[PATCH] D96708: [llvm] Bug fix: tightlyNested() of Loop interchange

[Masakazu Ueno via Phabricator via llvm-commits]

masakazu.ueno created this revision.
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There is an error in the determination of whether the loop is tightly nested. 
Therefore, the execution result may not be correct.
So I added a check to see if there are instructions between nested loops.

- Example

  #include <stdio.h>
  static   long  int i,j,k=3,m=3;
  unsigned long  int x[50];
  long double a[50][50];
  
  int main() {
  
  for (i=0; i<50; i++) for (j=0; j<50; j++) a[i][j]=i; for (i=0; i<50; i++) x[i]=1;
  
  j=0;
  for(i=40;i>1;i--) {
    k=20;
    while(k++<40) x[k]=x[j]%40;
    a[i][i]+=a[i][j]+a[j][i];
    for(m=9;;) if (--m<5) break;
  }
  
  for (i=0; i<50; i++) for (j=0; j<50; j++)
    printf("a:%ld %5.5Le \n",i,a[i][j]);
  
    return 0;
  }

*) Specified options are "-Ofast -mllvm -enable-loopinterchange=true -flegacy-pass-manager".

The difference between the results when loop interchange is enabled and disabled is shown below.

  $ clang -O2 -flegacy-pass-manager -mllvm -enable-loopinterchange=true -Rpass=loop-interchange bug1362_tightlynested.c -o a_error.out
  bug1362_tightlynested.c:13:3: remark: Loop interchanged with enclosing loop. [-Rpass=loop-interchange]
    while(k++<40) x[k]=x[j]%40;
    ^
  $ clang -O2 -flegacy-pass-manager -mllvm -enable-loopinterchange=false -Rpass=loop-interchange bug1362_tightlynested.c -o a_ok.out
  $ ./a_error.out > a_error.res
  $ ./a_ok.out > a_ok.res
  $ diff a_ok.res a_error.res 
  103c103
  < a:2 4.00000e+00 
  ---
  > a:2 4.20000e+01 
  154c154
  < a:3 6.00000e+00 
  ---
  > a:3 6.30000e+01 
  205c205
  < a:4 8.00000e+00 
  ---
  > a:4 8.40000e+01
   :
   Omit the following.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D96708

Files:
  llvm/lib/Transforms/Scalar/LoopInterchange.cpp


Index: llvm/lib/Transforms/Scalar/LoopInterchange.cpp
===================================================================
--- llvm/lib/Transforms/Scalar/LoopInterchange.cpp
+++ llvm/lib/Transforms/Scalar/LoopInterchange.cpp
@@ -347,6 +347,7 @@
 
 private:
   bool tightlyNested(Loop *Outer, Loop *Inner);
+  bool containsUnsafeInstructionsInInnerLoop(void);
   bool containsUnsafeInstructions(BasicBlock *BB);
 
   /// Discover induction and reduction PHIs in the header of \p L. Induction
@@ -578,6 +579,33 @@
 
 } // end anonymous namespace
 
+/// Returns true if there are unsafe instructions above or below
+/// the inner loop.
+bool LoopInterchangeLegality::containsUnsafeInstructionsInInnerLoop() {
+  BasicBlock *InnerLoopPreHeader = InnerLoop->getLoopPreheader();
+  BasicBlock *InnerLoopExit = InnerLoop->getExitBlock();
+  BasicBlock *OuterLoopHeader = OuterLoop->getHeader();
+  BasicBlock *OuterLoopLatch = OuterLoop->getLoopLatch();
+
+  // Check loop preheader.
+  if (InnerLoopPreHeader != OuterLoopHeader) {
+    for (Instruction &I : *InnerLoopPreHeader) {
+      if (!isa<BranchInst>(&I) && !isa<DbgInfoIntrinsic>(&I))
+        return true;
+    }
+  }
+  // Check loop latch
+  if (InnerLoopExit != OuterLoopLatch) {
+    for (Instruction &I : *InnerLoopExit) {
+      if (!isa<BranchInst>(&I) && !isa<PHINode>(&I) &&
+          !isa<DbgInfoIntrinsic>(&I))
+        return true;
+    }
+  }
+
+  return false;
+}
+
 bool LoopInterchangeLegality::containsUnsafeInstructions(BasicBlock *BB) {
   return any_of(*BB, [](const Instruction &I) {
     return I.mayHaveSideEffects() || I.mayReadFromMemory();
@@ -618,6 +646,9 @@
       containsUnsafeInstructions(InnerLoopPreHeader))
     return false;
 
+  if (containsUnsafeInstructionsInInnerLoop())
+    return false;
+
   LLVM_DEBUG(dbgs() << "Loops are perfectly nested\n");
   // We have a perfect loop nest.
   return true;


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