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

[David Majnemer]

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.


>
> - PB
>
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