[LLVMbugs] [Bug 16903] New: clang misscompiles “|?===?UTF-8?Q?” if the right side is a int{char{T}}

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=16903

            Bug ID: 16903
           Summary: clang misscompiles “|=” if the right side is a
                    int{char{T}}
           Product: clang
           Version: 3.3
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++
          Assignee: unassignedclangbugs at nondot.org
          Reporter: florian.weber at sfz-bw.de
                CC: dgregor at apple.com, llvmbugs at cs.uiuc.edu
    Classification: Unclassified

Created attachment 11041
  --> http://llvm.org/bugs/attachment.cgi?id=11041&action=edit
the preprocessed source

The following program causes clang to create wrong code (plattform:
x86_64-unknown-linux-gnu):

#include <iostream>

template<typename T>
void fun (T it) {
    int m = 0;
    for(int i = 0; i < 4; ++i, ++it){
        m |= int{char{*it}};
        std::cout << int(*it) << ", " << m << ", " << i << std::endl;
    }
}

int main () {
    char in[4] = {0,0,0,0};
    fun(in);
}

Expected output:
0, 0, 0
0, 0, 1
0, 0, 2
0, 0, 3

The real output depends on the optimization-level, eg with -O0:
0, 256, 0
0, 110578432, 1
0, 110578432, 2
0, 110578432, 3

(110578432 is a number unique to every run if adress-space-randomisation is
enabled, otherwise it is always -7936)

with -O3 the output is always:
0, 224, 0
0, 224, 1
0, 224, 2
0, 224, 3

Additional information:

My commandline is:
clang++ -O0 -std=c++11 main.cpp / clang++ -O3 -std=c++11 main.cpp

The preprocessed source is attached.

As the Bug has some (though small) similarities, it may be related to #16894.

A look into the assembler showed that the only “or”-instruction in the TU
involves the register %ecx which is not used in any other place of the
object-file (I don't known details of the x64-assembler, so this might be
unrelated but it took long enough to find and looks suspicious enough that I
note it here).

clang++ --version:
clang version 3.3 (tags/RELEASE_33/final)
Target: x86_64-unknown-linux-gnu
Thread model: posix

plattform: Arch Linux

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