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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 - Regression caused by CodeGenPrepare patch rL354298"
href="https://bugs.llvm.org/show_bug.cgi?id=41129">41129</a>
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</tr>
<tr>
<th>Summary</th>
<td>Regression caused by CodeGenPrepare patch rL354298
</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>All
</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>
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<th>Component</th>
<td>Scalar Optimizations
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>carrot@google.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>Compile following code with
opt -codegenprepare -mtriple=x86_64-unknown-linux-gnu -S test3.ll
define void @foo(i64* %p64) {
entry:
%key = load i64, i64* %p64, align 8
%cond17 = icmp eq i64 %key, 0
br i1 %cond17, label %true, label %false
false:
%andval = and i64 %key, 7
store i64 %andval, i64* %p64
br label %exit
true:
%svalue = add i64 %key, -1
store i64 %svalue, i64* %p64
br label %exit
exit:
ret void
}
Then generate code for x86, then I can get
foo: # @foo
.cfi_startproc
# %bb.0: # %entry
movq (%rdi), %rax
movq %rax, %rcx // Extra copy
subq $1, %rcx
jae .LBB0_1
# %bb.2: # %true
movq %rcx, (%rdi)
retq
.LBB0_1: # %false
andl $7, %eax
movq %rax, (%rdi)
retq
.Lfunc_end0:
.size foo, .Lfunc_end0-foo
An extra copy is generated in the entry block. It is needed for a 2-address ISA
like x86, and the operand has other uses. In this case, the extra COPY is
inserted in BB entry, a SUB is eliminated in BB true, BB entry is executed more
times than BB true, so it is always a lose.</pre>
</div>
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