[LLVMbugs] [Bug 20644] New: Too strong optimization with -O2 generates invalid code

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=20644

            Bug ID: 20644
           Summary: Too strong optimization with -O2 generates invalid
                    code
           Product: clang
           Version: unspecified
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++
          Assignee: unassignedclangbugs at nondot.org
          Reporter: sylvestre at debian.org
                CC: dgregor at apple.com, llvmbugs at cs.uiuc.edu
    Classification: Unclassified

The following code compiled with -O2:
clang++ -O2 -Wall -g -o segfault segfault.cpp && ./segfault 
is segfaulting with 3.4, 3.5 and trunk. gcc is fine.

Commenting some items in the switch/case or commenting the cout removes the
segfault.

LLDB is confused too. It seems that fractional is wrongly optimized:

Process 10217 launched: '/tmp/segfault' (x86_64)
0x2
00
Process 10217 stopped
* thread #1: tid = 10217, 0x0000000000400afb
segfault`InterPrediction::test(SampleYVector const&) [inlined]
getMargins(fractional=<unavailable>) + 6 at segault.cpp:47, name = 'segfault',
stop reason = invalid address (fault address: 0x1000400c08)
    frame #0: 0x0000000000400afb segfault`InterPrediction::test(SampleYVector
const&) [inlined] getMargins(fractional=<unavailable>) + 6 at segault.cpp:47
   44      void getMargins(const int fractional, int &topLeftMargin)
   45      {
   46          switch (fractional)
-> 47          {
   48              case 0:
   49                  topLeftMargin = 0;
   50                  break;
(lldb) p fractional
error: Couldn't materialize struct: size of variable fractional (4) disagrees
with the ValueObject's size (0)
Errored out in Execute, couldn't PrepareToExecuteJITExpression


------------------------
#include <iostream>

class SampleYVector
{

public:
    SampleYVector()
        :x(0)
        ,y(0)
        {}
    SampleYVector(int x_, int y_)
        :x(x_)
        ,y(y_)
        {}

    SampleYVector& operator&= (const int mask)
        {
            x &= mask;
            y &= mask;
            return *this;
        }

    int x;
    int y;
};

SampleYVector operator& (const SampleYVector &lhs, const int mask)
{
    SampleYVector temp(lhs);
    temp &= mask;
    return temp;
}

struct InterPrediction
{
public:

    int leftDataMargin;

    void test(const SampleYVector &mv);

};

void getMargins(const int fractional, int &topLeftMargin)
{
    switch (fractional)
    {
        case 0:
            topLeftMargin = 0;
            break;

        case 1:
            topLeftMargin = 3;
            break;

        case 2:
            topLeftMargin = 3;
            break;

        case 3:
            topLeftMargin = 2;
            break;

    }
}

void InterPrediction::test(const SampleYVector &mv)
{
    const SampleYVector mvFrac = mv & 3;
    std::cout << mvFrac.x << "x" << mvFrac.y << '\n';

    const int fractionalX = 0;
    std::cout << fractionalX << mvFrac.x << std::endl;

    getMargins(mvFrac.x, leftDataMargin);
}

int main()
{
    SampleYVector mv(0, 2);
    InterPrediction interPrediction;
    interPrediction.test(mv);
    return 0;
}
------------------------


Initially reported:
http://bugs.debian.org/758007

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