[LoopInterchange] Bail out when finding a dependency with all * elements (#149049)

If a direction vector with all `*` elements, like `[* * *]`, is present,
it indicates that none of the loop pairs are legal to interchange. In
such cases, continuing the analysis is meaningless.
This patch introduces a check to detect such direction vectors and exits
early when one is found. This slightly reduces compile time.

Author: kasuga-fj

llvm/lib/Transforms/Scalar/LoopInterchange.cpp

@@ -260,6 +260,17 @@ static bool populateDependencyMatrix(CharMatrix &DepMatrix, unsigned Level,
           Dep.push_back('I');
         }
 
+        // If all the elements of any direction vector have only '*', legality
+        // can't be proven. Exit early to save compile time.
+        if (all_of(Dep, [](char C) { return C == '*'; })) {
+          ORE->emit([&]() {
+            return OptimizationRemarkMissed(DEBUG_TYPE, "Dependence",
+                                            L->getStartLoc(), L->getHeader())
+                   << "All loops have dependencies in all directions.";
+          });
+          return false;
+        }
+
         // Test whether the dependency is forward or not.
         bool IsKnownForward = true;
         if (Src->getParent() != Dst->getParent()) {

llvm/test/Transforms/LoopInterchange/bail-out-all-deps.ll

@@ -0,0 +1,44 @@
+; RUN: opt < %s -passes=loop-interchange -pass-remarks-output=%t \
+; RUN:     -disable-output
+; RUN: FileCheck -input-file %t %s
+
+; Check that loop interchange bails out early when finding a direction vector
+; with all '*' elements.
+;
+; for (int i = 0; i < 4; i++)
+;   for (int j = 0; j < 4; j++)
+;     A[i & val][j & val] = 0;
+
+; CHECK:      --- !Missed
+; CHECK-NEXT: Pass:            loop-interchange
+; CHECK-NEXT: Name:            Dependence
+; CHECK-NEXT: Function:        f
+; CHECK-NEXT: Args:
+; CHECK-NEXT:   - String:          All loops have dependencies in all directions.
+; CHECK-NEXT: ...
+define void @f(ptr %A, i64 %val) {
+entry:
+  br label %for.i.header
+
+for.i.header:
+  %i = phi i64 [ 0, %entry ], [ %i.next, %for.i.latch ]
+  br label %for.j
+
+for.j:
+  %j = phi i64 [ 0, %for.i.header ], [ %j.next, %for.j ]
+  %subscript.0 = and i64 %i, %val
+  %subscript.1 = and i64 %j, %val
+  %idx = getelementptr inbounds [4 x i8], ptr %A, i64 %subscript.0, i64 %subscript.1
+  store i8 0, ptr %idx
+  %j.next = add nuw nsw i64 %j, 1
+  %exit.j = icmp eq i64 %j.next, 4
+  br i1 %exit.j, label %for.i.latch, label %for.j
+
+for.i.latch:
+  %i.next = add nuw nsw i64 %i, 1
+  %exit.i = icmp eq i64 %i.next, 4
+  br i1 %exit.i, label %exit, label %for.i.header
+
+exit:
+  ret void
+}

llvm/test/Transforms/LoopInterchange/confused-dependence.ll

@@ -1,6 +1,6 @@
-; REQUIRES: asserts
-; RUN: opt < %s -passes=loop-interchange -verify-dom-info -verify-loop-info \
-; RUN:     -disable-output -debug 2>&1 | FileCheck %s
+; RUN: opt < %s -passes=loop-interchange -pass-remarks-output=%t \
+; RUN:     -disable-output
+; RUN: FileCheck -input-file %t %s
 
 ;; In the following case, p0 and p1 may alias, so the direction vector must be [* *].
 ;;
@@ -10,9 +10,13 @@
 ;;       p0[4 * i + j] = p1[4 * j + i];
 ;; }
 
-; CHECK:      Dependency matrix before interchange:
-; CHECK-NEXT: * *
-; CHECK-NEXT: Processing InnerLoopId = 1 and OuterLoopId = 0
+; CHECK:      --- !Missed
+; CHECK-NEXT: Pass:            loop-interchange
+; CHECK-NEXT: Name:            Dependence
+; CHECK-NEXT: Function:        may_alias
+; CHECK-NEXT: Args:
+; CHECK-NEXT:   - String:          All loops have dependencies in all directions.
+; CHECK-NEXT: ...
 define void @may_alias(ptr %p0, ptr %p1) {
 entry:
   br label %for.i.header

llvm/test/Transforms/LoopInterchange/legality-for-scalar-deps.ll

@@ -21,13 +21,13 @@
 ; CHECK-NEXT: Name:            Dependence
 ; CHECK-NEXT: Function:        issue46867
 ; CHECK-NEXT: Args:
-; CHECK-NEXT:  - String:       Cannot interchange loops due to dependences.
+; CHECK-NEXT:  - String:          All loops have dependencies in all directions.
 ; CHECK:      --- !Missed
 ; CHECK-NEXT: Pass:            loop-interchange
 ; CHECK-NEXT: Name:            Dependence
 ; CHECK-NEXT: Function:        issue46867
 ; CHECK-NEXT: Args:
-; CHECK-NEXT:  - String:       Cannot interchange loops due to dependences.
+; CHECK-NEXT:  - String:          All loops have dependencies in all directions.
 define void @issue46867(ptr noundef captures(none) %s, i32 noundef %c, ptr noundef readonly captures(none) %ff) {
 entry:
   %tobool7.not = icmp eq i32 %c, 0
@@ -121,7 +121,7 @@ land.end:
 ; CHECK-NEXT: Name:            Dependence
 ; CHECK-NEXT: Function:        issue47401
 ; CHECK-NEXT: Args:
-; CHECK-NEXT:  - String:       Cannot interchange loops due to dependences.
+; CHECK-NEXT:  - String:          All loops have dependencies in all directions.
 define void @issue47401(ptr noundef writeonly captures(none) %e, ptr noundef readonly captures(none) %bb) {
 entry:
   br label %for.cond1.preheader
@@ -175,7 +175,7 @@ land.end:
 ; CHECK-NEXT: Name:            Dependence
 ; CHECK-NEXT: Function:        issue47295
 ; CHECK-NEXT: Args:
-; CHECK-NEXT:  - String:       Cannot interchange loops due to dependences.
+; CHECK-NEXT:  - String:          All loops have dependencies in all directions.
 define void @issue47295(ptr noundef captures(none) %f, ptr noundef writeonly captures(none) %cc) {
 entry:
   br label %for.cond1.preheader
@@ -221,7 +221,7 @@ for.body4:
 ; CHECK-NEXT: Name:            Dependence
 ; CHECK-NEXT: Function:        issue54176
 ; CHECK-NEXT: Args:
-; CHECK-NEXT:  - String:       Cannot interchange loops due to dependences.
+; CHECK-NEXT:  - String:          All loops have dependencies in all directions.
 define void @issue54176(i32 noundef %n, i32 noundef %m, ptr noundef captures(none) %aa, ptr noundef readonly captures(none) %bb, ptr noundef writeonly captures(none) %cc) {
 
 entry:

llvm/test/Transforms/LoopInterchange/loop-interchange-optimization-remarks.ll

@@ -71,7 +71,7 @@ for.end19:
 ; CHECK-NEXT: Name:            Dependence
 ; CHECK-NEXT: Function:        test01
 ; CHECK-NEXT: Args:
-; CHECK-NEXT:   - String:          Cannot interchange loops due to dependences.
+; CHECK-NEXT:   - String:          All loops have dependencies in all directions.
 ; CHECK-NEXT: ...
 
 ; DELIN: --- !Analysis
@@ -147,7 +147,7 @@ define void @test02(i32 %k, i32 %N) {
 ; CHECK-NEXT: Name:            Dependence
 ; CHECK-NEXT: Function:        test02
 ; CHECK-NEXT: Args:
-; CHECK-NEXT:   - String:          Cannot interchange loops due to dependences.
+; CHECK-NEXT:   - String:          All loops have dependencies in all directions.
 ; CHECK-NEXT: ...
 
 ; DELIN: --- !Analysis
@@ -290,7 +290,7 @@ for.end17:
 ; CHECK-NEXT: Name:            Dependence
 ; CHECK-NEXT: Function:        test04
 ; CHECK-NEXT: Args:
-; CHECK-NEXT:   - String:          Cannot interchange loops due to dependences.
+; CHECK-NEXT:   - String:          All loops have dependencies in all directions.
 ; CHECK-NEXT: ...
 
 ; DELIN: --- !Missed

llvm/test/Transforms/LoopInterchange/unique-dep-matrix.ll

@@ -2,14 +2,13 @@
 ; RUN: opt < %s -passes=loop-interchange -S -debug 2>&1 | FileCheck %s
 
 ; CHECK:       Dependency matrix before interchange:
-; CHECK-NEXT:  * *
 ; CHECK-NEXT:  = *
 ; CHECK-NEXT:  < *
 ; CHECK-NEXT:  Processing InnerLoopId
 
 ; This example is taken from github issue #54176
 ;
-define void @foo(i32 noundef %n, i32 noundef %m, ptr nocapture noundef %aa, ptr nocapture noundef readonly %bb, ptr nocapture noundef writeonly %cc) {
+define void @foo(i32 noundef %n, i32 noundef %m, ptr nocapture noundef noalias %aa, ptr nocapture noundef readonly noalias %bb, ptr nocapture noundef writeonly noalias %cc) {
 entry:
   %arrayidx7 = getelementptr inbounds i8, ptr %aa, i64 512
   br label %for.cond1.preheader