Initial commit for the rewrite of the inline cost analysis to operate

on a per-callsite walk of the called function's instructions, in
breadth-first order over the potentially reachable set of basic blocks.

This is a major shift in how inline cost analysis works to improve the
accuracy and rationality of inlining decisions. A brief outline of the
algorithm this moves to:

- Build a simplification mapping based on the callsite arguments to the
  function arguments.
- Push the entry block onto a worklist of potentially-live basic blocks.
- Pop the first block off of the *front* of the worklist (for
  breadth-first ordering) and walk its instructions using a custom
  InstVisitor.
- For each instruction's operands, re-map them based on the
  simplification mappings available for the given callsite.
- Compute any simplification possible of the instruction after
  re-mapping, and store that back int othe simplification mapping.
- Compute any bonuses, costs, or other impacts of the instruction on the
  cost metric.
- When the terminator is reached, replace any conditional value in the
  terminator with any simplifications from the mapping we have, and add
  any successors which are not proven to be dead from these
  simplifications to the worklist.
- Pop the next block off of the front of the worklist, and repeat.
- As soon as the cost of inlining exceeds the threshold for the
  callsite, stop analyzing the function in order to bound cost.

The primary goal of this algorithm is to perfectly handle dead code
paths. We do not want any code in trivially dead code paths to impact
inlining decisions. The previous metric was *extremely* flawed here, and
would always subtract the average cost of two successors of
a conditional branch when it was proven to become an unconditional
branch at the callsite. There was no handling of wildly different costs
between the two successors, which would cause inlining when the path
actually taken was too large, and no inlining when the path actually
taken was trivially simple. There was also no handling of the code
*path*, only the immediate successors. These problems vanish completely
now. See the added regression tests for the shiny new features -- we
skip recursive function calls, SROA-killing instructions, and high cost
complex CFG structures when dead at the callsite being analyzed.

Switching to this algorithm required refactoring the inline cost
interface to accept the actual threshold rather than simply returning
a single cost. The resulting interface is pretty bad, and I'm planning
to do lots of interface cleanup after this patch.

Several other refactorings fell out of this, but I've tried to minimize
them for this patch. =/ There is still more cleanup that can be done
here. Please point out anything that you see in review.

I've worked really hard to try to mirror at least the spirit of all of
the previous heuristics in the new model. It's not clear that they are
all correct any more, but I wanted to minimize the change in this single
patch, it's already a bit ridiculous. One heuristic that is *not* yet
mirrored is to allow inlining of functions with a dynamic alloca *if*
the caller has a dynamic alloca. I will add this back, but I think the
most reasonable way requires changes to the inliner itself rather than
just the cost metric, and so I've deferred this for a subsequent patch.
The test case is XFAIL-ed until then.

As mentioned in the review mail, this seems to make Clang run about 1%
to 2% faster in -O0, but makes its binary size grow by just under 4%.
I've looked into the 4% growth, and it can be fixed, but requires
changes to other parts of the inliner.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@153812 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 48 insertions, 40 deletions

diff --git a/lib/Analysis/CodeMetrics.cpp b/lib/Analysis/CodeMetrics.cpp
index 6c93f78629..316e7bc934 100644
--- a/lib/Analysis/CodeMetrics.cpp
+++ b/lib/Analysis/CodeMetrics.cpp
@@ -50,6 +50,52 @@ bool llvm::callIsSmall(const Function *F) {
   return false;
 }
 
+bool llvm::isInstructionFree(const Instruction *I, const TargetData *TD) {
+  if (isa<PHINode>(I))
+    return true;
+
+  // If a GEP has all constant indices, it will probably be folded with
+  // a load/store.
+  if (const GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(I))
+    return GEP->hasAllConstantIndices();
+
+  if (const IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
+    switch (II->getIntrinsicID()) {
+    default:
+      return false;
+    case Intrinsic::dbg_declare:
+    case Intrinsic::dbg_value:
+    case Intrinsic::invariant_start:
+    case Intrinsic::invariant_end:
+    case Intrinsic::lifetime_start:
+    case Intrinsic::lifetime_end:
+    case Intrinsic::objectsize:
+    case Intrinsic::ptr_annotation:
+    case Intrinsic::var_annotation:
+      // These intrinsics don't count as size.
+      return true;
+    }
+  }
+
+  if (const CastInst *CI = dyn_cast<CastInst>(I)) {
+    // Noop casts, including ptr <-> int,  don't count.
+    if (CI->isLosslessCast() || isa<IntToPtrInst>(CI) || isa<PtrToIntInst>(CI))
+      return true;
+    // trunc to a native type is free (assuming the target has compare and
+    // shift-right of the same width).
+    if (TD && isa<TruncInst>(CI) &&
+        TD->isLegalInteger(TD->getTypeSizeInBits(CI->getType())))
+      return true;
+    // Result of a cmp instruction is often extended (to be used by other
+    // cmp instructions, logical or return instructions). These are usually
+    // nop on most sane targets.
+    if (isa<CmpInst>(CI->getOperand(0)))
+      return true;
+  }
+
+  return false;
+}
+
 /// analyzeBasicBlock - Fill in the current structure with information gleaned
 /// from the specified block.
 void CodeMetrics::analyzeBasicBlock(const BasicBlock *BB,
@@ -58,27 +104,11 @@ void CodeMetrics::analyzeBasicBlock(const BasicBlock *BB,
   unsigned NumInstsBeforeThisBB = NumInsts;
   for (BasicBlock::const_iterator II = BB->begin(), E = BB->end();
        II != E; ++II) {
-    if (isa<PHINode>(II)) continue;           // PHI nodes don't count.
+    if (isInstructionFree(II, TD))
+      continue;
 
     // Special handling for calls.
     if (isa<CallInst>(II) || isa<InvokeInst>(II)) {
-      if (const IntrinsicInst *IntrinsicI = dyn_cast<IntrinsicInst>(II)) {
-        switch (IntrinsicI->getIntrinsicID()) {
-        default: break;
-        case Intrinsic::dbg_declare:
-        case Intrinsic::dbg_value:
-        case Intrinsic::invariant_start:
-        case Intrinsic::invariant_end:
-        case Intrinsic::lifetime_start:
-        case Intrinsic::lifetime_end:
-        case Intrinsic::objectsize:
-        case Intrinsic::ptr_annotation:
-        case Intrinsic::var_annotation:
-          // These intrinsics don't count as size.
-          continue;
-        }
-      }
-
       ImmutableCallSite CS(cast<Instruction>(II));
 
       if (const Function *F = CS.getCalledFunction()) {
@@ -115,28 +145,6 @@ void CodeMetrics::analyzeBasicBlock(const BasicBlock *BB,
     if (isa<ExtractElementInst>(II) || II->getType()->isVectorTy())
       ++NumVectorInsts;
 
-    if (const CastInst *CI = dyn_cast<CastInst>(II)) {
-      // Noop casts, including ptr <-> int,  don't count.
-      if (CI->isLosslessCast() || isa<IntToPtrInst>(CI) ||
-          isa<PtrToIntInst>(CI))
-        continue;
-      // trunc to a native type is free (assuming the target has compare and
-      // shift-right of the same width).
-      if (isa<TruncInst>(CI) && TD &&
-          TD->isLegalInteger(TD->getTypeSizeInBits(CI->getType())))
-        continue;
-      // Result of a cmp instruction is often extended (to be used by other
-      // cmp instructions, logical or return instructions). These are usually
-      // nop on most sane targets.
-      if (isa<CmpInst>(CI->getOperand(0)))
-        continue;
-    } else if (const GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(II)){
-      // If a GEP has all constant indices, it will probably be folded with
-      // a load/store.
-      if (GEPI->hasAllConstantIndices())
-        continue;
-    }
-
     ++NumInsts;
   }