https://llvm.org/bugs/show_bug.cgi?id=27866
Bug ID: 27866
Summary: Alias to Linkonce global / canonical representation
for Aliases
Product: new-bugs
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: enhancement
Priority: P
Component: new bugs
Assignee: unassignedb...@nondot.org
Reporter: mehdi.am...@apple.com
CC: llvm-bugs@lists.llvm.org
Classification: Unclassified
I spent some time with Duncan today trying to understand aliases and we came to
the conclusion that:
@a = global i32 0
@b = alias @a
should be equivalent to:
@0 = private i32 0
@a = alias @0
@b = alias @0
We may even consider this as a canonical representation of aliases and have
global-opt actually take care of doing this transformation.
However some behavior is unexpected, right now running llvm-link with the
following IR:
@A = linkonce_odr global i8 1
@B = alias i8, i8* @A
will generate the same IR. It means that linking @B actually introduce the
symbol @A as if the alias is an actual use of the other name. Running
globalopt/global-dce does not change the IR.
However if we "canonicalize" to:
@0 = private global i8 1
@A = linkonce_odr alias i8, i8* @0
@B = alias i8, i8* @0
Then llvm-link generates:
@0 = private global i8 1
@B = alias i8, i8* @0
In this case the symbol @A will *not* be pulled in.
Also, if instead of running llvm-link we run globaldce, we end up with the same
result (@A is dropped).
Finally globalopt is able to turn this IR into:
@B = constant i8 1
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