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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 - PHI/Aggregate merging"
href="https://bugs.llvm.org/show_bug.cgi?id=47060">47060</a>
</td>
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<tr>
<th>Summary</th>
<td>PHI/Aggregate merging
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>Linux
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>Scalar Optimizations
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>lebedev.ri@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>I'm seeing roughly the following as remnants of clang C++ exception handling
on unwind branch:
<a href="https://godbolt.org/z/hxMTs6">https://godbolt.org/z/hxMTs6</a>
declare void @something0()
declare void @something1()
declare void @something2()
define {i8*, i32} @square({i8*, i32} %arg0, {i8*, i32} %arg1, {i8*, i32} %arg2,
i1 %cond) {
entry:
br i1 %cond, label %v0, label %v1
v0:
%i121 = extractvalue { i8*, i32 } %arg0, 0
%i122 = extractvalue { i8*, i32 } %arg0, 1
call void @something0()
br label %end
v1:
%i1212 = extractvalue { i8*, i32 } %arg1, 0
%i1222 = extractvalue { i8*, i32 } %arg1, 1
call void @something1()
br label %end
end:
%exn.slot.4 = phi i8* [ %i121, %v0 ], [ %i1212, %v1 ]
%ehselector.slot.4 = phi i32 [ %i122, %v0 ], [ %i1222, %v1 ]
call void @something2()
%lpad.val = insertvalue { i8*, i32 } undef, i8* %exn.slot.4, 0
%lpad.val302 = insertvalue { i8*, i32 } %lpad.val, i32 %ehselector.slot.4, 1
ret {i8*, i32} %lpad.val302
}
All that is redundant and should roughly just be:
<a href="https://godbolt.org/z/a6q1Ea">https://godbolt.org/z/a6q1Ea</a>
declare void @something0()
declare void @something1()
declare void @something2()
define {i8*, i32} @square({i8*, i32} %arg0, {i8*, i32} %arg1, {i8*, i32} %arg2,
i1 %cond) {
entry:
br i1 %cond, label %v0, label %v1
v0:
call void @something0()
br label %end
v1:
call void @something1()
br label %end
end:
%r = phi {i8*, i32} [ %arg0, %v0 ], [ %arg1, %v1 ]
call void @something2()
ret {i8*, i32} %r
}
If that happens, the resume block of an exception will be recognized as being
empty,
and invoke will be simplified into a call. (not depicted in examples,
see <a href="http://reviews.llvm.org/rGe492f0e03b01a5e4ec4b6333abb02d303c3e479e">http://reviews.llvm.org/rGe492f0e03b01a5e4ec4b6333abb02d303c3e479e</a>)
I guess, this is a missing fold in instcombine.
Question is: *what* fold is missing? How lax are we okay with it being?
The most simple solution is to look for 'insert of phi', where each incoming
value is an 'extract',
where types of the destination and source match and the lanes match, and just
merge all those extracts
into the block with insert, by forming a PHI for the sources of extracts and
extracting from said new PHI.
This will be rather simple do to.
If that happens for all new inserts, we already know how to fold the rest away:
<a href="https://godbolt.org/z/jcY1zo">https://godbolt.org/z/jcY1zo</a>
More complicated (but more proper) solution would be to start looking from the
final insert,
and ensure that we can just replace chain of inserts with the PHI of extract
sources.
Do we want to go with the former or latter approach?</pre>
</div>
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