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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] FastISel miscompiles comparison used in different block"
href="https://bugs.llvm.org/show_bug.cgi?id=51651">51651</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>[WebAssembly] FastISel miscompiles comparison used in different block
</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>
<tr>
<th>OS</th>
<td>Linux
</td>
</tr>
<tr>
<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>Backend: WebAssembly
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>nikita.ppv@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>target triple = "wasm32-unknown-unknown"
define i32 @test(i8* %p, i8* %p2) {
%v = load i8, i8* %p
%v.ext = zext i8 %v to i32
%cond = icmp eq i32 %v.ext, 0
; Cause FastISel abort.
%shl = shl i8 %v, 0
store i8 %shl, i8* %p2
br label %bb2
bb2:
br i1 %cond, label %bb3, label %bb4
bb4:
ret i32 0
bb3:
ret i32 1
}
; RUN: llc -O0
# %bb.0:
local.get 0
i32.load8_u 0
local.set 2
local.get 2
i32.eqz
drop
local.get 1
local.get 2
i32.store8 0
# %bb.1: # %bb2
block
local.get 3
i32.eqz
br_if 0 # 0: down to label0
# %bb.2: # %bb4
i32.const 0
local.set 4
local.get 4
return
.LBB0_3: # %bb3
end_block # label0:
i32.const 1
local.set 5
local.get 5
return
end_function
Note the local.get 3 reading an uninitialized register.
This is because
<a href="https://github.com/llvm/llvm-project/blob/2f69c82cec1ae05b4fdcef4ac48f48e9e2bad32b/llvm/lib/Target/WebAssembly/WebAssemblyFastISel.cpp#L422-L427">https://github.com/llvm/llvm-project/blob/2f69c82cec1ae05b4fdcef4ac48f48e9e2bad32b/llvm/lib/Target/WebAssembly/WebAssemblyFastISel.cpp#L422-L427</a>
does not check that the icmp operand is in the same block. FastISel doesn't
allow this kind of cross-block optimization.</pre>
</div>
</p>
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