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

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Merged
merged 5 commits into from
Sep 25, 2025
Merged

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

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24a7257
[LoopInterchange] Exit early when all loops have deps in all directions
kasuga-fj Jul 15, 2025
144582a
Merge branch 'main' into feature/loop-interchange-early-exit
kasuga-fj Aug 7, 2025
5502f32
Merge branch 'main' into loop-interchange-early-exit
kasuga-fj Sep 25, 2025
c51edff
fix test
kasuga-fj Sep 25, 2025
0e4e759
adjust comment
kasuga-fj Sep 25, 2025
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11 changes: 11 additions & 0 deletions llvm/lib/Transforms/Scalar/LoopInterchange.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -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 == '*'; })) {
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@madhur13490 madhur13490 Sep 25, 2025

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Can be reworded to the following for clarity.

// If all the elements of any direction vector have only '*', legality can't be proven.
// Exit early to save compile time.

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@kasuga-fj kasuga-fj Sep 25, 2025

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fixed, thanks

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()) {
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44 changes: 44 additions & 0 deletions llvm/test/Transforms/LoopInterchange/bail-out-all-deps.ll
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@@ -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
}
16 changes: 10 additions & 6 deletions llvm/test/Transforms/LoopInterchange/confused-dependence.ll
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@@ -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 [* *].
;;
Expand All @@ -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: ...
Comment on lines -13 to +19
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@kasuga-fj kasuga-fj Jul 16, 2025 •
edited
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Changed because the previous CHECK directives check the debug outputs, which are no longer printed due to this change.

define void @may_alias(ptr %p0, ptr %p1) {
entry:
br label %for.i.header
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10 changes: 5 additions & 5 deletions llvm/test/Transforms/LoopInterchange/legality-for-scalar-deps.ll
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Expand Up @@ -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
Expand Down Expand Up @@ -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
Expand Down Expand Up @@ -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
Expand Down Expand Up @@ -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:
Expand Down
6 changes: 3 additions & 3 deletions llvm/test/Transforms/LoopInterchange/loop-interchange-optimization-remarks.ll
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Expand Up @@ -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
Expand Down Expand Up @@ -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
Expand Down Expand Up @@ -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
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3 changes: 1 addition & 2 deletions llvm/test/Transforms/LoopInterchange/unique-dep-matrix.ll
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Expand Up @@ -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) {
Comment on lines -5 to +11
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@kasuga-fj kasuga-fj Jul 16, 2025

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Added noalias to all ptr arguments to eliminate the * * dependency.

entry:
%arrayidx7 = getelementptr inbounds i8, ptr %aa, i64 512
br label %for.cond1.preheader
Expand Down