[LLVMbugs] [Bug 6426] New: Structure layout is not correct in big endian.

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=6426

           Summary: Structure layout is not correct in big endian.
           Product: libraries
           Version: trunk
          Platform: PC
        OS/Version: Linux
            Status: NEW
          Severity: critical
          Priority: P5
         Component: Target Description Classes
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: dodohack at gmail.com
                CC: llvmbugs at cs.uiuc.edu


This bug can be reproduced when you compile 2003-05-07-VarArgs.c for big endian
target.
There is a structure "DWordS dw = { 18, 'a' };" that past as va_arg to function
test, LowerFormalArgument can currectly deal with "dw" and store it in register
or on stack, but llvm IR that expanded from macro va_arg failed to access char
'a' in big endian, because 'a' stored at least significant byte, but generated
IR assume 'a' stored at most significant byte.

I have read the code about struct layout, and tried to fix this bug like this:
original code: TargetData.cpp
=================================================================
StructLayout::StructLayout(const StructType *ST, const TargetData &TD) {      
  StructAlignment = 0;                                                        
  StructSize = 0;                                                             
  NumElements = ST->getNumElements();  

  // Loop over each of the elements, placing them in memory.                  
  for (unsigned i = 0, e = NumElements; i != e; ++i) {                        
    const Type *Ty = ST->getElementType(i);                                   
    unsigned TyAlign = ST->isPacked() ? 1 : TD.getABITypeAlignment(Ty);       

    // Add padding if necessary to align the data element properly.           
    if ((StructSize & (TyAlign-1)) != 0)                                      
      StructSize = TargetData::RoundUpAlignment(StructSize, TyAlign);         

    // Keep track of maximum alignment constraint.                            
    StructAlignment = std::max(TyAlign, StructAlignment);                     

    MemberOffsets[i] = StructSize; 
    ......
=================================================================

updated code: TargetData.cpp
=================================================================
StructLayout::StructLayout(const StructType *ST, const TargetData &TD) {      
  StructAlignment = 0;                                                        
  StructSize = 0;                                                             
  NumElements = ST->getNumElements();                                         
  bool isBigEndian = TD.isBigEndian();

  // Loop over each of the elements, placing them in memory.                  
  for (unsigned i = 0, e = NumElements; i != e; ++i) {                        
    const Type *Ty = ST->getElementType(i);                                   
    unsigned TyAlign = ST->isPacked() ? 1 : TD.getABITypeAlignment(Ty);       

    // Add padding if necessary to align the data element properly.           
    if ((StructSize & (TyAlign-1)) != 0)                                      
      StructSize = TargetData::RoundUpAlignment(StructSize, TyAlign);         

    // Keep track of maximum alignment constraint.                            
    StructAlignment = std::max(TyAlign, StructAlignment);                     

    if (isBigEndian && !Ty->isAggregateType() &&                              
        Ty->getScalarSizeInBits() < 32 && Ty->getScalarSizeInBits() > 0) {    
      MemberOffsets[i] = StructSize + 4 - Ty->getScalarSizeInBits()/8;        
    } else {                                                                  
      MemberOffsets[i] = StructSize;                                          
    }                                                                         
    ......                                                                      

Add, other places that related to structure padding should be fixed, say
function EmitGlobalConstantStruct in AsmPrinter.cpp.

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