[LLVMdev] extractelement causes memory access violation - what to do?

[Hal Finkel]

----- Original Message -----
> From: "Paweł Bylica" <chfast at gmail.com>
> To: "David Majnemer" <david.majnemer at gmail.com>
> Cc: "LLVMdev" <llvmdev at cs.uiuc.edu>
> Sent: Tuesday, June 30, 2015 5:42:24 AM
> Subject: Re: [LLVMdev] extractelement causes memory access violation - what	to do?
> 
> 
> 
> 
> 
> On Fri, Jun 26, 2015 at 5:42 PM David Majnemer <
> david.majnemer at gmail.com > wrote:
> 
> 
> 
> 
> 
> On Fri, Jun 26, 2015 at 7:00 AM, Paweł Bylica < chfast at gmail.com >
> wrote:
> 
> 
> 
> Hi,
> 
> 
> Let's have a simple program:
> 
> define i32 @main(i32 %n, i64 %idx) {
> %idxSafe = trunc i64 %idx to i5
> %r = extractelement <4 x i32> <i32 -1, i32 -1, i32 -1, i32 -1>, i64
> %idx
> ret i32 %r
> }
> 
> 
> The assembly of that would be:
> 
> pcmpeqd %xmm0, %xmm0
> movdqa %xmm0, -24(%rsp)
> movl -24(%rsp,%rsi,4), %eax
> retq
> 
> 
> The language reference states that the extractelement instruction
> produces undefined value in case the index argument is invalid (our
> case). But the implementation simply dumps the vector to the stack
> memory, calculates the memory offset out of the index value and
> tries to access the memory. That causes the crash.
> 
> 
> The workaround is to trunc the index value before extractelement (see
> %idxSafe). But what should be the ultimate solution?
> 
> 
> 
> 
> 
> We could fix this by specifying that out of bounds access on an
> extractelement leads to full-on undefined behavior, no need to force
> everyone to eat the cost of a mask.
> 
> 
> I don't have preference for any of the solutions.
> 
> 
> I have a side question. It is not stated explicitly in the reference
> but I would assume the index of extractelement is processed as an
> unsigned value. However, the DAG Builder extends the index with
> sext. Is it correct?

Hrmm. Given that only (small) positive numbers are valid, it shouldn't matter. Unless we can find a reason that it works better to be sext, it seems conceptually cleaner to make it zext.

 -Hal

> 
> 
> - PB
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-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory