[llvm-dev] The bit pattern after stripPointerCasts() (original) (raw)
Doerfert, Johannes Rudolf via llvm-dev llvm-dev at lists.llvm.org
Tue Dec 11 16:51:45 PST 2018
- Previous message: [llvm-dev] Using LLD to link against third-party libraries? How?
- Next message: [llvm-dev] The bit pattern after stripPointerCasts()
- Messages sorted by: [ date ] [ thread ] [ subject ] [ author ]
Hi,
in a recent review [0], Florian Hahn helped me to realize something that was rather surprising to me:
The widely popular and very useful function llvm::Value::stripPointerCasts() can return a value with a different bit pattern than the input.
Now, I think this should not be the case but I want the hear other opinions. Before I go on, please not that there is at least one location in the code base [1] that makes a wrong assumption about the bit pattern preservation.
The Problem
StripPointerCasts will peel of address space casts but these can potentially change the bit pattern.
Possible Solutions
Make stripPointerCasts not look through address space casts. A new function (or parameter) would then be introduced to do the same thing and peel of address space casts.
Keep looking through address space casts in stripPointerCasts. A new function (or parameter) would then be introduced to do the same but with the guarantee that the bit pattern is preserved.
Note that the module implementation of stripPointerCasts and friends makes basically all the changes to them easy.
Solution one has the benefit that our code is "more sound" than before because the stripPointerCasts function is more restrictive. We might lose performance though. Solution two will not disturb any existing code but we might need to revisit the uses of stripPointerCasts to fix existing bugs.
Suggestions welcome!
Thanks, Johannes
[0] https://reviews.llvm.org/D54956?vs=on&id=175572&whitespace=ignore-most#inline-485807
[1] LazyValueInfo::getPredicateAt() line ~1714, isKnownNull(...) is used after the stripPointerCasts() function might have changed the bit pattern (and thereby the null-ness).
--
Johannes Doerfert Researcher
Argonne National Laboratory Lemont, IL 60439, USA
jdoerfert at anl.gov -------------- next part -------------- A non-text attachment was scrubbed... Name: signature.asc Type: application/pgp-signature Size: 228 bytes Desc: not available URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20181212/8ae3993e/attachment.sig>
- Previous message: [llvm-dev] Using LLD to link against third-party libraries? How?
- Next message: [llvm-dev] The bit pattern after stripPointerCasts()
- Messages sorted by: [ date ] [ thread ] [ subject ] [ author ]