[LoopInterchange] Make isProfitableForVectorization slightly more conservative.

[fhahn]

Summary

After D43236, we started interchanging loops with empty dependence
matrices. In isProfitableForVectorization, we try to determine if
interchanging makes the loop dependences more friendly to the
vectorizer. If there are no dependences, we should not interchange,
based on that heuristic.

[mcrosier]

This make sense to me. LGTM.

[fhahn]

rebased

lib/Transforms/Scalar/LoopInterchange.cpp

for (auto &Row : DepMatrix) {
     if (Row[InnerLoopId] != 'S' && Row[InnerLoopId] != 'I')
       return false;
     // TODO: We need to improve this heuristic.
     if (Row[OuterLoopId] != '=')
       return false;
   }
   // If outer loop has dependence and inner loop is loop independent then it is
   // profitable to interchange to enable parallelism.
-  return true;
+  // If there are no dependences, interchanging will not improve anything.
+  return !DepMatrix.empty();
 }
 
 bool LoopInterchangeProfitability::isProfitable(unsigned InnerLoopId,
                                                 unsigned OuterLoopId,
                                                 CharMatrix &DepMatrix) {
   // TODO: Add better profitability checks.
   // e.g
   // 1) Construct dependency matrix and move the one with no loop carried dep

test/Transforms/LoopInterchange/interchange-no-deps.ll

-; RUN: opt < %s -loop-interchange -simplifycfg -S -pass-remarks=loop-interchange 2>&1 | FileCheck %s
-; CHECK: Loop interchanged with enclosing loop.
+; RUN: opt < %s -loop-interchange -simplifycfg -pass-remarks-output=%t \
+; RUN:     -pass-remarks=loop-interchange -pass-remarks-missed=loop-interchange -S
+; RUN: FileCheck -input-file %t %s
 
-; no_deps_interchange just access a single nested array and can be interchange.
 
-define i32 @no_deps_interchange([1024 x i32]* nocapture %Arr, i32 %k) local_unnamed_addr #0 {
+; no_deps_interchange just accesses a single nested array and can be interchange.
+; CHECK:      Name:       Interchanged
+; CHECK-NEXT: Function:   no_deps_interchange
+define i32 @no_deps_interchange([1024 x i32]* nocapture %Arr) local_unnamed_addr #0 {
 entry:
-  br label %for.body
+  br label %for1.header
 
-for.body:                                         ; preds = %entry, %for.cond.cleanup3
-  %indvars.iv19 = phi i64 [ 0, %entry ], [ %indvars.iv.next20, %for.cond.cleanup3 ]
-  br label %for.body4
+for1.header:                                         ; preds = %entry, %for1.inc
+  %indvars.iv19 = phi i64 [ 0, %entry ], [ %indvars.iv.next20, %for1.inc ]
+  br label %for2
 
-for.body4:                                        ; preds = %for.body, %for.body4
-  %indvars.iv = phi i64 [ 0, %for.body ], [ %indvars.iv.next, %for.body4 ]
+for2:                                        ; preds = %for1.header, %for2
+  %indvars.iv = phi i64 [ 0, %for1.header ], [ %indvars.iv.next, %for2 ]
   %arrayidx6 = getelementptr inbounds [1024 x i32], [1024 x i32]* %Arr, i64 %indvars.iv, i64 %indvars.iv19
   store i32 0, i32* %arrayidx6, align 4
   %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
   %exitcond = icmp ne i64 %indvars.iv.next, 1024
-  br i1 %exitcond, label %for.body4, label %for.cond.cleanup3
+  br i1 %exitcond, label %for2, label %for1.inc
 
-for.cond.cleanup3:                                ; preds = %for.body4
+for1.inc:
   %indvars.iv.next20 = add nuw nsw i64 %indvars.iv19, 1
   %exitcond21 = icmp ne i64 %indvars.iv.next20, 1024
-  br i1 %exitcond21, label %for.body, label %for.cond.cleanup
+  br i1 %exitcond21, label %for1.header, label %exit
 
+exit:                                 ; preds = %for1.inc
+  ret i32 0
+
+}
+
+; Only the inner loop induction variable is used for memory accesses.
+; Interchanging is not beneficial.
+; CHECK:      Name:       InterchangeNotProfitable
+; CHECK-NEXT: Function:   no_bad_order
+define i32 @no_bad_order(i32* %Arr) {
+entry:
+  br label %for1.header
+
+for1.header:                                         ; preds = %entry, %for1.inc
+  %indvars.iv19 = phi i64 [ 0, %entry ], [ %indvars.iv.next20, %for1.inc ]
+  br label %for2
+
+for2:                                        ; preds = %for1.header, %for2
+  %indvars.iv = phi i64 [ 0, %for1.header ], [ %indvars.iv.next, %for2 ]
+  %arrayidx6 = getelementptr inbounds i32, i32* %Arr, i64 %indvars.iv
+  store i32 0, i32* %arrayidx6, align 4
+  %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
+  %exitcond = icmp ne i64 %indvars.iv.next, 1024
+  br i1 %exitcond, label %for2, label %for1.inc
+
+for1.inc:
+  %indvars.iv.next20 = add nuw nsw i64 %indvars.iv19, 1
+  %exitcond21 = icmp ne i64 %indvars.iv.next20, 1024
+  br i1 %exitcond21, label %for1.header, label %exit
+
+exit:                                 ; preds = %for1.inc
+  ret i32 0
+}
+
+; No memory access using any induction variables, interchanging not beneficial.
+; CHECK:      Name:        InterchangeNotProfitable
+; CHECK-NEXT: Function:    no_mem_instrs
+define i32 @no_mem_instrs(i64* %ptr) {
+entry:
+  br label %for1.header
+
+for1.header:                                         ; preds = %entry, %for1.inc
+  %indvars.iv19 = phi i64 [ 0, %entry ], [ %indvars.iv.next20, %for1.inc ]
+  br label %for2
+
+for2:                                        ; preds = %for1.header, %for2
+  %indvars.iv = phi i64 [ 0, %for1.header ], [ %indvars.iv.next, %for2 ]
+  store i64 %indvars.iv, i64* %ptr, align 4
+  %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
+  %exitcond = icmp ne i64 %indvars.iv.next, 1024
+  br i1 %exitcond, label %for2, label %for1.inc
+
+for1.inc:
+  %indvars.iv.next20 = add nuw nsw i64 %indvars.iv19, 1
+  %exitcond21 = icmp ne i64 %indvars.iv.next20, 1024
+  br i1 %exitcond21, label %for1.header, label %exit
 
-for.cond.cleanup:                                 ; preds = %for.cond.cleanup3
+exit:                                 ; preds = %for1.inc
   ret i32 0
 }