[PATCH] D141361: [IndVars] Improve handling of multi-exit loops with known symbolic counts

[Max Kazantsev via Phabricator via llvm-commits]

mkazantsev updated this revision to Diff 488074.
mkazantsev added a comment.

Typo fix


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D141361/new/

https://reviews.llvm.org/D141361

Files:
  llvm/lib/Transforms/Scalar/IndVarSimplify.cpp
  llvm/test/Transforms/IndVarSimplify/turn-to-invariant.ll


Index: llvm/test/Transforms/IndVarSimplify/turn-to-invariant.ll
===================================================================
--- llvm/test/Transforms/IndVarSimplify/turn-to-invariant.ll
+++ llvm/test/Transforms/IndVarSimplify/turn-to-invariant.ll
@@ -842,13 +842,14 @@
   ret i32 -3
 }
 
-; TODO: Same as test_litter_conditions, but an extra check with known exact exit count is preventing the opt.
+; Same as test_litter_conditions, but an extra check with known exact exit count is preventing the opt.
 define i32 @test_litter_conditions_constant(i32 %start, i32 %len) {
 ; CHECK-LABEL: @test_litter_conditions_constant(
 ; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[TMP0:%.*]] = add i32 [[START:%.*]], -1
 ; CHECK-NEXT:    br label [[LOOP:%.*]]
 ; CHECK:       loop:
-; CHECK-NEXT:    [[IV:%.*]] = phi i32 [ [[START:%.*]], [[ENTRY:%.*]] ], [ [[IV_NEXT:%.*]], [[BACKEDGE:%.*]] ]
+; CHECK-NEXT:    [[IV:%.*]] = phi i32 [ [[START]], [[ENTRY:%.*]] ], [ [[IV_NEXT:%.*]], [[BACKEDGE:%.*]] ]
 ; CHECK-NEXT:    [[CANONICAL_IV:%.*]] = phi i32 [ 0, [[ENTRY]] ], [ [[CANONICAL_IV_NEXT:%.*]], [[BACKEDGE]] ]
 ; CHECK-NEXT:    [[CONSTANT_CHECK:%.*]] = icmp ult i32 [[CANONICAL_IV]], 65635
 ; CHECK-NEXT:    br i1 [[CONSTANT_CHECK]], label [[CONSTANT_CHECK_PASSED:%.*]], label [[CONSTANT_CHECK_FAILED:%.*]]
@@ -858,10 +859,9 @@
 ; CHECK-NEXT:    [[AND_1:%.*]] = and i1 [[ZERO_CHECK]], [[FAKE_1]]
 ; CHECK-NEXT:    br i1 [[AND_1]], label [[RANGE_CHECK_BLOCK:%.*]], label [[FAILED_1:%.*]]
 ; CHECK:       range_check_block:
-; CHECK-NEXT:    [[IV_MINUS_1:%.*]] = add i32 [[IV]], -1
-; CHECK-NEXT:    [[RANGE_CHECK:%.*]] = icmp ult i32 [[IV_MINUS_1]], [[LEN:%.*]]
+; CHECK-NEXT:    [[RANGE_CHECK_FIRST_ITER:%.*]] = icmp ult i32 [[TMP0]], [[LEN:%.*]]
 ; CHECK-NEXT:    [[FAKE_2:%.*]] = call i1 @cond()
-; CHECK-NEXT:    [[AND_2:%.*]] = and i1 [[RANGE_CHECK]], [[FAKE_2]]
+; CHECK-NEXT:    [[AND_2:%.*]] = and i1 [[RANGE_CHECK_FIRST_ITER]], [[FAKE_2]]
 ; CHECK-NEXT:    br i1 [[AND_2]], label [[BACKEDGE]], label [[FAILED_2:%.*]]
 ; CHECK:       backedge:
 ; CHECK-NEXT:    [[IV_NEXT]] = add i32 [[IV]], -1
Index: llvm/lib/Transforms/Scalar/IndVarSimplify.cpp
===================================================================
--- llvm/lib/Transforms/Scalar/IndVarSimplify.cpp
+++ llvm/lib/Transforms/Scalar/IndVarSimplify.cpp
@@ -1408,8 +1408,17 @@
   }
 
   if (SkipLastIter) {
-    const SCEV *One = SE->getOne(MaxIter->getType());
-    MaxIter = SE->getMinusSCEV(MaxIter, One);
+    // Semantically skip last iter is "subtract 1, do not bother about unsigned
+    // wrap". getLoopInvariantExitCondDuringFirstIterations knows how to deal
+    // with umin in a smart way, but umin(a, b) - 1 will likely not simplify.
+    // So we manually construct umin(a - 1, b - 1).
+    SmallVector<const SCEV *, 4> Elements;
+    if (auto *UMin = dyn_cast<SCEVUMinExpr>(MaxIter)) {
+      for (auto *Op : UMin->operands())
+        Elements.push_back(SE->getMinusSCEV(Op, SE->getOne(Op->getType())));
+      MaxIter = SE->getUMinFromMismatchedTypes(Elements);
+    } else
+      MaxIter = SE->getMinusSCEV(MaxIter, SE->getOne(MaxIter->getType()));
   }
 
   // Check if there is a loop-invariant predicate equivalent to our check.
@@ -1726,6 +1735,19 @@
 
   bool Changed = false;
   bool SkipLastIter = false;
+  const SCEV *CurrMaxExit = SE->getCouldNotCompute();
+  auto UpdateSkipLastIter = [&](const SCEV *MaxExitCount) {
+    if (SkipLastIter || isa<SCEVCouldNotCompute>(MaxExitCount))
+      return;
+    if (isa<SCEVCouldNotCompute>(CurrMaxExit))
+      CurrMaxExit = MaxExitCount;
+    else
+      CurrMaxExit = SE->getUMinFromMismatchedTypes(CurrMaxExit, MaxExitCount);
+    // If the loop has more than 1 iteration, all further checks will be
+    // executed 1 iteration less.
+    if (CurrMaxExit == MaxBECount)
+      SkipLastIter = true;
+  };
   SmallSet<const SCEV *, 8> DominatingExactExitCounts;
   for (BasicBlock *ExitingBB : ExitingBlocks) {
     const SCEV *ExactExitCount = SE->getExitCount(L, ExitingBB);
@@ -1761,17 +1783,11 @@
         Changed = true;
       else if (SkipLastIter && OptimizeCond(true))
         Changed = true;
-      if (MaxBECount == MaxExitCount)
-        // If the loop has more than 1 iteration, all further checks will be
-        // executed 1 iteration less.
-        SkipLastIter = true;
+      UpdateSkipLastIter(MaxExitCount);
       continue;
     }
 
-    if (MaxBECount == MaxExitCount)
-      // If the loop has more than 1 iteration, all further checks will be
-      // executed 1 iteration less.
-      SkipLastIter = true;
+    UpdateSkipLastIter(ExactExitCount);
 
     // If we know we'd exit on the first iteration, rewrite the exit to
     // reflect this.  This does not imply the loop must exit through this


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