[MergeFunc] Remove discardables function before writing alias or thunk. (#128865)

Update writeThunkOrAlias to only create an alias or thunk if it is
actually needed. Drop discardable linkone_odr functions if they are not
used before.

PR: https://github.com/llvm/llvm-project/pull/128865

1 files changed, 21 insertions, 14 deletions

diff --git a/llvm/lib/Transforms/IPO/MergeFunctions.cpp b/llvm/lib/Transforms/IPO/MergeFunctions.cpp
index 9cc1598..d2443a8 100644
--- a/llvm/lib/Transforms/IPO/MergeFunctions.cpp
+++ b/llvm/lib/Transforms/IPO/MergeFunctions.cpp
@@ -283,9 +283,10 @@ private:
   // Replace G with an alias to F (deleting function G)
   void writeAlias(Function *F, Function *G);
 
-  // Replace G with an alias to F if possible, or a thunk to F if possible.
-  // Returns false if neither is the case.
-  bool writeThunkOrAlias(Function *F, Function *G);
+  // If needed, replace G with an alias to F if possible, or a thunk to F if
+  // profitable. Returns false if neither is the case. If \p G is not needed
+  // (i.e. it is discardable and not used), \p G is removed directly.
+  bool writeThunkOrAliasIfNeeded(Function *F, Function *G);
 
   /// Replace function F with function G in the function tree.
   void replaceFunctionInTree(const FunctionNode &FN, Function *G);
@@ -875,9 +876,14 @@ void MergeFunctions::writeAlias(Function *F, Function *G) {
   ++NumAliasesWritten;
 }
 
-// Replace G with an alias to F if possible, or a thunk to F if
-// profitable. Returns false if neither is the case.
-bool MergeFunctions::writeThunkOrAlias(Function *F, Function *G) {
+// If needed, replace G with an alias to F if possible, or a thunk to F if
+// profitable. Returns false if neither is the case. If \p G is not needed (i.e.
+// it is discardable and unused), \p G is removed directly.
+bool MergeFunctions::writeThunkOrAliasIfNeeded(Function *F, Function *G) {
+  if (G->isDiscardableIfUnused() && G->use_empty() && !MergeFunctionsPDI) {
+    G->eraseFromParent();
+    return true;
+  }
   if (canCreateAliasFor(G)) {
     writeAlias(F, G);
     return true;
@@ -904,9 +910,10 @@ void MergeFunctions::mergeTwoFunctions(Function *F, Function *G) {
     assert((!isODR(G) || isODR(F)) &&
            "if G is ODR, F must also be ODR due to ordering");
 
-    // Both writeThunkOrAlias() calls below must succeed, either because we can
-    // create aliases for G and NewF, or because a thunk for F is profitable.
-    // F here has the same signature as NewF below, so that's what we check.
+    // Both writeThunkOrAliasIfNeeded() calls below must succeed, either because
+    // we can create aliases for G and NewF, or because a thunk for F is
+    // profitable. F here has the same signature as NewF below, so that's what
+    // we check.
     if (!canCreateThunkFor(F) &&
         (!canCreateAliasFor(F) || !canCreateAliasFor(G)))
       return;
@@ -930,13 +937,13 @@ void MergeFunctions::mergeTwoFunctions(Function *F, Function *G) {
     if (isODR(F))
       replaceDirectCallers(NewF, F);
 
-    // We collect alignment before writeThunkOrAlias that overwrites NewF and
-    // G's content.
+    // We collect alignment before writeThunkOrAliasIfNeeded that overwrites
+    // NewF and G's content.
     const MaybeAlign NewFAlign = NewF->getAlign();
     const MaybeAlign GAlign = G->getAlign();
 
-    writeThunkOrAlias(F, G);
-    writeThunkOrAlias(F, NewF);
+    writeThunkOrAliasIfNeeded(F, G);
+    writeThunkOrAliasIfNeeded(F, NewF);
 
     if (NewFAlign || GAlign)
       F->setAlignment(std::max(NewFAlign.valueOrOne(), GAlign.valueOrOne()));
@@ -975,7 +982,7 @@ void MergeFunctions::mergeTwoFunctions(Function *F, Function *G) {
       return;
     }
 
-    if (writeThunkOrAlias(F, G)) {
+    if (writeThunkOrAliasIfNeeded(F, G)) {
       ++NumFunctionsMerged;
     }
   }