https://bugs.llvm.org/show_bug.cgi?id=34775
Bug ID: 34775
Summary: Different unrolling behavior with manual hoisting
Product: tools
Version: 5.0
Hardware: PC
OS: All
Status: NEW
Severity: enhancement
Priority: P
Component: opt
Assignee: unassignedb...@nondot.org
Reporter: piotr.padlew...@gmail.com
CC: llvm-bugs@lists.llvm.org
Hey,
I am not sure if this is a bug or it is by design, but I recently found out
that
clang unrolls these two loops in different way:
#include <vector>
void bar(std::vector<int> &v, int* p) {
auto size = v.size();
for (int i = 0 ; i < size; i++)
p[i] = v[i];
}
void bar2(std::vector<int> &v, int* p) {
for (int i = 0 ; i < v.size(); i++)
p[i] = v[i];
}
https://godbolt.org/g/U1kH8P
I would probably guess that LICM hoists the size after the unrolling, but I
haven't checked that and also have no idea which one is faster. It is just
weird to me that we were able to hoist v.size() in the bar2, but did not
perform the same unrolling.
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