[PATCH] D99240: [ConstantFolding] Look through local aliases when simplify GEPs

[Arthur Eubanks via Phabricator via llvm-commits]

aeubanks added a comment.

Ah I see, https://github.com/llvm/llvm-project/blob/61a55c8812e790842799ba1de5bd81fe8afb3b16/llvm/lib/Transforms/IPO/GlobalOpt.cpp#L2953. It only works if the aliasee is a global value. Extending globalopt to handle aliasees that may be a constant expression containing a global value, rather than just a global value, would probably work as well. Either way is fine by me.

globalopt specifically deletes aliases when possible, so it seems fair to do that here as well, even if that may remove entries from the symbol table.



================
Comment at: llvm/lib/IR/Value.cpp:612-613
       V = cast<GlobalAlias>(V)->getAliasee();
+    } else if (StripKind == PSK_ZeroIndicesAndLocalAliases &&
+               isa<GlobalAlias>(V) && cast<GlobalAlias>(V)->hasLocalLinkage()) {
+      V = cast<GlobalAlias>(V)->getAliasee();
----------------
pcc wrote:
> aeubanks wrote:
> > pcc wrote:
> > > rnk wrote:
> > > > I wonder if it would be safe to power up all of the other versions of pointer stripping to look through local aliases.
> > > I don't think we should. It wouldn't be safe to allow passes to replace `@alias` with `@aliasee` with this IR:
> > > ```
> > > @alias = internal alias @aliasee
> > > @aliasee = linkonce_odr global i32 42
> > > ```
> > > We should also avoid the QoI issue (missing symbol table entry) that would result from such replacement given this IR:
> > > ```
> > > @alias = internal alias @aliasee
> > > @aliasee = private global i32 42
> > > ```
> > Why is the first transform not safe?
> Because it would replace a reference to this object file's copy of `@aliasee` with a reference to the copy of `@aliasee` selected by the linker, and those could potentially be different (and have different addresses, which is observable).
It doesn't end up using the copy of `@aliasee` selected by the linker?

globalopt actually performs this transform:
```
$ cat /tmp/y.ll
@alias = internal alias i32, i32* @aliasee
@aliasee = linkonce_odr global i32 42

define i32 @foo() {
  %i = load i32, i32* @alias
  ret i32 %i
}
$ opt -passes=globalopt /tmp/y.ll -S
@aliasee = linkonce_odr local_unnamed_addr global i32 42

define i32 @foo() local_unnamed_addr {
  %i = load i32, i32* @aliasee, align 4
  ret i32 %i
}
```


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D99240/new/

https://reviews.llvm.org/D99240