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IR: Do not consider available_externally linkage to be linker-weak.

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From the linker's perspective, an available_externally global is equivalent
to an external declaration (per isDeclarationForLinker()), so it is incorrect
to consider it to be a weak definition.

Also clean up some logic in the dead argument elimination pass and clarify
its comments to better explain how its behavior depends on linkage,
introduce GlobalValue::isStrongDefinitionForLinker() and start using
it throughout the optimizers and backend.

Differential Revision: http://reviews.llvm.org/D10941

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@241413 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Peter Collingbourne
2015-07-05 20:52:35 +00:00
parent 3b19ca3a2e
commit ce2b45bd7d
14 changed files with 55 additions and 71 deletions
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26
lib/Transforms/IPO/DeadArgumentElimination.cpp
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@@ -326,7 +326,18 @@ bool DAE::DeleteDeadVarargs(Function &Fn) {
/// instead.
bool DAE::RemoveDeadArgumentsFromCallers(Function &Fn)
{
if (Fn.isDeclaration() || Fn.mayBeOverridden())
// We cannot change the arguments if this TU does not define the function or
// if the linker may choose a function body from another TU, even if the
// nominal linkage indicates that other copies of the function have the same
// semantics. In the below example, the dead load from %p may not have been
// eliminated from the linker-chosen copy of f, so replacing %p with undef
// in callers may introduce undefined behavior.
//
// define linkonce_odr void @f(i32* %p) {
// %v = load i32 %p
// ret void
// }
if (!Fn.isStrongDefinitionForLinker())
return false;
// Functions with local linkage should already have been handled, except the
@@ -334,19 +345,6 @@ bool DAE::RemoveDeadArgumentsFromCallers(Function &Fn)
if (Fn.hasLocalLinkage() && !Fn.getFunctionType()->isVarArg())
return false;
// If a function seen at compile time is not necessarily the one linked to
// the binary being built, it is illegal to change the actual arguments
// passed to it. These functions can be captured by isWeakForLinker().
// *NOTE* that mayBeOverridden() is insufficient for this purpose as it
// doesn't include linkage types like AvailableExternallyLinkage and
// LinkOnceODRLinkage. Take link_odr* as an example, it indicates a set of
// *EQUIVALENT* globals that can be merged at link-time. However, the
// semantic of *EQUIVALENT*-functions includes parameters. Changing
// parameters breaks this assumption.
//
if (Fn.isWeakForLinker())
return false;
if (Fn.use_empty())
return false;