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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - UBSan breaks target intrinsics taking constants: should we fold overflow intrinsics? Where?"
href="https://llvm.org/bugs/show_bug.cgi?id=23517">23517</a>
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<th>Summary</th>
<td>UBSan breaks target intrinsics taking constants: should we fold overflow intrinsics? Where?
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<th>Product</th>
<td>clang
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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>All
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<th>Status</th>
<td>NEW
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<th>Severity</th>
<td>normal
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>LLVM Codegen
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>ahmed.bougacha@gmail.com
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<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
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<th>Classification</th>
<td>Unclassified
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<p>
<div>
<pre>Consider:
#include <arm_neon.h>
int32x2_t test(int64x2_t a) {
return vqrshrn_n_s64(a, 31 - 2);
}
clang -fsanitize=undefined -target arm64-apple-darwin -O0
will generate:
%3 = call { i32, i1 } @llvm.ssub.with.overflow.i32(i32 31, i32 2)
%4 = extractvalue { i32, i1 } %3, 0
...
%vqrshrn_n1 = call <2 x i32> @llvm.aarch64.neon.sqrshrn.v2i32(<2 x i64>
%vqrshrn_n, i32 %4)
Which is not OK, and breaks various assumptions in clang and LLVM (and will
causes us to eventually crash).
With -O0, we can't expect any pass to fold these; in fact, clang itself
assumes these intrinsics *will* take Constants, so we have to deal with this
during IR gen.
I see two alternatives:
- teach ConstantFolder to deal with overflow intrinsics (messy, because it's
implemented in terms of ConstantExprs)
- teach the UBSan clang CodeGen to do the folding (which sounds wrong)</pre>
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