[llvm-dev] RFC: Killing undef and spreading poison (original) (raw)

Michael Kuperstein via llvm-dev llvm-dev at lists.llvm.org
Thu Oct 20 14:05:46 PDT 2016

The fact two IR values are defined the same way doesn't necessarily imply they are actually the same value. e.g.

%a = load volatile i32, i32* %p %b = load volatile i32, i32* %p

As Sanjoy said, though, it should always be legal to optimize all uses of different "freeze(%x)" values to use the same def - this is equivalent to choosing the same value for all freezes. It's just not necessary to do so.

On Thu, Oct 20, 2016 at 1:58 PM, Krzysztof Parzyszek via llvm-dev < llvm-dev at lists.llvm.org> wrote:

In both of these cases, the expression tree in the IR is going to look like == (freeze(%x), freeze(%x))

The %a and %b are just labels on values, which are defined in the exact same way. How do you differentiate these two? If %a = freeze(%x), is %a+1 == %a+1? -Krzysztof

On 10/20/2016 3:36 PM, Sanjoy Das wrote: Hi Krzysztof, Krzysztof Parzyszek wrote:

On 10/18/2016 4:29 PM, Nuno Lopes wrote:

Even %a and %b might not be the same in "%a = freeze(%x), %b = freeze(%x)" (each freeze returns an arbitrary, but fixed, value).

Assume that %x is known to be a poison value and have: %a = freeze(%x) %b = freeze(%x) Is %a == %a true? Yes, %a is always == %a. It is a normal SSA value with some unspecific content. Is %a == %b true? Not necessarily; but the compiler can make it true by (consistently) choosing equal values for %a and %b. By consistently I mean it can't fold one instance of %a == %b to true and fold another instance of %a == %b to false. -- Sanjoy -- Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, hosted by The Linux Foundation

LLVM Developers mailing list llvm-dev at lists.llvm.org http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20161020/0c2fb0bb/attachment.html>

More information about the llvm-dev mailing list