[LLVMdev] load widening conflicts with AddressSanitizer

[Chris Lattner]

On Dec 16, 2011, at 2:27 PM, Kostya Serebryany wrote:
> This is a good question.  Would it be possible for ASan to do its instrumentation earlier?  
> 
> It would be possible but undesirable. 
> First, asan blows up the IR and running asan early will increase the compile-time. 
> Second, asan greatly benefits from all optimizations running before it because it needs to instrument less memory accesses. 
> It actually benefits from load widening too: in the test case above asan instruments only one load instead of two. 

You certainly wouldn't want to run asan before mem2reg/SRoA, but after that, the benefits are probably small.  I'd guess that there is some non-zero value to exposing the code generated by asan to the optimizer.  Have you looked at that at all?

> In this case, we have an array of 22 bytes which is 16-aligned. 
> I suspect that load widening changed the alignment of alloca instruction to make the transformation legal. Right? 
> Can we change the load widening algorithm to also change the size of alloca instruction to be dividable by 16? 
> This will solve the problem, at least the variant I observe now. 

I believe it is 16-byte aligned based on ABI requirements for x86-64, though you're right that the optimizer will increase alignment in other cases.  In any case, we don't want to increase the size of the object, because that would prevent packing some other data in after it.  For example, a 2-byte aligned 10 byte object can be placed after it in memory if we keep it 22-bytes in size.

-Chris

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