[LoopInterchange] Handle confused dependence correctly (#140709)

This patch fixes the handling of a confused `Dependence` object. Such an
object doesn’t contain any information about dependencies, so we must
process it conservatively. However, it was converted into a direction
vector like `[I I ... I]`. As a result, it was treated as if there are
no loop-carried dependencies, which can lead to illegal loop exchanges.

Fixes #140238

Author: kasuga-fj

llvm/lib/Transforms/Scalar/LoopInterchange.cpp

@@ -209,6 +209,14 @@ static bool populateDependencyMatrix(CharMatrix &DepMatrix, unsigned Level,
             Direction = '*';
           Dep.push_back(Direction);
         }
+
+        // If the Dependence object doesn't have any information, fill the
+        // dependency vector with '*'.
+        if (D->isConfused()) {
+          assert(Dep.empty() && "Expected empty dependency vector");
+          Dep.assign(Level, '*');
+        }
+
         while (Dep.size() != Level) {
           Dep.push_back('I');
         }

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

@@ -0,0 +1,41 @@
+; REQUIRES: asserts
+; RUN: opt < %s -passes=loop-interchange -verify-dom-info -verify-loop-info \
+; RUN:     -disable-output -debug 2>&1 | FileCheck %s
+
+;; In the following case, p0 and p1 may alias, so the direction vector must be [* *].
+;;
+;; void may_alias(float *p0, float *p1) {
+;;   for (int i = 0; i < 4; i++)
+;;     for (int j = 0; j < 4; j++)
+;;       p0[4 * i + j] = p1[4 * j + i];
+;; }
+
+; CHECK:      Dependency matrix before interchange:
+; CHECK-NEXT: * *
+; CHECK-NEXT: Processing InnerLoopId = 1 and OuterLoopId = 0
+define void @may_alias(ptr %p0, ptr %p1) {
+entry:
+  br label %for.i.header
+
+for.i.header:
+  %i = phi i32 [ 0, %entry ], [ %i.inc, %for.i.latch ]
+  br label %for.j
+
+for.j:
+  %j = phi i32 [ 0, %for.i.header ], [ %j.inc, %for.j ]
+  %idx.0 = getelementptr inbounds [4 x [4 x float]], ptr %p0, i32 0, i32 %j, i32 %i
+  %idx.1 = getelementptr inbounds [4 x [4 x float]], ptr %p1, i32 0, i32 %i, i32 %j
+  %0 = load float, ptr %idx.0, align 4
+  store float %0, ptr %idx.1, align 4
+  %j.inc = add nuw nsw i32 %j, 1
+  %cmp.j = icmp slt i32 %j.inc, 4
+  br i1 %cmp.j, label %for.j, label %for.i.latch
+
+for.i.latch:
+  %i.inc = add nuw nsw i32 %i, 1
+  %cmp.i = icmp slt i32 %i.inc, 4
+  br i1 %cmp.i, label %for.i.header, label %exit
+
+exit:
+  ret void
+}

llvm/test/Transforms/LoopInterchange/pr43326-ideal-access-pattern.ll

@@ -4,7 +4,7 @@
 
 ; Triply nested loop, should be able to do interchange three times
 ; to get the ideal access pattern.
-; void f(int e[10][10][10], int f[10][10][10]) {
+; void f(int e[restrict 10][10][10], int f[restrict 10][10][10]) {
 ;   for (int a = 0; a < 10; a++) {
 ;     for (int b = 0; b < 10; b++) {
 ;       for (int c = 0; c < 10; c++) {
@@ -35,7 +35,7 @@
 ; REMARKS-NEXT: Name:            Interchanged
 ; REMARKS-NEXT: Function:        pr43326-triply-nested
 
-define void @pr43326-triply-nested(ptr %e, ptr %f) {
+define void @pr43326-triply-nested(ptr noalias %e, ptr noalias %f) {
 entry:
   br label %for.outermost.header
 

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

@@ -2,7 +2,7 @@
 ; RUN: opt < %s -passes=loop-interchange -S -debug 2>&1 | FileCheck %s
 
 ; CHECK:       Dependency matrix before interchange:
-; CHECK-NEXT:  I I
+; CHECK-NEXT:  * *
 ; CHECK-NEXT:  = *
 ; CHECK-NEXT:  < *
 ; CHECK-NEXT:  Processing InnerLoopId