[llvm-dev] Hoisting in the presence of volatile loads.

[Krzysztof Parzyszek via llvm-dev]

Hi Alina,
The testcase was inspired by a situation that a customer reported to us 
a while back.
There was a function taking two pointers, one was used to load something 
from device-mapped memory (volatile), while the other was accessing 
regular memory.  These two pointers were aliased (in LLVM), but not 
because they were proven to alias, but because they weren't proved to be 
different.
However, according to the C rules, accessing a volatile object through a 
non-volatile pointer results in an undefined behavior, so we could have 
assumed that the program was well-formed and thus that the pointers 
didn't alias after all.
In the end, what was important was that the loop was running as fast as 
possible, and any invariant code should have been moved out of it.  The 
non-volatile load was not (because of how AliasSetTracker was treating 
volatile loads), and the customer reported it to us as a missed 
performance opportunity.

-Krzysztof

On 12/19/2017 7:14 PM, Alina Sbirlea wrote:
> Hi Krzysztof,
> 
> Could I get some background info on reasoning about hoisting in the 
> presence of volatile loads?
> I was looking at this testcase: test/Transforms/LICM/volatile-alias.ll
> 
> Context: MemorySSA treats volatile loads as defs. I'm looking to better 
> understand expected behavior in the presence of volatile accesses.
> More context: https://reviews.llvm.org/D40375.
> 
> Thanks in advance,
> Alina

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