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[unroll-and-jam] Document dependencies_multidims.ll and fix loop bounds (NFC) #156578

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sebpop merged 2 commits into from
Nov 21, 2025
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[unroll-and-jam] Document dependencies_multidims.ll and fix loop bounds (NFC) #156578

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[unroll-and-jam] Document dependencies_multidims.ll and fix loop boun…
sebpop Sep 3, 2025
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Merge branch 'main' into unj-doc
sebpop Nov 21, 2025
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118 changes: 101 additions & 17 deletions llvm/test/Transforms/LoopUnrollAndJam/dependencies_multidims.ll
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Original file line number Diff line number Diff line change
@@ -1,15 +1,17 @@
; RUN: opt -da-disable-delinearization-checks -passes=loop-unroll-and-jam -allow-unroll-and-jam -unroll-and-jam-count=4 < %s -S | FileCheck %s
; RUN: opt -da-disable-delinearization-checks -aa-pipeline=basic-aa -passes='loop-unroll-and-jam' -allow-unroll-and-jam -unroll-and-jam-count=4 < %s -S | FileCheck %s

target datalayout = "e-m:e-p:32:32-i64:64-v128:64:128-a:0:32-n32-S64"

; XFAIL: *
; The transformation seems to have succeeded "accidentally". It should be fixed
; by PR #156578.

; CHECK-LABEL: sub_sub_less
; CHECK: %j = phi
; CHECK-NOT: %j.1 = phi
;
; sub_sub_less should NOT be unroll-and-jammed due to a loop-carried dependency.
; Memory accesses:
; - A[i][j] = 1 (write to current iteration)
; - A[i+1][j-1] = add (write to next i iteration, previous j iteration)
; The dependency: A[i+1][j-1] from iteration (i,j) may conflict with A[i'][j']
; from a later iteration when i'=i+1 and j'=j-1, creating a backward dependency
; in the j dimension that prevents safe unroll-and-jam.
define void @sub_sub_less(ptr noalias nocapture %A, i32 %N, ptr noalias nocapture readonly %B) {
entry:
%cmp = icmp sgt i32 %N, 0
Expand All @@ -20,7 +22,7 @@ for.outer:
br label %for.inner

for.inner:
%j = phi i32 [ %add6, %for.inner ], [ 0, %for.outer ]
%j = phi i32 [ %add6, %for.inner ], [ 1, %for.outer ]
%sum = phi i32 [ %add, %for.inner ], [ 0, %for.outer ]
%arrayidx5 = getelementptr inbounds i32, ptr %B, i32 %j
%0 = load i32, ptr %arrayidx5, align 4
Expand Down Expand Up @@ -51,6 +53,31 @@ cleanup:
; CHECK: %j.1 = phi
; CHECK: %j.2 = phi
; CHECK: %j.3 = phi
;
; sub_sub_eq SHOULD be unroll-and-jammed (count=4) as it's safe.
; Memory accesses:
; - A[i][j] = 1 (write to current iteration)
; - A[i+1][j] = add (write to next i iteration, same j iteration)
; No dependency conflict: When unroll-and-jamming with count=4, the i loop
; iterations (i, i+1, i+2, i+3) are unrolled and their j loops are jammed
; together. Unroll-and-jam factor 4:
;
; for (int i = 0; i < N; i += 4)
; for (int j = 0; j < N; ++j) {
; // i iteration
; A[i][j] = 1; A[i+1][j] = sum_i;
; // i+1 iteration
; A[i+1][j] = 1; A[i+2][j] = sum_i1;
; // i+2 iteration
; A[i+2][j] = 1; A[i+3][j] = sum_i2;
; // i+3 iteration
; A[i+3][j] = 1; A[i+4][j] = sum_i3;
; }
;
; A[i+1][j] from iteration i doesn't conflict with A[i'][j'] from unrolled
; iterations since each unrolled i iteration accesses its own row i+1, i+2, i+3.
; j' values are identical, but accesses happen to different rows in the same j
; iteration before moving to the next j value.
define void @sub_sub_eq(ptr noalias nocapture %A, i32 %N, ptr noalias nocapture readonly %B) {
entry:
%cmp = icmp sgt i32 %N, 0
Expand Down Expand Up @@ -92,6 +119,29 @@ cleanup:
; CHECK: %j.1 = phi
; CHECK: %j.2 = phi
; CHECK: %j.3 = phi
;
; sub_sub_more SHOULD be unroll-and-jammed (count=4) as it's safe.
; Memory accesses:
; - A[i][j] = 1 (write to current iteration)
; - A[i+1][j+1] = add (write to next i iteration, next j iteration)
; No dependency conflict: The forward dependency pattern (j+1 in i dimension)
; is safe. Unroll-and-jam factor 4:
;
; for (int i = 0; i < N; i += 4)
; for (int j = 0; j < N; ++j) {
; // i iteration
; A[i][j] = 1; A[i+1][j+1] = sum_i;
; // i+1 iteration
; A[i+1][j] = 1; A[i+2][j+1] = sum_i1;
; // i+2 iteration
; A[i+2][j] = 1; A[i+3][j+1] = sum_i2;
; // i+3 iteration
; A[i+3][j] = 1; A[i+4][j+1] = sum_i3;
; }
;
; A[i+1][j+1] from iteration i accesses row i+1 and column j+1, which is
; disjoint from the accesses in the same iteration. The forward dependency
; pattern doesn't create conflicts between unrolled i iterations.
define void @sub_sub_more(ptr noalias nocapture %A, i32 %N, ptr noalias nocapture readonly %B) {
entry:
%cmp = icmp sgt i32 %N, 0
Expand Down Expand Up @@ -130,12 +180,21 @@ cleanup:
; CHECK-LABEL: sub_sub_less_3d
; CHECK: %k = phi
; CHECK-NOT: %k.1 = phi

;
; sub_sub_less_3d should NOT be unroll-and-jammed due to a loop-carried dependency.
; Memory accesses:
; - A3d[i][j][k] = 0 (write to current iteration)
; - A3d[i+1][j][k-1] = 0 (write to next i iteration, previous k iteration)
; The dependency: A[i+1][j][k-1] from iteration (i,j,k) may conflict with
; A[i'][j'][k'] from a later iteration when i'=i+1 and k'=k-1, creating a
; backward dependency in the k dimension that prevents safe unroll-and-jam.
; This is a 3D version of the same pattern as sub_sub_less.
;
; for (long i = 0; i < 100; ++i)
; for (long j = 0; j < 100; ++j)
; for (long k = 0; k < 100; ++k) {
; A[i][j][k] = 0;
; A[i+1][j][k-1] = 0;
; for (long k = 1; k < 100; ++k) {
; A[i][j][k] = 5;
; A[i+1][j][k-1] = 10;
; }

define void @sub_sub_less_3d(ptr noalias %A) {
Expand All @@ -151,13 +210,13 @@ for.j:
br label %for.k

for.k:
%k = phi i32 [ 0, %for.j ], [ %inc.k, %for.k ]
%k = phi i32 [ 1, %for.j ], [ %inc.k, %for.k ]
%arrayidx = getelementptr inbounds [100 x [100 x i32]], ptr %A, i32 %i, i32 %j, i32 %k
store i32 0, ptr %arrayidx, align 4
store i32 5, ptr %arrayidx, align 4
%add.i = add nsw i32 %i, 1
%sub.k = add nsw i32 %k, -1
%arrayidx2 = getelementptr inbounds [100 x [100 x i32]], ptr %A, i32 %add.i, i32 %j, i32 %sub.k
store i32 0, ptr %arrayidx2, align 4
store i32 10, ptr %arrayidx2, align 4
%inc.k = add nsw i32 %k, 1
%cmp.k = icmp slt i32 %inc.k, 100
br i1 %cmp.k, label %for.k, label %for.j.latch
Expand All @@ -178,8 +237,33 @@ for.end:

; CHECK-LABEL: sub_sub_outer_scalar
; CHECK: %k = phi
; CHECK-NOT: %k.1 = phi

; CHECK: %k.1 = phi
; CHECK: %k.2 = phi
; CHECK: %k.3 = phi
;
; sub_sub_outer_scalar SHOULD be unroll-and-jammed (count=4) as it's safe.
; Memory accesses:
; - load from A[j][k] (read from current j iteration)
; - store to A[j-1][k] (write to previous j iteration)
; The dependency: reading A[j][k] and writing A[j-1][k] creates a backward
; dependency, but execution order is preserved. Unroll-and-jam factor 4:
;
; for (int i = 0; i < 100; i++)
; for (int j = 1; j < 100; j += 4)
; for (int k = 0; k < 100; k++) {
; // j iteration
; temp0 = A[j][k]; A[j-1][k] = temp0;
; // j+1 iteration
; temp1 = A[j+1][k]; A[j][k] = temp1;
; // j+2 iteration
; temp2 = A[j+2][k]; A[j+1][k] = temp2;
; // j+3 iteration
; temp3 = A[j+3][k]; A[j+2][k] = temp3;
; }
;
; All k iterations for each j iteration (including j+1, j+2, j+3) are completed
; before moving to the next j group, so j+1's store to A[j][k] doesn't conflict
; with j's load from A[j][k] because they happen in different k loop invocations.
define void @sub_sub_outer_scalar(ptr %A) {
entry:
br label %for.i
Expand All @@ -189,7 +273,7 @@ for.i:
br label %for.j

for.j:
%j = phi i64 [ 0, %for.i ], [ %inc.j, %for.j.latch ]
%j = phi i64 [ 1, %for.i ], [ %inc.j, %for.j.latch ]
br label %for.k

for.k:
Expand Down