https://bugs.llvm.org/show_bug.cgi?id=42897
Bug ID: 42897
Summary: LLVM optimizer eliminates zero check if return
statement is missing
Product: new-bugs
Version: 8.0
Hardware: PC
OS: Windows NT
Status: NEW
Severity: normal
Priority: P
Component: new bugs
Assignee: unassignedb...@nondot.org
Reporter: timo.str...@emmtrix.com
CC: htmldevelo...@gmail.com, llvm-bugs@lists.llvm.org
Created attachment 22344
--> https://bugs.llvm.org/attachment.cgi?id=22344&action=edit
C++ file to reproduce bug
Dear LLVM developers,
I think I have found a bug inside the LLVM optimizer for O1.
We have the following function that is missing the return statement:
int func1_broken(long long count) {
Test t1[10][count];
}
The code should call the constructor of Test class 10*count times. In the IR
generated by the clang front end, there is a zero check followed by a do/while
construct to call the constructor. The bug is now that the optimizer eliminates
the zero check.
If we change the return type to void or add the missing return statement
void func2_correct(long long count) {
Test t1[10][count];
}
the zero check is not eliminated.
You can reproduce the bug by compiling the attached llvmbug.cpp. Just run
clang++ -S -emit-llvm -O1 llvmbug.cpp -o llvmbug.ll
and take a look at llvmbug.ll. Different results for func1_broken and
func2_correct/func3_correct are generated.
I have tested it with clang 5.0.1 from cygwin as well as a self-compiled
clang/llvm version 8.0.0.
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