https://llvm.org/bugs/show_bug.cgi?id=31801
Bug ID: 31801
Summary: changeToUnreachable makes questionable decisions (and
modifies the CFG)
Product: libraries
Version: trunk
Hardware: PC
OS: Windows NT
Status: NEW
Severity: normal
Priority: P
Component: Scalar Optimizations
Assignee: unassignedb...@nondot.org
Reporter: dav...@freebsd.org
CC: chandl...@gmail.com, dber...@dberlin.org,
fil...@gmail.com, florian.h...@arm.com,
llvm-bugs@lists.llvm.org, llvm-...@redking.me.uk,
simon.f.whitta...@gmail.com
Classification: Unclassified
Some recent changes in NewGVN revealed a couple of issues in
changeToUnreachable().
1) It's declared in Local.h, although it modifies the CFG (it shouldn't :().
2) It takes a `preserveLCSSA` boolean argument which prevents removal of
useless PHI nodes (the argument is passed down to
BasicBlock::removePredecessor). As discussed with Danny, this doesn't seem
enough to preserve LCSSA, but it could just be that the arugment of the
function is misnamed.
We should:
1) Modify the function so that it preserves the CFG
Danny suggested we could probably just insert before the terminator
new StoreInst(UndefValue::get(Int8Ty), Constant::getNullValue(Int8Ty\
getPointerTo()), TI)
so that SimplifyCFG knows how to remove it.
2) Understand how `preserveLCSSA` is used, and either fix to really preserve
LCSSA or change the parameter name (and document it, while we're there:)
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