[PowerPC] Check hot loop exit edge in PPCCTRLoops

[inouehrs]

Summary

PPCCTRLoops transform loops using mtctr/bdnz instructions if loop trip count is known and big enough to compensate for the cost of mtctr.
But if there is a loop exit edge which is known to be frequently taken (by builtin_expect or by PGO), we should not transform the loop to avoid the cost of mtctr instruction. Here is an example of a loop with hot exit edge:

for (unsigned i = 0; i < TripCount; i++) {
  // do something
  if (__builtin_expect(check(), 1))
    break;
  // do something
}

[inouehrs]

• simplified the implementation

[nemanjai]

This seems like a good thing to do. Would you be able to provide some benchmark data to show the impact?

lib/Target/PowerPC/PPCCTRLoops.cpp

533

Is there a reason this isn't a range for loop? Also, we typically use I for instructions or iterators over them. Seems like BB might be a clearer name here.

test/CodeGen/PowerPC/ctrloops-hot-exit.ll

23

Is it possible to also construct a test case where the true branch is the exit branch?

[inouehrs]

• addressed comments from @nemanjai

[inouehrs]

This seems like a good thing to do. Would you be able to provide some benchmark data to show the impact?

In a micro benchmark with a small loop always exit at the first iteration due to a hot exit branch, this patch gives about 20% performance improvements. I am testing SPEC benchmarks with PGO.

lib/Target/PowerPC/PPCCTRLoops.cpp

533

Thanks. This is much clearer.
I used iterator to be consistent with other loops in this method.

test/CodeGen/PowerPC/ctrloops-hot-exit.ll

23

I added variations where the true branch is for the exit branch.

[nemanjai]

LGTM.

lib/Target/PowerPC/PPCCTRLoops.cpp

#endif
   for (Loop::block_iterator I = L->block_begin(), IE = L->block_end();
        I != IE; ++I)
     if (mightUseCTR(*I))
       return MadeChange;
 
   SmallVector<BasicBlock*, 4> ExitingBlocks;
   L->getExitingBlocks(ExitingBlocks);
 
+  // If there is an exit edge known to be frequently taken,
+  // we should not transform this loop.
+  for (SmallVectorImpl<BasicBlock *>::iterator I = ExitingBlocks.begin(),
+       IE = ExitingBlocks.end(); I != IE; ++I) {
+    Instruction *TI = (*I)->getTerminator();
+    if (!TI) continue;
+
+    if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
+      uint64_t TrueWeight, FalseWeight;
+      if (!BI->isConditional() ||
+          !BI->extractProfMetadata(TrueWeight, FalseWeight))
+        continue;
+
+      // If the exit path is more frequent than the loop path,
+      // we return here without further analysis for this loop.
+      bool TrueIsExit = !L->contains(BI->getSuccessor(0));
+      if (( TrueIsExit && FalseWeight < TrueWeight) ||
+          (!TrueIsExit && FalseWeight > TrueWeight))
+        return MadeChange;
+    }
+  }
+
   BasicBlock *CountedExitBlock = nullptr;
   const SCEV *ExitCount = nullptr;
   BranchInst *CountedExitBranch = nullptr;
   for (SmallVectorImpl<BasicBlock *>::iterator I = ExitingBlocks.begin(),
        IE = ExitingBlocks.end(); I != IE; ++I) {
     const SCEV *EC = SE->getExitCount(L, *I);
     DEBUG(dbgs() << "Exit Count for " << *L << " from block " <<
                     (*I)->getName() << ": " << *EC << "\n");

test/CodeGen/PowerPC/ctrloops-hot-exit.ll

+; RUN: llc -verify-machineinstrs -mtriple=powerpc64le-unknown-linux-gnu -mcpu=pwr9 < %s | FileCheck %s
+
+; If there is an exit edge known to be frequently taken,
+; we should not transform this loop.
+
+; A loop having a hot exit edge
+define signext i64 @func() {
+; CHECK: @func
+; CHECK-NOT: mtctr
+; CHECK-NOT: bdnz
+
+entry:
+  %a = alloca [1000 x i32], align 4
+  %0 = bitcast [1000 x i32]* %a to i8*
+  br label %for.body
+
+for.body:
+  %i.013 = phi i64 [ 0, %entry ], [ %inc, %if.end ]
+  %b.012 = phi i64 [ 0, %entry ], [ %xor, %if.end ]
+  %arrayidx = getelementptr inbounds [1000 x i32], [1000 x i32]* %a, i64 0, i64 %i.013
+  %1 = load i32, i32* %arrayidx, align 4
+  %tobool = icmp eq i32 %1, 0
+  br i1 %tobool, label %if.end, label %cleanup, !prof !1
+
+if.end:
+  %xor = xor i64 %i.013, %b.012
+  %inc = add nuw nsw i64 %i.013, 1
+  %cmp = icmp ult i64 %inc, 1000
+  br i1 %cmp, label %for.body, label %cleanup
+
+cleanup:
+  %res = phi i64 [ %b.012, %for.body ], [ %xor, %if.end ]
+  ret i64 %res
+}
+
+; A loop having a cold exit edge
+define signext i64 @func2() {
+; CHECK: @func2
+; CHECK: mtctr
+; CHECK: bdnz
+
+entry:
+  %a = alloca [1000 x i32], align 4
+  %0 = bitcast [1000 x i32]* %a to i8*
+  br label %for.body
+
+for.body:
+  %i.013 = phi i64 [ 0, %entry ], [ %inc, %if.end ]
+  %b.012 = phi i64 [ 0, %entry ], [ %xor, %if.end ]
+  %arrayidx = getelementptr inbounds [1000 x i32], [1000 x i32]* %a, i64 0, i64 %i.013
+  %1 = load i32, i32* %arrayidx, align 4
+  %tobool = icmp eq i32 %1, 0
+  br i1 %tobool, label %if.end, label %cleanup, !prof !2
+
+if.end:
+  %xor = xor i64 %i.013, %b.012
+  %inc = add nuw nsw i64 %i.013, 1
+  %cmp = icmp ult i64 %inc, 1000
+  br i1 %cmp, label %for.body, label %cleanup
+
+cleanup:
+  %res = phi i64 [ %b.012, %for.body ], [ %xor, %if.end ]
+  ret i64 %res
+}
+
+; A loop having an exit edge without profile data
+define signext i64 @func3() {
+; CHECK: @func3
+; CHECK: mtctr
+; CHECK: bdnz
+
+entry:
+  %a = alloca [1000 x i32], align 4
+  %0 = bitcast [1000 x i32]* %a to i8*
+  br label %for.body
+
+for.body:
+  %i.013 = phi i64 [ 0, %entry ], [ %inc, %if.end ]
+  %b.012 = phi i64 [ 0, %entry ], [ %xor, %if.end ]
+  %arrayidx = getelementptr inbounds [1000 x i32], [1000 x i32]* %a, i64 0, i64 %i.013
+  %1 = load i32, i32* %arrayidx, align 4
+  %tobool = icmp eq i32 %1, 0
+  br i1 %tobool, label %if.end, label %cleanup
+
+if.end:
+  %xor = xor i64 %i.013, %b.012
+  %inc = add nuw nsw i64 %i.013, 1
+  %cmp = icmp ult i64 %inc, 1000
+  br i1 %cmp, label %for.body, label %cleanup
+
+cleanup:
+  %res = phi i64 [ %b.012, %for.body ], [ %xor, %if.end ]
+  ret i64 %res
+}
+
+!1 = !{!"branch_weights", i32 1, i32 2000}
+!2 = !{!"branch_weights", i32 2000, i32 1}