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<base href="http://llvm.org/bugs/" />
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<body><table border="1" cellspacing="0" cellpadding="8">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - StackColoring can wrongly merge stack slots due to incorrect liveness calculation"
href="http://llvm.org/bugs/show_bug.cgi?id=15707">15707</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>StackColoring can wrongly merge stack slots due to incorrect liveness calculation
</td>
</tr>
<tr>
<th>Product</th>
<td>tools
</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>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>llc
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>mseaborn@chromium.org
</td>
</tr>
<tr>
<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>StackColoring::calculateLiveIntervals() does not correctly handle the case
where the following sequence occurs in a basic block:
llvm.lifetime.start(%buf)
...
llvm.lifetime.end(%buf)
...
llvm.lifetime.start(%buf)
calculateLiveIntervals() ignores the second lifetime.start() and regards %buf
as not being live after the lifetime.end(). This will cause StackColoring to
merge stack slots that shouldn't be merged.
Example:
declare void @llvm.lifetime.start(i64, i8*)
declare void @llvm.lifetime.end(i64, i8*)
declare void @ext(i8*)
define void @foo() {
%buf1 = alloca i8, i32 10000, align 16
%buf2 = alloca i8, i32 10000, align 16
call void @llvm.lifetime.start(i64 -1, i8* %buf1)
call void @llvm.lifetime.end(i64 -1, i8* %buf1)
call void @llvm.lifetime.start(i64 -1, i8* %buf1)
call void @llvm.lifetime.start(i64 -1, i8* %buf2)
call void @ext(i8* %buf1)
call void @ext(i8* %buf2)
ret void
}
Expected output:
$ llc -mtriple=x86_64-none-gnu llvm_lifetime_bug.ll -O1 -o -
...
subq $20008, %rsp # imm = 0x4E28
.Ltmp1:
.cfi_def_cfa_offset 20016
leaq 10000(%rsp), %rdi
callq ext
leaq (%rsp), %rdi
callq ext
addq $20008, %rsp # imm = 0x4E28
ret
Actual output:
$ llc -mtriple=x86_64-none-gnu llvm_lifetime_bug.ll -O1 -o -
...
subq $10008, %rsp # imm = 0x2718
.Ltmp1:
.cfi_def_cfa_offset 10016
leaq (%rsp), %rdi
callq ext
leaq (%rsp), %rdi
callq ext
addq $10008, %rsp # imm = 0x2718
ret
The following change fixes the bug:
@@ -434,10 +446,8 @@ void StackColoring::calculateLiveIntervals(unsigned
NumSlots) {
if (Alive.any()) {
for (int pos = Alive.find_first(); pos != -1;
pos = Alive.find_next(pos)) {
- if (!Starts[pos].isValid())
- Starts[pos] = Indexes->getMBBStartIdx(MBB);
- if (!Finishes[pos].isValid())
- Finishes[pos] = Indexes->getMBBEndIdx(MBB);
+ Starts[pos] = Indexes->getMBBStartIdx(MBB);
+ Finishes[pos] = Indexes->getMBBEndIdx(MBB);
}
}</pre>
</div>
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