[cfe-dev] Crash on alignments >= 2**16 (PR26444)

[Philip Reames via cfe-dev]

 From the LLVM perspective, the maximum alignment is specified as:

In Value.h:
00520/// \brief The maximum alignment for instructions.
00521///
00522/// This is the greatest alignment value supported by load, store, and 
alloca
00523/// instructions, and global values.
00524 
<http://llvm.org/docs/doxygen/html/classllvm_1_1Value.html#a50e02a54cc6f87a00a671265e1447e61>     static  const  unsigned  MaxAlignmentExponent 
<http://llvm.org/docs/doxygen/html/classllvm_1_1Value.html#a50e02a54cc6f87a00a671265e1447e61>  = 29;
00525 
<http://llvm.org/docs/doxygen/html/classllvm_1_1Value.html#a2885f6c5817ff68bc24bd1e22e221740>    static  const  unsigned  MaximumAlignment 
<http://llvm.org/docs/doxygen/html/classllvm_1_1Value.html#a2885f6c5817ff68bc24bd1e22e221740>  = 1u <<MaxAlignmentExponent 
<http://llvm.org/docs/doxygen/html/classllvm_1_1Value.html#a50e02a54cc6f87a00a671265e1447e61>;

This is checked in the Verifier via:
  Assert(GV.getAlignment() <= Value::MaximumAlignment,
           "huge alignment values are unsupported", &GV);
(and similiar checks for Load, Store, Alloca, but not the various atomic 
instructions)





On 02/14/2016 01:30 PM, don hinton via cfe-dev wrote:
> Sema::AddAlignment() defines MaxValidAlignment like this:
>    // Alignment calculations can wrap around if it's greater than 2**28.
>    unsigned MaxValidAlignment =
>        Context.getTargetInfo().getTriple().isOSBinFormatCOFF() ? 8192
>                                                                : 268435456;
>
> But AggValueSlot stores Alignment as an unsigned short:
>
> class AggValueSlot {
> ...
>    unsigned short Alignment;
>
> The max value for an unsigned short is 2**16 -1, but you are passing 2**16, which AddeAlignment says is okay.  However, it ends up getting stored as 0 in an unsigned short.
>
> The fix is to make these sizes consistent, but I'm not sure which should be changed  (though I'm guessing Alignment should be unsigned instead of unsigned short).
>
>
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