Teach IRCE to look at branch weights when recognizing range checks

Splitting a loop to make range checks redundant is profitable only if
the range check "never" fails. Make this fact a part of recognizing a
range check -- a branch is a range check only if it is expected to
pass (via branch_weights metadata).

Differential Revision: http://reviews.llvm.org/D7192



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@227249 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/Scalar/InductiveRangeCheckElimination.cpp

@@ -43,6 +43,7 @@
 
 #include "llvm/ADT/Optional.h"
 
+#include "llvm/Analysis/BranchProbabilityInfo.h"
 #include "llvm/Analysis/InstructionSimplify.h"
 #include "llvm/Analysis/LoopInfo.h"
 #include "llvm/Analysis/LoopPass.h"
@@ -169,7 +170,8 @@ public:
   /// Create an inductive range check out of BI if possible, else return
   /// nullptr.
   static InductiveRangeCheck *create(AllocatorTy &Alloc, BranchInst *BI,
-                                     Loop *L, ScalarEvolution &SE);
+                                     Loop *L, ScalarEvolution &SE,
+                                     BranchProbabilityInfo &BPI);
 };
 
 class InductiveRangeCheckElimination : public LoopPass {
@@ -187,6 +189,7 @@ public:
     AU.addRequiredID(LoopSimplifyID);
     AU.addRequiredID(LCSSAID);
     AU.addRequired<ScalarEvolution>();
+    AU.addRequired<BranchProbabilityInfo>();
   }
 
   bool runOnLoop(Loop *L, LPPassManager &LPM) override;
@@ -354,13 +357,20 @@ static bool SplitRangeCheckCondition(Loop *L, ScalarEvolution &SE,
   return true;
 }
 
+
 InductiveRangeCheck *
 InductiveRangeCheck::create(InductiveRangeCheck::AllocatorTy &A, BranchInst *BI,
-                            Loop *L, ScalarEvolution &SE) {
+                            Loop *L, ScalarEvolution &SE,
+                            BranchProbabilityInfo &BPI) {
 
   if (BI->isUnconditional() || BI->getParent() == L->getLoopLatch())
     return nullptr;
 
+  BranchProbability LikelyTaken(15, 16);
+
+  if (BPI.getEdgeProbability(BI->getParent(), (unsigned) 0) < LikelyTaken)
+    return nullptr;
+
   Value *Length = nullptr;
   const SCEV *IndexSCEV = nullptr;
 
@@ -1175,11 +1185,12 @@ bool InductiveRangeCheckElimination::runOnLoop(Loop *L, LPPassManager &LPM) {
   InductiveRangeCheck::AllocatorTy IRCAlloc;
   SmallVector<InductiveRangeCheck *, 16> RangeChecks;
   ScalarEvolution &SE = getAnalysis<ScalarEvolution>();
+  BranchProbabilityInfo &BPI = getAnalysis<BranchProbabilityInfo>();
 
   for (auto BBI : L->getBlocks())
     if (BranchInst *TBI = dyn_cast<BranchInst>(BBI->getTerminator()))
       if (InductiveRangeCheck *IRC =
-              InductiveRangeCheck::create(IRCAlloc, TBI, L, SE))
+          InductiveRangeCheck::create(IRCAlloc, TBI, L, SE, BPI))
         RangeChecks.push_back(IRC);
 
   if (RangeChecks.empty())

test/Transforms/IRCE/multiple-access-no-preloop.ll

@@ -13,13 +13,13 @@ define void @multiple_access_no_preloop(
   %idx = phi i32 [ 0, %entry ] , [ %idx.next, %in.bounds.b ]
   %idx.next = add i32 %idx, 1
   %abc.a = icmp slt i32 %idx, %len.a
-  br i1 %abc.a, label %in.bounds.a, label %out.of.bounds
+  br i1 %abc.a, label %in.bounds.a, label %out.of.bounds, !prof !1
 
  in.bounds.a:
   %addr.a = getelementptr i32* %arr_a, i32 %idx
   store i32 0, i32* %addr.a
   %abc.b = icmp slt i32 %idx, %len.b
-  br i1 %abc.b, label %in.bounds.b, label %out.of.bounds
+  br i1 %abc.b, label %in.bounds.b, label %out.of.bounds, !prof !1
 
  in.bounds.b:
   %addr.b = getelementptr i32* %arr_b, i32 %idx
@@ -57,3 +57,4 @@ define void @multiple_access_no_preloop(
 ; CHECK: br i1 %next.postloop, label %loop.postloop, label %exit.loopexit
 
 !0 = !{i32 0, i32 2147483647}
+!1 = !{!"branch_weights", i32 64, i32 4}

test/Transforms/IRCE/not-likely-taken.ll

@@ -0,0 +1,40 @@
+; RUN: opt -verify-loop-info -irce-print-changed-loops -irce < %s 2>&1 | FileCheck %s
+
+; CHECK-NOT: constrained Loop
+
+define void @multiple_access_no_preloop(
+    i32* %arr_a, i32* %a_len_ptr, i32* %arr_b, i32* %b_len_ptr, i32 %n) {
+
+ entry:
+  %len.a = load i32* %a_len_ptr, !range !0
+  %len.b = load i32* %b_len_ptr, !range !0
+  %first.itr.check = icmp sgt i32 %n, 0
+  br i1 %first.itr.check, label %loop, label %exit
+
+ loop:
+  %idx = phi i32 [ 0, %entry ] , [ %idx.next, %in.bounds.b ]
+  %idx.next = add i32 %idx, 1
+  %abc.a = icmp slt i32 %idx, %len.a
+  br i1 %abc.a, label %in.bounds.a, label %out.of.bounds, !prof !1
+
+ in.bounds.a:
+  %addr.a = getelementptr i32* %arr_a, i32 %idx
+  store i32 0, i32* %addr.a
+  %abc.b = icmp slt i32 %idx, %len.b
+  br i1 %abc.b, label %in.bounds.b, label %out.of.bounds, !prof !1
+
+ in.bounds.b:
+  %addr.b = getelementptr i32* %arr_b, i32 %idx
+  store i32 -1, i32* %addr.b
+  %next = icmp slt i32 %idx.next, %n
+  br i1 %next, label %loop, label %exit
+
+ out.of.bounds:
+  ret void
+
+ exit:
+  ret void
+}
+
+!0 = !{i32 0, i32 2147483647}
+!1 = !{!"branch_weights", i32 1, i32 1}

test/Transforms/IRCE/single-access-no-preloop.ll

@@ -10,7 +10,7 @@ define void @single_access_no_preloop_no_offset(i32 *%arr, i32 *%a_len_ptr, i32
   %idx = phi i32 [ 0, %entry ] , [ %idx.next, %in.bounds ]
   %idx.next = add i32 %idx, 1
   %abc = icmp slt i32 %idx, %len
-  br i1 %abc, label %in.bounds, label %out.of.bounds
+  br i1 %abc, label %in.bounds, label %out.of.bounds, !prof !1
 
  in.bounds:
   %addr = getelementptr i32* %arr, i32 %idx
@@ -65,7 +65,7 @@ define void @single_access_no_preloop_with_offset(i32 *%arr, i32 *%a_len_ptr, i3
   %idx.next = add i32 %idx, 1
   %idx.for.abc = add i32 %idx, 4
   %abc = icmp slt i32 %idx.for.abc, %len
-  br i1 %abc, label %in.bounds, label %out.of.bounds
+  br i1 %abc, label %in.bounds, label %out.of.bounds, !prof !1
 
  in.bounds:
   %addr = getelementptr i32* %arr, i32 %idx.for.abc
@@ -108,3 +108,4 @@ define void @single_access_no_preloop_with_offset(i32 *%arr, i32 *%a_len_ptr, i3
 ; CHECK: br i1 %next.postloop, label %loop.postloop, label %exit.loopexit
 
 !0 = !{i32 0, i32 2147483647}
+!1 = !{!"branch_weights", i32 64, i32 4}

test/Transforms/IRCE/single-access-with-preloop.ll

@@ -13,7 +13,7 @@ define void @single_access_with_preloop(i32 *%arr, i32 *%a_len_ptr, i32 %n, i32
   %abc.high = icmp slt i32 %array.idx, %len
   %abc.low = icmp sge i32 %array.idx, 0
   %abc = and i1 %abc.low, %abc.high
-  br i1 %abc, label %in.bounds, label %out.of.bounds
+  br i1 %abc, label %in.bounds, label %out.of.bounds, !prof !1
 
  in.bounds:
   %addr = getelementptr i32* %arr, i32 %array.idx
@@ -57,3 +57,4 @@ define void @single_access_with_preloop(i32 *%arr, i32 *%a_len_ptr, i32 %n, i32
 ; CHECK: br i1 %next.postloop, label %loop.postloop, label %exit.loopexit
 
 !0 = !{i32 0, i32 2147483647}
+!1 = !{!"branch_weights", i32 64, i32 4}

test/Transforms/IRCE/with-parent-loops.ll

@@ -16,7 +16,7 @@ loop:                                             ; preds = %in.bounds, %entry
   %idx = phi i32 [ 0, %entry ], [ %idx.next, %in.bounds ]
   %idx.next = add i32 %idx, 1
   %abc = icmp slt i32 %idx, %len
-  br i1 %abc, label %in.bounds, label %out.of.bounds
+  br i1 %abc, label %in.bounds, label %out.of.bounds, !prof !1
 
 in.bounds:                                        ; preds = %loop
   %addr = getelementptr i32* %arr, i32 %idx
@@ -50,7 +50,7 @@ loop.i:                                           ; preds = %in.bounds.i, %loop
   %idx.i = phi i32 [ 0, %loop ], [ %idx.next.i, %in.bounds.i ]
   %idx.next.i = add i32 %idx.i, 1
   %abc.i = icmp slt i32 %idx.i, %len.i
-  br i1 %abc.i, label %in.bounds.i, label %out.of.bounds.i
+  br i1 %abc.i, label %in.bounds.i, label %out.of.bounds.i, !prof !1
 
 in.bounds.i:                                      ; preds = %loop.i
   %addr.i = getelementptr i32* %arr, i32 %idx.i
@@ -96,7 +96,7 @@ loop.i.i:                                         ; preds = %in.bounds.i.i, %loo
   %idx.i.i = phi i32 [ 0, %loop.i ], [ %idx.next.i.i, %in.bounds.i.i ]
   %idx.next.i.i = add i32 %idx.i.i, 1
   %abc.i.i = icmp slt i32 %idx.i.i, %len.i.i
-  br i1 %abc.i.i, label %in.bounds.i.i, label %out.of.bounds.i.i
+  br i1 %abc.i.i, label %in.bounds.i.i, label %out.of.bounds.i.i, !prof !1
 
 in.bounds.i.i:                                    ; preds = %loop.i.i
   %addr.i.i = getelementptr i32* %arr, i32 %idx.i.i
@@ -140,7 +140,7 @@ loop.i:                                           ; preds = %in.bounds.i, %loop
   %idx.i = phi i32 [ 0, %loop ], [ %idx.next.i, %in.bounds.i ]
   %idx.next.i = add i32 %idx.i, 1
   %abc.i = icmp slt i32 %idx.i, %len.i
-  br i1 %abc.i, label %in.bounds.i, label %out.of.bounds.i
+  br i1 %abc.i, label %in.bounds.i, label %out.of.bounds.i, !prof !1
 
 in.bounds.i:                                      ; preds = %loop.i
   %addr.i = getelementptr i32* %arr, i32 %idx.i
@@ -163,7 +163,7 @@ loop.i6:                                          ; preds = %in.bounds.i9, %inne
   %idx.i3 = phi i32 [ 0, %inner_loop.exit ], [ %idx.next.i4, %in.bounds.i9 ]
   %idx.next.i4 = add i32 %idx.i3, 1
   %abc.i5 = icmp slt i32 %idx.i3, %len.i1
-  br i1 %abc.i5, label %in.bounds.i9, label %out.of.bounds.i10
+  br i1 %abc.i5, label %in.bounds.i9, label %out.of.bounds.i10, !prof !1
 
 in.bounds.i9:                                     ; preds = %loop.i6
   %addr.i7 = getelementptr i32* %arr, i32 %idx.i3
@@ -210,7 +210,7 @@ loop.i.i:                                         ; preds = %in.bounds.i.i, %loo
   %idx.i.i = phi i32 [ 0, %loop.i ], [ %idx.next.i.i, %in.bounds.i.i ]
   %idx.next.i.i = add i32 %idx.i.i, 1
   %abc.i.i = icmp slt i32 %idx.i.i, %len.i.i
-  br i1 %abc.i.i, label %in.bounds.i.i, label %out.of.bounds.i.i
+  br i1 %abc.i.i, label %in.bounds.i.i, label %out.of.bounds.i.i, !prof !1
 
 in.bounds.i.i:                                    ; preds = %loop.i.i
   %addr.i.i = getelementptr i32* %arr, i32 %idx.i.i
@@ -242,7 +242,7 @@ loop.i.i10:                                       ; preds = %in.bounds.i.i13, %l
   %idx.i.i7 = phi i32 [ 0, %loop.i6 ], [ %idx.next.i.i8, %in.bounds.i.i13 ]
   %idx.next.i.i8 = add i32 %idx.i.i7, 1
   %abc.i.i9 = icmp slt i32 %idx.i.i7, %len.i.i4
-  br i1 %abc.i.i9, label %in.bounds.i.i13, label %out.of.bounds.i.i14
+  br i1 %abc.i.i9, label %in.bounds.i.i13, label %out.of.bounds.i.i14, !prof !1
 
 in.bounds.i.i13:                                  ; preds = %loop.i.i10
   %addr.i.i11 = getelementptr i32* %arr, i32 %idx.i.i7
@@ -286,7 +286,7 @@ loop.i:                                           ; preds = %in.bounds.i, %loop
   %idx.i = phi i32 [ 0, %loop ], [ %idx.next.i, %in.bounds.i ]
   %idx.next.i = add i32 %idx.i, 1
   %abc.i = icmp slt i32 %idx.i, %len.i
-  br i1 %abc.i, label %in.bounds.i, label %out.of.bounds.i
+  br i1 %abc.i, label %in.bounds.i, label %out.of.bounds.i, !prof !1
 
 in.bounds.i:                                      ; preds = %loop.i
   %addr.i = getelementptr i32* %arr, i32 %idx.i
@@ -315,7 +315,7 @@ loop.i.i:                                         ; preds = %in.bounds.i.i, %loo
   %idx.i.i = phi i32 [ 0, %loop.i4 ], [ %idx.next.i.i, %in.bounds.i.i ]
   %idx.next.i.i = add i32 %idx.i.i, 1
   %abc.i.i = icmp slt i32 %idx.i.i, %len.i.i
-  br i1 %abc.i.i, label %in.bounds.i.i, label %out.of.bounds.i.i
+  br i1 %abc.i.i, label %in.bounds.i.i, label %out.of.bounds.i.i, !prof !1
 
 in.bounds.i.i:                                    ; preds = %loop.i.i
   %addr.i.i = getelementptr i32* %arr, i32 %idx.i.i
@@ -342,3 +342,4 @@ exit:                                             ; preds = %with_parent.exit
 attributes #0 = { alwaysinline }
 
 !0 = !{i32 0, i32 2147483647}
+!1 = !{!"branch_weights", i32 64, i32 4}