[PATCH] Don't merge tiny functions.

[Matt Arsenault]

Hi sdyatkovskiy,

It's silly to merge functions like these:
    
define void @foo(i32 %x) {
    ret void
}
    
define void @bar(i32 %x) {
    ret void
}
    
to get
    
define void @bar(i32) {
    tail call void @foo(i32 %0)
    ret void
}


http://llvm-reviews.chandlerc.com/D1702

Files:
  lib/Transforms/IPO/MergeFunctions.cpp
  test/Transforms/MergeFunc/merge-ptr-and-int.ll
  test/Transforms/MergeFunc/too-small.ll

Index: lib/Transforms/IPO/MergeFunctions.cpp
===================================================================
--- lib/Transforms/IPO/MergeFunctions.cpp
+++ lib/Transforms/IPO/MergeFunctions.cpp
@@ -844,6 +844,16 @@
 
   const ComparableFunction &OldF = *Result.first;
 
+  // Don't merge tiny functions, since it can just end up making the function
+  // larger.
+  if (NewF.getFunc()->size() == 1) {
+    if (NewF.getFunc()->front().size() <= 2) {
+      DEBUG(dbgs() << NewF.getFunc()->getName()
+            << " is to small to bother merging\n");
+      return false;
+    }
+  }
+
   // Never thunk a strong function to a weak function.
   assert(!OldF.getFunc()->mayBeOverridden() ||
          NewF.getFunc()->mayBeOverridden());
Index: test/Transforms/MergeFunc/merge-ptr-and-int.ll
===================================================================
--- test/Transforms/MergeFunc/merge-ptr-and-int.ll
+++ test/Transforms/MergeFunc/merge-ptr-and-int.ll
@@ -1,18 +1,27 @@
 ; RUN: opt -S -mergefunc < %s | FileCheck %s
 target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64-S128"
 
-; CHECK-LABEL: f0
+
+declare void @stuff()
+
+; CHECK-LABEL: @f0(
 define void @f0(i64 %p0) {
 entry:
+  call void @stuff()
+  call void @stuff()
+  call void @stuff()
   ret void
 }
 
-; CHECK-LABEL: f1
+; CHECK-LABEL: @f1(
 ; CHECK: ptrtoint i64*
 ; CHECK: tail call void @f0(i64
 
 define void @f1(i64* %p0) {
 entry:
+  call void @stuff()
+  call void @stuff()
+  call void @stuff()
   ret void
 }
 
Index: test/Transforms/MergeFunc/too-small.ll
===================================================================
--- /dev/null
+++ test/Transforms/MergeFunc/too-small.ll
@@ -0,0 +1,14 @@
+; RUN: opt -S -mergefunc < %s | FileCheck %s
+
+define void @foo(i32 %x) {
+; CHECK-LABEL: @foo(
+; CHECK-NOT: call
+  ret void
+}
+
+define void @bar(i32 %x) {
+; CHECK-LABEL: @bar(
+; CHECK-NOT: call
+  ret void
+}
+
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