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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 - branches containing only llvm.assume are not fully eliminated"
href="https://bugs.llvm.org/show_bug.cgi?id=49403">49403</a>
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<th>Summary</th>
<td>branches containing only llvm.assume are not fully eliminated
</td>
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<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
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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>
</tr>
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<th>Severity</th>
<td>enhancement
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Common Code Generator Code
</td>
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<th>Assignee</th>
<td>jannh@google.com
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<th>Reporter</th>
<td>jannh@google.com
</td>
</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
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<p>
<div>
<pre>[patch in progress]
When a branch contains only a call to `llvm.assume`, the generated machine code
still contains the condition of that branch, even though the branch itself and
its body are gone.
The issue seems to be that the call to `llvm.assume` is only removed very late,
in the "CodeGen Prepare (codegenprepare)" pass; the resulting empty branch
survives the move into MIR and only disappears in the MIR pass "Control Flow
Optimizer (branch-folder)".
Reproducer:
```
define dso_local void @a(i64 %addr, i64 %addr2) {
entry:
%cmp = icmp ne i64 %addr, %addr2
%cmp1 = icmp eq i64 %addr, 0
br i1 %cmp1, label %do_assume, label %after_assume
do_assume:
tail call void @llvm.assume(i1 %cmp)
br label %after_assume
after_assume:
br i1 %cmp, label %end, label %call_b
call_b:
tail call void @b(i64 %addr)
br label %end
end:
ret void
}
declare void @llvm.assume(i1 noundef)
declare void @b(i64)
```
Resulting X86 assembly:
```
# %bb.0: # %entry
testq %rdi, %rdi
cmpq %rsi, %rdi
je .LBB0_2
# %bb.1: # %end
retq
.LBB0_2: # %call_b
jmp b@PLT # TAILCALL
```
If you remove the call to @llvm.assume from the testcase, the entire branch
including its condition disappears in the codegenprepare pass.
So the issue is probably that after CodeGenPrepare::optimizeCallInst() may have
killed the call instruction, we don't try to eliminate the block with
CodeGenPrepare::eliminateMostlyEmptyBlocks()? And this is the case because
CodeGenPrepare::runOnFunction() first calls eliminateMostlyEmptyBlocks(), then
runs the loop that calls optimizeBlock().
I'll try to send a patch for this in a bit.</pre>
</div>
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