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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - [AArch64][GlobalSel] Incorrect half to single precision code gen"
href="https://bugs.llvm.org/show_bug.cgi?id=36106">36106</a>
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<th>Summary</th>
<td>[AArch64][GlobalSel] Incorrect half to single precision code gen
</td>
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<th>Product</th>
<td>new-bugs
</td>
</tr>
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<th>Version</th>
<td>trunk
</td>
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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>
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<th>Severity</th>
<td>normal
</td>
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>new bugs
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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</tr>
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<th>Reporter</th>
<td>carey.williams@arm.com
</td>
</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
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<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=19756" name="attach_19756" title="Generated ASM">attachment 19756</a> <a href="attachment.cgi?id=19756&action=edit" title="Generated ASM">[details]</a></span>
Generated ASM
I believe there is a bug in the GlobalISel -0O code generation that affects
AArch64's half-precision (FP16) mechanics.
The generated code tries to convert a float value from single precision to
half-precision via FCVT. The problem is that in this case the immediate value
it is operating on is actually already stored/encoded in the half-precision
format.
Minimal reproducer:
int main() {
_Float16 b = 1024.0f16;
return (float)b;
}
Compiled with: clang -S -O0 -mllvm -aarch64-enable-global-isel-at-O=0
--target=aarch64-arm-none-eabi -march=armv8.2-a+fp16
Passing -mllvm -aarch64-enable-global-isel-at-O=-1 instead will generate
correctly functioning code.
---
I've attached the problematic generation with some comments.</pre>
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