[LoopUnroll] Fold add chains during unrolling

Loop unrolling tends to produce chains of
`%x1 = add %x0, 1; %x2 = add %x1, 1; ...` with one add per unrolled
iteration. This patch simplifies these adds to `%xN = add %x0, N`
directly during unrolling, rather than waiting for InstCombine to do so.

The motivation for this is that having a single add (rather than
an add chain) on the induction variable makes it a simple recurrence,
which we specially recognize in a number of places. This allows
InstCombine to directly perform folds with that knowledge, instead
of first folding the add chains, and then doing other folds in another
InstCombine iteration.

Due to the reduced number of InstCombine iterations, this also
results in a small compile-time improvement.

Differential Revision: https://reviews.llvm.org/D153540

1 files changed, 27 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/Utils/LoopUnroll.cpp b/llvm/lib/Transforms/Utils/LoopUnroll.cpp
index 29661cd..511dd61 100644
--- a/llvm/lib/Transforms/Utils/LoopUnroll.cpp
+++ b/llvm/lib/Transforms/Utils/LoopUnroll.cpp
@@ -45,6 +45,7 @@
 #include "llvm/IR/IntrinsicInst.h"
 #include "llvm/IR/Metadata.h"
 #include "llvm/IR/Module.h"
+#include "llvm/IR/PatternMatch.h"
 #include "llvm/IR/Use.h"
 #include "llvm/IR/User.h"
 #include "llvm/IR/ValueHandle.h"
@@ -216,6 +217,8 @@ void llvm::simplifyLoopAfterUnroll(Loop *L, bool SimplifyIVs, LoopInfo *LI,
                                    ScalarEvolution *SE, DominatorTree *DT,
                                    AssumptionCache *AC,
                                    const TargetTransformInfo *TTI) {
+  using namespace llvm::PatternMatch;
+
   // Simplify any new induction variables in the partially unrolled loop.
   if (SE && SimplifyIVs) {
     SmallVector<WeakTrackingVH, 16> DeadInsts;
@@ -241,6 +244,30 @@ void llvm::simplifyLoopAfterUnroll(Loop *L, bool SimplifyIVs, LoopInfo *LI,
           Inst.replaceAllUsesWith(V);
       if (isInstructionTriviallyDead(&Inst))
         DeadInsts.emplace_back(&Inst);
+
+      // Fold ((add X, C1), C2) to (add X, C1+C2). This is very common in
+      // unrolled loops, and handling this early allows following code to
+      // identify the IV as a "simple recurrence" without first folding away
+      // a long chain of adds.
+      {
+        Value *X;
+        const APInt *C1, *C2;
+        if (match(&Inst, m_Add(m_Add(m_Value(X), m_APInt(C1)), m_APInt(C2)))) {
+          auto *InnerI = dyn_cast<Instruction>(Inst.getOperand(0));
+          auto *InnerOBO = cast<OverflowingBinaryOperator>(Inst.getOperand(0));
+          bool SignedOverflow;
+          APInt NewC = C1->sadd_ov(*C2, SignedOverflow);
+          Inst.setOperand(0, X);
+          Inst.setOperand(1, ConstantInt::get(Inst.getType(), NewC));
+          Inst.setHasNoUnsignedWrap(Inst.hasNoUnsignedWrap() &&
+                                    InnerOBO->hasNoUnsignedWrap());
+          Inst.setHasNoSignedWrap(Inst.hasNoSignedWrap() &&
+                                  InnerOBO->hasNoSignedWrap() &&
+                                  !SignedOverflow);
+          if (InnerI && isInstructionTriviallyDead(InnerI))
+            DeadInsts.emplace_back(InnerI);
+        }
+      }
     }
     // We can't do recursive deletion until we're done iterating, as we might
     // have a phi which (potentially indirectly) uses instructions later in