[X86TTI] Remove the unrolling branch limits

The loop stream detector (LSD) on modern Intel cores, which optimizes the
execution of small loops, has limits on the number of taken branches in
addition to uop-count limits (modern AMD cores have similar limits).
Unfortunately, at the IR level, estimating the number of branches that will be
taken is difficult. For one thing, it strongly depends on later passes (block
placement, etc.). The original implementation took a conservative approach and
limited the maximal BB DFS depth of the loop.  However, fairly-extensive
benchmarking by several of us has revealed that this is the wrong approach. In
fact, there are zero known cases where the branch limit prevents a detrimental
unrolling (but plenty of cases where it does prevent beneficial unrolling).

While we could improve the current branch counting logic by incorporating
branch probabilities, this further complication seems unjustified without a
motivating regression. Instead, unless and until a regression appears, the
branch counting will be removed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208255 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Target/X86/X86TargetTransformInfo.cpp b/lib/Target/X86/X86TargetTransformInfo.cpp
index ce060aba64446d36349d910bc8ada7e2dd5fa80a..cad8dfd5221151534152a6d6d31c7d53df40b51f 100644 (file)
--- a/lib/Target/X86/X86TargetTransformInfo.cpp
+++ b/lib/Target/X86/X86TargetTransformInfo.cpp
@@ -41,10 +41,6 @@ UsePartialUnrolling("x86-use-partial-unrolling", cl::init(true),
 static cl::opt<unsigned>
 PartialUnrollingThreshold("x86-partial-unrolling-threshold", cl::init(0),
   cl::desc("Threshold for X86 partial unrolling"), cl::Hidden);
-static cl::opt<unsigned>
-PartialUnrollingMaxBranches("x86-partial-max-branches", cl::init(2),
-  cl::desc("Threshold for taken branches in X86 partial unrolling"),
-  cl::Hidden);
 
 namespace {
 
@@ -172,49 +168,38 @@ void X86TTI::getUnrollingPreferences(Loop *L, UnrollingPreferences &UP) const {
   //  - The loop must have fewer than 16 branches
   //  - The loop must have less than 40 uops in all executed loop branches
 
-  unsigned MaxBranches, MaxOps;
+  // The number of taken branches in a loop is hard to estimate here, and
+  // benchmarking has revealed that it is better not to be conservative when
+  // estimating the branch count. As a result, we'll ignore the branch limits
+  // until someone finds a case where it matters in practice.
+
+  unsigned MaxOps;
   if (PartialUnrollingThreshold.getNumOccurrences() > 0) {
-    MaxBranches = PartialUnrollingMaxBranches;
     MaxOps = PartialUnrollingThreshold;
   } else if (ST->isAtom()) {
     // On the Atom, the throughput for taken branches is 2 cycles. For small
     // simple loops, expand by a small factor to hide the backedge cost.
-    MaxBranches = 2;
     MaxOps = 10;
   } else if (ST->hasFSGSBase() && ST->hasXOP() /* Steamroller and later */) {
-    MaxBranches = 16;
     MaxOps = 40;
   } else if (ST->hasFMA4() /* Any other recent AMD */) {
     return;
   } else if (ST->hasAVX() || ST->hasSSE42() /* Nehalem and later */) {
-    MaxBranches = 8;
     MaxOps = 28;
   } else if (ST->hasSSSE3() /* Intel Core */) {
-    MaxBranches = 4;
     MaxOps = 18;
   } else {
     return;
   }
 
-  // Scan the loop: don't unroll loops with calls, and count the potential
-  // number of taken branches (this is somewhat conservative because we're
-  // counting all block transitions as potential branches while in reality some
-  // of these will become implicit via block placement).
-  unsigned MaxDepth = 0;
-  for (df_iterator<BasicBlock*> DI = df_begin(L->getHeader()),
-       DE = df_end(L->getHeader()); DI != DE;) {
-    if (!L->contains(*DI)) {
-      DI.skipChildren();
-      continue;
-    }
-
-    MaxDepth = std::max(MaxDepth, DI.getPathLength());
-    if (MaxDepth > MaxBranches)
-      return;
+  // Scan the loop: don't unroll loops with calls.
+  for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
+       I != E; ++I) {
+    BasicBlock *BB = *I;
 
-    for (BasicBlock::iterator I = DI->begin(), IE = DI->end(); I != IE; ++I)
-      if (isa<CallInst>(I) || isa<InvokeInst>(I)) {
-        ImmutableCallSite CS(I);
+    for (BasicBlock::iterator J = BB->begin(), JE = BB->end(); J != JE; ++J)
+      if (isa<CallInst>(J) || isa<InvokeInst>(J)) {
+        ImmutableCallSite CS(J);
         if (const Function *F = CS.getCalledFunction()) {
           if (!isLoweredToCall(F))
             continue;
@@ -222,23 +207,11 @@ void X86TTI::getUnrollingPreferences(Loop *L, UnrollingPreferences &UP) const {
 
         return;
       }
-
-    ++DI;
   }
 
   // Enable runtime and partial unrolling up to the specified size.
   UP.Partial = UP.Runtime = true;
   UP.PartialThreshold = UP.PartialOptSizeThreshold = MaxOps;
-
-  // Set the maximum count based on the loop depth. The maximum number of
-  // branches taken in a loop (including the backedge) is equal to the maximum
-  // loop depth (the DFS path length from the loop header to any block in the
-  // loop). When the loop is unrolled, this depth (except for the backedge
-  // itself) is multiplied by the unrolling factor. This new unrolled depth
-  // must be less than the target-specific maximum branch count (which limits
-  // the number of taken branches in the uop buffer).
-  if (MaxDepth > 1)
-    UP.MaxCount = (MaxBranches-1)/(MaxDepth-1);
 }
 
 unsigned X86TTI::getNumberOfRegisters(bool Vector) const {