ThinLTO: Don't import aliases of any kind (even linkonce_odr)

Summary:
Until a more advanced version of importing can be implemented for
aliases (one that imports an alias as an available_externally definition
of the aliasee), skip the narrow subset of cases that was possible but
came at a cost: aliases of linkonce_odr functions could be imported
because the linkonce_odr function could be safely duplicated from the
source module. This came/comes at the cost of not being able to 'home'
imported linkonce functions (they had to be emitted linkonce_odr in all
the destination modules (even if they weren't used by an alias) rather
than as available_externally - causing extra object size).

Tangentially, this also was the only reason ThinLTO would emit multiple
CUs in to the resulting DWARF - which happens to be a problem for
Fission (there's a fix for this in GDB but not released yet, etc).
(actually it's not the only reason - but I'm sending a patch to fix the
other reason shortly)

There's no reason to believe this particularly narrow alias importing
was especially/meaningfully important, only that it was /possible/ to
implement in this way. When a more general solution is done, it should
still satisfy the DWARF concerns above, since the import will still be
available_externally, and thus not create extra CUs.

Since now all aliases are treated the same, I removed/simplified some
test cases since they were testing corner cases where there are no
longer any corners.

Reviewers: tejohnson, mehdi_amini

Differential Revision: https://reviews.llvm.org/D35875

llvm-svn: 309278

1 files changed, 9 insertions, 19 deletions

diff --git a/llvm/lib/Transforms/IPO/FunctionImport.cpp b/llvm/lib/Transforms/IPO/FunctionImport.cpp
index 233a36d2bc5..a36314ca205 100644
--- a/llvm/lib/Transforms/IPO/FunctionImport.cpp
+++ b/llvm/lib/Transforms/IPO/FunctionImport.cpp
@@ -132,16 +132,11 @@ selectCallee(const ModuleSummaryIndex &Index,
         if (GlobalValue::isInterposableLinkage(GVSummary->linkage()))
           // There is no point in importing these, we can't inline them
           return false;
-        if (auto *AS = dyn_cast<AliasSummary>(GVSummary)) {
-          GVSummary = &AS->getAliasee();
-          // Alias can't point to "available_externally". However when we import
-          // linkOnceODR the linkage does not change. So we import the alias
-          // and aliasee only in this case.
+        if (auto *AS = dyn_cast<AliasSummary>(GVSummary))
+          // Aliases can't point to "available_externally".
           // FIXME: we should import alias as available_externally *function*,
-          // the destination module does need to know it is an alias.
-          if (!GlobalValue::isLinkOnceODRLinkage(GVSummary->linkage()))
-            return false;
-        }
+          // the destination module does not need to know it is an alias.
+          return false;
 
         auto *Summary = cast<FunctionSummary>(GVSummary);
 
@@ -227,16 +222,11 @@ static void computeImportForFunction(
       DEBUG(dbgs() << "ignored! No qualifying callee with summary found.\n");
       continue;
     }
-    // "Resolve" the summary, traversing alias,
-    const FunctionSummary *ResolvedCalleeSummary;
-    if (isa<AliasSummary>(CalleeSummary)) {
-      ResolvedCalleeSummary = cast<FunctionSummary>(
-          &cast<AliasSummary>(CalleeSummary)->getAliasee());
-      assert(
-          GlobalValue::isLinkOnceODRLinkage(ResolvedCalleeSummary->linkage()) &&
-          "Unexpected alias to a non-linkonceODR in import list");
-    } else
-      ResolvedCalleeSummary = cast<FunctionSummary>(CalleeSummary);
+
+    // "Resolve" the summary
+    assert(!isa<AliasSummary>(CalleeSummary) &&
+           "Unexpected alias in import list");
+    const auto *ResolvedCalleeSummary = cast<FunctionSummary>(CalleeSummary);
 
     assert(ResolvedCalleeSummary->instCount() <= NewThreshold &&
            "selectCallee() didn't honor the threshold");