1 files changed, 44 insertions, 32 deletions

diff --git a/clang/lib/Sema/SemaOpenMP.cpp b/clang/lib/Sema/SemaOpenMP.cpp
index 89eb2e26452..36048a38b99 100644
--- a/clang/lib/Sema/SemaOpenMP.cpp
+++ b/clang/lib/Sema/SemaOpenMP.cpp
@@ -5579,31 +5579,59 @@ checkOpenMPLoop(OpenMPDirectiveKind DKind, Expr *CollapseLoopCountExpr,
   Built.Updates.resize(NestedLoopCount);
   Built.Finals.resize(NestedLoopCount);
   {
-    ExprResult Div;
-    // Go from inner nested loop to outer.
-    for (int Cnt = NestedLoopCount - 1; Cnt >= 0; --Cnt) {
+    // We implement the following algorithm for obtaining the
+    // original loop iteration variable values based on the
+    // value of the collapsed loop iteration variable IV.
+    //
+    // Let n+1 be the number of collapsed loops in the nest.
+    // Iteration variables (I0, I1, .... In)
+    // Iteration counts (N0, N1, ... Nn)
+    //
+    // Acc = IV;
+    //
+    // To compute Ik for loop k, 0 <= k <= n, generate:
+    //    Prod = N(k+1) * N(k+2) * ... * Nn;
+    //    Ik = Acc / Prod;
+    //    Acc -= Ik * Prod;
+    //
+    ExprResult Acc = IV;
+    for (unsigned int Cnt = 0; Cnt < NestedLoopCount; ++Cnt) {
       LoopIterationSpace &IS = IterSpaces[Cnt];
       SourceLocation UpdLoc = IS.IncSrcRange.getBegin();
-      // Build: Iter = (IV / Div) % IS.NumIters
-      // where Div is product of previous iterations' IS.NumIters.
       ExprResult Iter;
-      if (Div.isUsable()) {
-        Iter =
-            SemaRef.BuildBinOp(CurScope, UpdLoc, BO_Div, IV.get(), Div.get());
-      } else {
-        Iter = IV;
-        assert((Cnt == (int)NestedLoopCount - 1) &&
-               "unusable div expected on first iteration only");
-      }
 
-      if (Cnt != 0 && Iter.isUsable())
-        Iter = SemaRef.BuildBinOp(CurScope, UpdLoc, BO_Rem, Iter.get(),
-                                  IS.NumIterations);
+      // Compute prod
+      ExprResult Prod =
+          SemaRef.ActOnIntegerConstant(SourceLocation(), 1).get();
+      for (unsigned int K = Cnt+1; K < NestedLoopCount; ++K)
+        Prod = SemaRef.BuildBinOp(CurScope, UpdLoc, BO_Mul, Prod.get(),
+                                  IterSpaces[K].NumIterations);
+
+      // Iter = Acc / Prod
+      // If there is at least one more inner loop to avoid
+      // multiplication by 1.
+      if (Cnt + 1 < NestedLoopCount)
+        Iter = SemaRef.BuildBinOp(CurScope, UpdLoc, BO_Div,
+                                  Acc.get(), Prod.get());
+      else
+        Iter = Acc;
       if (!Iter.isUsable()) {
         HasErrors = true;
         break;
       }
 
+      // Update Acc:
+      // Acc -= Iter * Prod
+      // Check if there is at least one more inner loop to avoid
+      // multiplication by 1.
+      if (Cnt + 1 < NestedLoopCount)
+        Prod = SemaRef.BuildBinOp(CurScope, UpdLoc, BO_Mul,
+                                  Iter.get(), Prod.get());
+      else
+        Prod = Iter;
+      Acc = SemaRef.BuildBinOp(CurScope, UpdLoc, BO_Sub,
+                               Acc.get(), Prod.get());
+
       // Build update: IS.CounterVar(Private) = IS.Start + Iter * IS.Step
       auto *VD = cast<VarDecl>(cast<DeclRefExpr>(IS.CounterVar)->getDecl());
       DeclRefExpr *CounterVar = buildDeclRefExpr(
@@ -5632,22 +5660,6 @@ checkOpenMPLoop(OpenMPDirectiveKind DKind, Expr *CollapseLoopCountExpr,
         break;
       }
 
-      // Build Div for the next iteration: Div <- Div * IS.NumIters
-      if (Cnt != 0) {
-        if (Div.isUnset())
-          Div = IS.NumIterations;
-        else
-          Div = SemaRef.BuildBinOp(CurScope, UpdLoc, BO_Mul, Div.get(),
-                                   IS.NumIterations);
-
-        // Add parentheses (for debugging purposes only).
-        if (Div.isUsable())
-          Div = tryBuildCapture(SemaRef, Div.get(), Captures);
-        if (!Div.isUsable()) {
-          HasErrors = true;
-          break;
-        }
-      }
       if (!Update.isUsable() || !Final.isUsable()) {
         HasErrors = true;
         break;