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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - [aarch64] Redundant masks in downcast long multiply"
href="https://bugs.llvm.org/show_bug.cgi?id=47883">47883</a>
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<th>Summary</th>
<td>[aarch64] Redundant masks in downcast long multiply
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<th>Product</th>
<td>libraries
</td>
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<th>Version</th>
<td>10.0
</td>
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<th>Hardware</th>
<td>Other
</td>
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<th>OS</th>
<td>Linux
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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
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<th>Component</th>
<td>Backend: AArch64
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>husseydevin@gmail.com
</td>
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<th>CC</th>
<td>arnaud.degrandmaison@arm.com, llvm-bugs@lists.llvm.org, smithp352@googlemail.com, Ties.Stuij@arm.com
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<p>
<div>
<pre>All 64-bit backends appear to do this.
However, I only think this is important on aarch64; x86_64, RISC-V64, MIPS64,
and PPC64 don't appear to have similar instructions that are any more efficient
than the current method, so I am filing this under aarch64.
Code:
#include <stdint.h>
uint64_t umull(uint64_t x0, uint64_t x1) {
// Downcast + upcast works too
return (x0 & 0xffffffff) * (x1 & 0xffffffff);
}
int64_t smull(int64_t x0, int64_t x1) {
return (int64_t)(int32_t)x0 * (int64_t)(int32_t)x1;
}
Expected assembly (and what GCC 9.3.0 emits):
umull:
umull x0, w0, w1
ret
smull:
smull x0, w0, w1
ret
Clang 10.0.1:
umull:
and x8, x0, #0xffffffff
and x9, x1, #0xffffffff
mul x0, x8, x9
ret
smull:
sxtw x8, w0
sxtw x9, w1
mul x0, x8, x9
ret
Note that if the parameters are 32-bit integers, the expected code is emitted.
However, LLVM always turns this:
%2 = trunc i64 %0 to i32
%3 = zext i32 %2 to i64
to this:
%2 = and i64 %0, 0xffffffff
so this does not work.</pre>
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