[llvm-bugs] [Bug 34775] New: Different unrolling behavior with manual hoisting
via llvm-bugs
llvm-bugs at lists.llvm.org
Thu Sep 28 22:25:40 PDT 2017
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: unassignedbugs at nondot.org
Reporter: piotr.padlewski at gmail.com
CC: llvm-bugs at 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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