Give the always-inliner its own custom filter. It shouldn't have to pay

the very high overhead of the complex inline cost analysis when all it
wants to do is detect three patterns which must not be inlined. Comment
the code, clean it up, and leave some hints about possible performance
improvements if this ever shows up on a profile.

Moving this off of the (now more expensive) inline cost analysis is
particularly important because we have to run this inliner even at -O0.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@153814 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 63 insertions, 20 deletions

diff --git a/lib/Transforms/IPO/InlineAlways.cpp b/lib/Transforms/IPO/InlineAlways.cpp
index 4e666fb..a977027 100644
--- a/lib/Transforms/IPO/InlineAlways.cpp
+++ b/lib/Transforms/IPO/InlineAlways.cpp
@@ -32,7 +32,6 @@ namespace {
 
   // AlwaysInliner only inlines functions that are mark as "always inline".
   class AlwaysInliner : public Inliner {
-    InlineCostAnalyzer CA;
   public:
     // Use extremely low threshold.
     AlwaysInliner() : Inliner(ID, -2000000000, /*InsertLifetime*/true) {
@@ -43,24 +42,7 @@ namespace {
       initializeAlwaysInlinerPass(*PassRegistry::getPassRegistry());
     }
     static char ID; // Pass identification, replacement for typeid
-    InlineCost getInlineCost(CallSite CS) {
-      Function *Callee = CS.getCalledFunction();
-      // We assume indirect calls aren't calling an always-inline function.
-      if (!Callee) return InlineCost::getNever();
-
-      // We can't inline calls to external functions.
-      // FIXME: We shouldn't even get here.
-      if (Callee->isDeclaration()) return InlineCost::getNever();
-
-      // Return never for anything not marked as always inline.
-      if (!Callee->hasFnAttr(Attribute::AlwaysInline))
-        return InlineCost::getNever();
-
-      // We still have to check the inline cost in case there are reasons to
-      // not inline which trump the always-inline attribute such as setjmp and
-      // indirectbr.
-      return CA.getInlineCost(CS, getInlineThreshold(CS));
-    }
+    virtual InlineCost getInlineCost(CallSite CS);
     virtual bool doFinalization(CallGraph &CG) {
       return removeDeadFunctions(CG, /*AlwaysInlineOnly=*/true);
     }
@@ -81,9 +63,70 @@ Pass *llvm::createAlwaysInlinerPass(bool InsertLifetime) {
   return new AlwaysInliner(InsertLifetime);
 }
 
+/// \brief Minimal filter to detect invalid constructs for inlining.
+static bool isInlineViable(Function &F) {
+  bool ReturnsTwice = F.hasFnAttr(Attribute::ReturnsTwice);
+  for (Function::iterator BI = F.begin(), BE = F.end(); BI != BE; ++BI) {
+    // Disallow inlining of functions which contain an indirect branch.
+    if (isa<IndirectBrInst>(BI->getTerminator()))
+      return false;
+
+    for (BasicBlock::iterator II = BI->begin(), IE = BI->end(); II != IE;
+         ++II) {
+      CallSite CS(II);
+      if (!CS)
+        continue;
+
+      // Disallow recursive calls.
+      if (&F == CS.getCalledFunction())
+        return false;
+
+      // Disallow calls which expose returns-twice to a function not previously
+      // attributed as such.
+      if (ReturnsTwice && CS.isCall() &&
+          cast<CallInst>(CS.getInstruction())->canReturnTwice())
+        return false;
+    }
+  }
+
+  return true;
+}
+
+/// \brief Get the inline cost for the always-inliner.
+///
+/// The always inliner *only* handles functions which are marked with the
+/// attribute to force inlining. As such, it is dramatically simpler and avoids
+/// using the powerful (but expensive) inline cost analysis. Instead it uses
+/// a very simple and boring direct walk of the instructions looking for
+/// impossible-to-inline constructs.
+///
+/// Note, it would be possible to go to some lengths to cache the information
+/// computed here, but as we only expect to do this for relatively few and
+/// small functions which have the explicit attribute to force inlining, it is
+/// likely not worth it in practice.
+InlineCost AlwaysInliner::getInlineCost(CallSite CS) {
+  Function *Callee = CS.getCalledFunction();
+  // We assume indirect calls aren't calling an always-inline function.
+  if (!Callee) return InlineCost::getNever();
+
+  // We can't inline calls to external functions.
+  // FIXME: We shouldn't even get here.
+  if (Callee->isDeclaration()) return InlineCost::getNever();
+
+  // Return never for anything not marked as always inline.
+  if (!Callee->hasFnAttr(Attribute::AlwaysInline))
+    return InlineCost::getNever();
+
+  // Do some minimal analysis to preclude non-viable functions.
+  if (!isInlineViable(*Callee))
+    return InlineCost::getNever();
+
+  // Otherwise, force inlining.
+  return InlineCost::getAlways();
+}
+
 // doInitialization - Initializes the vector of functions that have not
 // been annotated with the "always inline" attribute.
 bool AlwaysInliner::doInitialization(CallGraph &CG) {
-  CA.setTargetData(getAnalysisIfAvailable<TargetData>());
   return false;
 }