note on integer-pointer transmutation

diff --git a/ralf/_posts/2022-04-11-provenance-exposed.md b/ralf/_posts/2022-04-11-provenance-exposed.md
index 6ab04f538b7c72e851dd600ec717816602ec159c..53d0b84f044db8c0a79d69cb9e2204b76725c778 100644 (file)
--- a/ralf/_posts/2022-04-11-provenance-exposed.md
+++ b/ralf/_posts/2022-04-11-provenance-exposed.md
@@ -421,6 +421,10 @@ My personal stance is that we should not let the cast synthesize a new provenanc
 This would entirely lose the benefit I discussed above of making pointer-integer round-trips a *local* concern -- if these round-trips produce new, never-before-seen kinds of provenance, then the entire rest of the memory model has to define how it deals with those provenances.
 We already have no choice but treat pointer-integer casts as an operation with side-effects; let's just do the same with integer-pointer casts and remain sure that no matter what the aliasing rules are, they will work fine even in the presence of pointer-integer round-trips.
 
 This would entirely lose the benefit I discussed above of making pointer-integer round-trips a *local* concern -- if these round-trips produce new, never-before-seen kinds of provenance, then the entire rest of the memory model has to define how it deals with those provenances.
 We already have no choice but treat pointer-integer casts as an operation with side-effects; let's just do the same with integer-pointer casts and remain sure that no matter what the aliasing rules are, they will work fine even in the presence of pointer-integer round-trips.
 

+That said, under this model integer-pointer casts still have no side-effect, in the sense that just removing them (if their result is unused) is fine.
+Hence, it *could* make sense to implicitly perform integer-pointer casts in some situations, like when an integer value (without provenance) is used in a pointer operation (due to an integer-to-pointer transmutation).
+This breaks some optimizations like load fusion (turning two loads into one assumes the same provenance was picked both times), but most optimizations (in particular dead code elimination) are unaffected.
+

 #### What about LLVM?
 
 I discussed above how my vision for Rust relates to the direction C is moving towards.
 #### What about LLVM?
 
 I discussed above how my vision for Rust relates to the direction C is moving towards.