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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 - WebAssembly: float-to-int intrinsics aren't const-propagated"
href="https://bugs.llvm.org/show_bug.cgi?id=46982">46982</a>
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<th>Summary</th>
<td>WebAssembly: float-to-int intrinsics aren't const-propagated
</td>
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<th>Product</th>
<td>libraries
</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>Windows NT
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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: WebAssembly
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>alex@crichton.co
</td>
</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>In the Rust compiler we've been updating the codegen for WebAssembly targets on
float-to-int casts recently. <a href="https://github.com/rust-lang/rust/issues/73591">https://github.com/rust-lang/rust/issues/73591</a> has
a high-level description of the problem, but the basic idea is that
rust-as-the-language defines our `as` operator as safe and saturating. This
causes quite a lot of codegen if we have our own branching and then LLVM has
its own set of branching for the `fptosi` and `fptoui` instructions. Put
another way, LLVM already emits quite a lot of instructions for `fptosi` and
`fptoui`, and we want to avoid that in codegen.
We recently switched to using the `llvm.wasm.trunc.signed.i32.f32` family of
intrinsics for implementing safe float-to-int casts. (this is when the
`+nontrapping-fptoint` argument is not provided, the default). In doing so,
though, we realized that LLVM won't optimize programs quite as much any more,
since the intrinsic is more opaque than the `fptosi` instruction itself. For
example this program:
define i32 @foo() {
start:
%0 = tail call i32 @llvm.wasm.trunc.signed.i32.f32(float 5.000000e+00)
ret i32 %0
}
declare i32 @llvm.wasm.trunc.signed.i32.f32(float)
won't get optimized any further, and the final wasm code has `i32.trunc_f32_s`,
generating this assembly:
foo: # @foo
f32.const 0x1.4p2
i32.trunc_f32_s
end_function
Would it be possible to teach the optimizer how to constant evaluate the wasm
float-to-int intrinsics? I'm not entirely sure how else `fptosi` interacts with
the optimizer but this is at least one case we're running into.</pre>
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