GVN-hoist: fix hoistingFromAllPaths for loops (PR29034)

It is invalid to hoist stores or loads if they are not executed on all paths
from the hoisting point to the exit of the function. In the testcase, there are
paths in the loop that do not execute the stores or the loads, and so hoisting
them within the loop is unsafe.

The problem is that the current implementation of hoistingFromAllPaths is
incomplete: it walks all blocks dominated by the hoisting point, and does not
return false when the loop contains a path on which the hoisted ld/st is
not executed.

Differential Revision: https://reviews.llvm.org/D23843

llvm-svn: 279732

1 files changed, 46 insertions, 31 deletions

diff --git a/llvm/lib/Transforms/Scalar/GVNHoist.cpp b/llvm/lib/Transforms/Scalar/GVNHoist.cpp
index 53f76dfcfe9..e287ba71e0b 100644
--- a/llvm/lib/Transforms/Scalar/GVNHoist.cpp
+++ b/llvm/lib/Transforms/Scalar/GVNHoist.cpp
@@ -273,24 +273,32 @@ private:
     return false;
   }
 
-  // Return true when all paths from A to the end of the function pass through
-  // either B or C.
-  bool hoistingFromAllPaths(const BasicBlock *A, const BasicBlock *B,
-                            const BasicBlock *C) {
-    // We fully copy the WL in order to be able to remove items from it.
-    SmallPtrSet<const BasicBlock *, 2> WL;
-    WL.insert(B);
-    WL.insert(C);
-
-    for (auto It = df_begin(A), E = df_end(A); It != E;) {
-      // There exists a path from A to the exit of the function if we are still
-      // iterating in DF traversal and we removed all instructions from the work
-      // list.
-      if (WL.empty())
+  // Return true when a successor of BB dominates A.
+  bool successorDominate(const BasicBlock *BB, const BasicBlock *A) {
+    for (const BasicBlock *Succ : BB->getTerminator()->successors())
+      if (DT->dominates(Succ, A))
+        return true;
+
+    return false;
+  }
+
+  // Return true when all paths from HoistBB to the end of the function pass
+  // through one of the blocks in WL.
+  bool hoistingFromAllPaths(const BasicBlock *HoistBB,
+                            SmallPtrSetImpl<const BasicBlock *> &WL) {
+
+    // Copy WL as the loop will remove elements from it.
+    SmallPtrSet<const BasicBlock *, 2> WorkList(WL.begin(), WL.end());
+
+    for (auto It = df_begin(HoistBB), E = df_end(HoistBB); It != E;) {
+      // There exists a path from HoistBB to the exit of the function if we are
+      // still iterating in DF traversal and we removed all instructions from
+      // the work list.
+      if (WorkList.empty())
         return false;
 
       const BasicBlock *BB = *It;
-      if (WL.erase(BB)) {
+      if (WorkList.erase(BB)) {
         // Stop DFS traversal when BB is in the work list.
         It.skipChildren();
         continue;
@@ -300,6 +308,11 @@ private:
       if (!isGuaranteedToTransferExecutionToSuccessor(BB->getTerminator()))
         return false;
 
+      // When reaching the back-edge of a loop, there may be a path through the
+      // loop that does not pass through B or C before exiting the loop.
+      if (successorDominate(BB, HoistBB))
+        return false;
+
       // Increment DFS traversal when not skipping children.
       ++It;
     }
@@ -476,23 +489,21 @@ private:
     return true;
   }
 
-  // Return true when it is safe to hoist scalar instructions from BB1 and BB2
-  // to HoistBB.
-  bool safeToHoistScalar(const BasicBlock *HoistBB, const BasicBlock *BB1,
-                         const BasicBlock *BB2, int &NBBsOnAllPaths) {
-    // Check that the hoisted expression is needed on all paths.  When HoistBB
-    // already contains an instruction to be hoisted, the expression is needed
-    // on all paths.  Enable scalar hoisting at -Oz as it is safe to hoist
-    // scalars to a place where they are partially needed.
-    if (!OptForMinSize && BB1 != HoistBB &&
-        !hoistingFromAllPaths(HoistBB, BB1, BB2))
+  // Return true when it is safe to hoist scalar instructions from all blocks in
+  // WL to HoistBB.
+  bool safeToHoistScalar(const BasicBlock *HoistBB,
+                         SmallPtrSetImpl<const BasicBlock *> &WL,
+                         int &NBBsOnAllPaths) {
+    // Check that the hoisted expression is needed on all paths.  Enable scalar
+    // hoisting at -Oz as it is safe to hoist scalars to a place where they are
+    // partially needed.
+    if (!OptForMinSize && !hoistingFromAllPaths(HoistBB, WL))
       return false;
 
-    if (hasEHOnPath(HoistBB, BB1, NBBsOnAllPaths) ||
-        hasEHOnPath(HoistBB, BB2, NBBsOnAllPaths))
-      return false;
+    for (const BasicBlock *BB : WL)
+      if (hasEHOnPath(HoistBB, BB, NBBsOnAllPaths))
+        return false;
 
-    // Safe to hoist scalars from BB1 and BB2 to HoistBB.
     return true;
   }
 
@@ -542,8 +553,12 @@ private:
           NewHoistPt = NewHoistBB->getTerminator();
       }
 
+      SmallPtrSet<const BasicBlock *, 2> WL;
+      WL.insert(HoistBB);
+      WL.insert(BB);
+
       if (K == InsKind::Scalar) {
-        if (safeToHoistScalar(NewHoistBB, HoistBB, BB, NBBsOnAllPaths)) {
+        if (safeToHoistScalar(NewHoistBB, WL, NBBsOnAllPaths)) {
           // Extend HoistPt to NewHoistPt.
           HoistPt = NewHoistPt;
           HoistBB = NewHoistBB;
@@ -557,7 +572,7 @@ private:
         // loading from the same address: for instance there may be a branch on
         // which the address of the load may not be initialized.
         if ((HoistBB == NewHoistBB || BB == NewHoistBB ||
-             hoistingFromAllPaths(NewHoistBB, HoistBB, BB)) &&
+             hoistingFromAllPaths(NewHoistBB, WL)) &&
             // Also check that it is safe to move the load or store from HoistPt
             // to NewHoistPt, and from Insn to NewHoistPt.
             safeToHoistLdSt(NewHoistPt, HoistPt, UD, K, NBBsOnAllPaths) &&