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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - ThinLTO missing optimization on Globals"
href="https://bugs.llvm.org/show_bug.cgi?id=32752">32752</a>
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<th>Summary</th>
<td>ThinLTO missing optimization on Globals
</td>
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<th>Product</th>
<td>new-bugs
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<th>Version</th>
<td>unspecified
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>All
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<th>Status</th>
<td>NEW
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<th>Severity</th>
<td>enhancement
</td>
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>new bugs
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>mehdi.amini@apple.com
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
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<p>
<div>
<pre>This examples shows some current limitations of ThinLTO with globals:
// globalopt0.c
int g0 = 0, g1 = 22, g2;
extern void foo1(void);
int main() {
g0 = 1;
g1 = 22;
g2 = 1;
foo1();
return g0 + g1;
}
// globalopt1.c
extern int g0, g1, g2;
void __attribute__((noinline)) foo1(void) {
g0 = 0;
g1 = 22;
g2 = 4;
}
Build with:
$ clang globalopt0.c -c -flto=thin -O3
$ clang globalopt1.c -c -flto=thin -O3
$ clang globalopt0.o globalopt1.o -O3 -o globalopt -Wl,-save-temps
Some potential optimizations:
- g0’s type can be changed to i1 because only two values (0 or 1) are stored to
it.
- g1 should be optimized away because only one value (22) is stored.
- g2 should be optimized away because it’s store-only.</pre>
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