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<base href="https://bugs.llvm.org/">
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<body><table border="1" cellspacing="0" cellpadding="8">
<tr>
<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - SCCP generates wrong code for ssa_copy"
href="https://bugs.llvm.org/show_bug.cgi?id=39814">39814</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>SCCP generates wrong code for ssa_copy
</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>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 test.ll -S -ipsccp
define void @foo(i64*, i64) {
entry:
%address = getelementptr inbounds i64, i64* %0, i64 2
%cond3 = icmp eq i64 %1, 10
br i1 %cond3, label %loop_header, label %exit
loop_header:
%v23 = phi i64 [ %ivar, %loop_last ], [ 0, %entry ]
%v25 = getelementptr inbounds i64, i64* %0, i64 %v23
%v26 = load i64, i64* %v25, align 4
%v27 = and i64 %v26, 1879048192
%cond2 = icmp eq i64 %v27, 1610612736
br i1 %cond2, label %loop_2, label %exit
loop_2:
%ivar = add nuw nsw i64 %v23, 1
%cond1 = icmp eq i64 %v26, %ivar
br i1 %cond1, label %dont_delete, label %exit
dont_delete:
br label %loop_last
loop_last:
%loop_cond = icmp ult i64 %ivar, 10
br i1 %loop_cond, label %loop_header, label %loop_exit
loop_exit:
store i64 %v27, i64* %address, align 8
br label %exit
exit:
ret void
}
I got:
define void @foo(i64*, i64) {
entry:
%address = getelementptr inbounds i64, i64* %0, i64 2
%cond3 = icmp eq i64 %1, 10
br i1 %cond3, label %loop_header, label %exit
loop_header: ; preds = %loop_last, %entry
%v23 = phi i64 [ 1, %loop_last ], [ 0, %entry ] // WRONG!
%v25 = getelementptr inbounds i64, i64* %0, i64 %v23
%v26 = load i64, i64* %v25, align 4
%v27 = and i64 %v26, 1879048192
%cond2 = icmp eq i64 %v27, 1610612736
br i1 %cond2, label %loop_2, label %exit
loop_2: ; preds = %loop_header
%ivar = add nuw nsw i64 %v23, 1
%cond1 = icmp eq i64 %v26, %ivar
br i1 %cond1, label %dont_delete, label %exit
dont_delete: ; preds = %loop_2
br label %loop_last
loop_last: ; preds = %dont_delete
%loop_cond = icmp ult i64 %ivar, 10
br i1 %loop_cond, label %loop_header, label %loop_exit
loop_exit: ; preds = %loop_last
store i64 1610612736, i64* %address, align 8
br label %exit
exit: ; preds = %loop_exit,
%loop_2, %loop_header, %entry
ret void
}
Note that
%v23 = phi i64 [ 1, %loop_last ], [ 0, %entry ]
the value comes from BB %loop_last should not be constant 1.
The problem is, when ssa_copy is inserted, we have
%ivar = add nuw nsw i64 %v23, 1
...
%ivar.0 = call i64 @llvm.ssa.copy.93825120544720(i64 %ivar)
...
%ivar.0.1 = call i64 @llvm.ssa.copy.93825120544720(i64 %ivar.0)
When llvm is processing the last instruction in function
SCCPSolver::visitCallSite, it uses constant information from the first
instruction.
return (void)mergeInValue(IV, I, getValueState(PBranch->OriginalOp));
At this time we have:
(gdb) p I->dump()
%ivar.0.1 = call i64 @llvm.ssa.copy.93825120544720(i64 %ivar.0)
$6 = void
(gdb) p CopyOf->dump()
%ivar.0 = call i64 @llvm.ssa.copy.93825120544720(i64 %ivar)
(gdb) p PBranch->OriginalOp->dump()
%ivar = add nuw nsw i64 %v23, 1
Basically the last instruction is a copy of %ivar.0, which is a copy of %ivar,
so which instruction is used to get constant information doesn't matter too
much, so current implementation uses the first instruction. But the problem is
the last instruction is not a direct user of the first instruction, so when the
constant information of %ivar is updated, the last instruction isn't updated
accordingly. So the correct fix should tell SCCP that I is a user of
PBranch->OriginalOp:
addAdditionalUser(PBranch->OriginalOp, I);
On the other hand, when computing the constant information of the second
instruction, it combines guarded condition information, so in theory it may
have more precise information than first instruction. So I think a better fix
is:
return (void)mergeInValue(IV, I, OriginalVal);</pre>
</div>
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