[PATCH] D34806: [MergeFunctions] Merge small functions if possible without a thunk

[whitequark via Phabricator via llvm-commits]

whitequark created this revision.

This can result in significant code size savings in some cases,
e.g. an interrupt table all filled with the same assembly stub
in a certain Cortex-M BSP results in code blowup by a factor of 2.5.


Repository:
  rL LLVM

https://reviews.llvm.org/D34806

Files:
  lib/Transforms/IPO/MergeFunctions.cpp
  test/Transforms/MergeFunc/merge-small-unnamed-addr.ll
  test/Transforms/MergeFunc/no-merge-small.ll


Index: test/Transforms/MergeFunc/no-merge-small.ll
===================================================================
--- /dev/null
+++ test/Transforms/MergeFunc/no-merge-small.ll
@@ -0,0 +1,11 @@
+; RUN: opt -S -mergefunc < %s | FileCheck %s
+
+; CHECK: @a
+define void @a() {
+  ret void
+}
+
+; CHECK: @b
+define void @b() {
+  ret void
+}
Index: test/Transforms/MergeFunc/merge-small-unnamed-addr.ll
===================================================================
--- /dev/null
+++ test/Transforms/MergeFunc/merge-small-unnamed-addr.ll
@@ -0,0 +1,14 @@
+; RUN: opt -S -mergefunc < %s | FileCheck %s
+
+; CHECK-NOT: @b
+
+ at x = constant { void ()*, void ()* } { void ()* @a, void ()* @b }
+; CHECK: { void ()* @a, void ()* @a }
+
+define internal void @a() unnamed_addr {
+  ret void
+}
+
+define internal void @b() unnamed_addr {
+  ret void
+}
Index: lib/Transforms/IPO/MergeFunctions.cpp
===================================================================
--- lib/Transforms/IPO/MergeFunctions.cpp
+++ lib/Transforms/IPO/MergeFunctions.cpp
@@ -647,6 +647,16 @@
     return;
   }
 
+  // Don't merge tiny functions using a thunk, since it can just end up
+  // making the function larger.
+  if (G->size() == 1) {
+    if (G->front().size() <= 2) {
+      DEBUG(dbgs() << "writeThunk: " << G->getName() 
+                   << " is too small to bother creating a thunk for\n");
+      return;
+    }
+  }
+
   BasicBlock *GEntryBlock = nullptr;
   std::vector<Instruction *> PDIUnrelatedWL;
   BasicBlock *BB = nullptr;
@@ -779,18 +789,6 @@
 
   const FunctionNode &OldF = *Result.first;
 
-  // Don't merge tiny functions, since it can just end up making the function
-  // larger.
-  // FIXME: Should still merge them if they are unnamed_addr and produce an
-  // alias.
-  if (NewFunction->size() == 1) {
-    if (NewFunction->front().size() <= 2) {
-      DEBUG(dbgs() << NewFunction->getName()
-                   << " is to small to bother merging\n");
-      return false;
-    }
-  }
-
   // Impose a total order (by name) on the replacement of functions. This is
   // important when operating on more than one module independently to prevent
   // cycles of thunks calling each other when the modules are linked together.


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