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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - [AssumeBundles] ValueTracking cannot use alignment assumptions"
href="https://bugs.llvm.org/show_bug.cgi?id=48224">48224</a>
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<th>Summary</th>
<td>[AssumeBundles] ValueTracking cannot use alignment assumptions
</td>
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<th>Product</th>
<td>libraries
</td>
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<th>Version</th>
<td>trunk
</td>
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<th>Hardware</th>
<td>All
</td>
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<th>OS</th>
<td>All
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>enhancement
</td>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Global Analyses
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>nikolai.bozhenov@intel.com
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
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<p>
<div>
<pre>This is a regression caused by switch to using operand bundles to encode
alignment assumptions
(<a href="https://reviews.llvm.org/rG78de7297abe2e8fa782682168989c70e3cb34a5c">https://reviews.llvm.org/rG78de7297abe2e8fa782682168989c70e3cb34a5c</a>).
It looks like the ValueTracking (computeKnownBits) cannot understand the new
format of alignment assumptions. As an example, consider compilation of the
following reproducer with clang-11 (old @llvm.assume format) and clang-trunk
(@llvm.assume with bundles):
#include <stdint.h>
int foo(int *p) {
__builtin_assume_aligned(p, 32);
return ((intptr_t) p) & 4;
}
clang-11 (<a href="https://godbolt.org/z/xGh1e4">https://godbolt.org/z/xGh1e4</a>) computes the returned value statically:
ret i32 0
clang-trunk (<a href="https://godbolt.org/z/1jWe5j">https://godbolt.org/z/1jWe5j</a>) fails to optimize the code:
%3 = ptrtoint i32* %0 to i64
%4 = trunc i64 %3 to i32
%5 = and i32 %4, 4
ret i32 %5
As one can see, after switching to operand bundles, clang-trunk lost its
ability to reason about low bits in aligned pointers.</pre>
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