
It optimizes to:
define dso_local i32 @main() local_unnamed_addr #0 {
%1 = load i64, i64* @byte, align 8
%2 = and i64 %1, 1073741824
%3 = icmp ne i64 %2, 0
%4 = tail call i32 (i8*, ...) @printf(i8* nonnull dereferenceable(1)
getelementptr inbounds ([4 x i8], [4 x i8]* @.str.newline, i64 0, i64 0), i1
%3)
ret i32 0
}
However, when bitmasking bit 31, for example:
define dso_local i32 @main() {
%rlo.1 = alloca i1
store i1 1, i1* %rlo.1
%1 = load i64, i64* @byte
%2 = shl i64 1, 31
%3 = and i64 %1, %2
%4 = icmp ne i64 %3, 0
%5 = load i1, i1* %rlo.1
%6 = and i1 %4, %5
store i1 %6, i1* %rlo.1
%7 = load i1, i1* %rlo.1
%8 = call i32 (i8*, ...) @printf(i8* getelementptr inbounds ([4 x i8], [4 x
i8]* @.str.newline, i64 0, i64 0), i1 %7)
ret i32 0
}
It gives me:
define dso_local i32 @main() local_unnamed_addr #0 {
%1 = load i64, i64* @byte, align 8
%2 = trunc i64 %1 to i32
%3 = icmp slt i32 %2, 0
%4 = tail call i32 (i8*, ...) @printf(i8* nonnull dereferenceable(1)
getelementptr inbounds ([4 x i8], [4 x i8]* @.str.newline, i64 0, i64 0), i1
%3)
ret i32 0
}
This holds for any of those bits mentioned before:
%2 = shl i64 1, 7
%3 = and i64 %1, %2
%4 = icmp ne i64 %3, 0
Optimizes to:
%1 = load i64, i64* @byte, align 8
%2 = trunc i64 %1 to i8
%3 = icmp slt i8 %2, 0
And so on...
Using "-print-before-all -print-after-all" on "opt -O3" I narrowed it down to
the following part (here on an another equivalent example):
*** IR Dump Before Combine redundant instructions ***
define dso_local i32 @main() local_unnamed_addr {
%1 = load i32, i32* @byte
%2 = and i32 %1, 32768
%3 = icmp ne i32 %2, 0
%4 = call i32 (i8*, ...) @printf(i8* getelementptr inbounds ([4 x i8], [4 x
i8]* @.str.newline, i64 0, i64 0), i1 %3)
ret i32 0
}
*** IR Dump After Combine redundant instructions ***
define dso_local i32 @main() local_unnamed_addr {
%1 = load i32, i32* @byte, align 4
%2 = trunc i32 %1 to i16
%3 = icmp slt i16 %2, 0
%4 = call i32 (i8*, ...) @printf(i8* nonnull dereferenceable(1) getelementptr
inbounds ([4 x i8], [4 x i8]* @.str.newline, i64 0, i64 0), i1 %3)
ret i32 0
}</pre>
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