From 823c70d06269a75f5877093b1e797bebfbc27380 Mon Sep 17 00:00:00 2001 From: Sebastian Pop Date: Tue, 2 Sep 2025 21:18:57 -0500 Subject: [PATCH] [unroll-and-jam] Document dependencies_multidims.ll and fix loop bounds (NFC) - Add detailed comments explaining why each function should/shouldn't be unroll-and-jammed based on memory access patterns and dependencies. - Fix loop bounds to ensure array accesses are within array bounds: * sub_sub_less: j starts from 1 (not 0) to ensure j-1 >= 0 * sub_sub_less_3d: k starts from 1 (not 0) to ensure k-1 >= 0 * sub_sub_outer_scalar: j starts from 1 (not 0) to ensure j-1 >= 0 --- .../dependencies_multidims.ll | 64 +++++++++++++++---- 1 file changed, 52 insertions(+), 12 deletions(-) diff --git a/llvm/test/Transforms/LoopUnrollAndJam/dependencies_multidims.ll b/llvm/test/Transforms/LoopUnrollAndJam/dependencies_multidims.ll index b95bbddf11d65..2a2b8c958b31d 100644 --- a/llvm/test/Transforms/LoopUnrollAndJam/dependencies_multidims.ll +++ b/llvm/test/Transforms/LoopUnrollAndJam/dependencies_multidims.ll @@ -1,11 +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" - ; 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 @@ -16,7 +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 @@ -47,6 +53,14 @@ 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: A[i+1][j] from iteration (i,j) doesn't conflict with +; any A[i'][j'] from unrolled j iterations since j' values are different and +; i+1 from current doesn't overlap with i' from unrolled iterations. define void @sub_sub_eq(ptr noalias nocapture %A, i32 %N, ptr noalias nocapture readonly %B) { entry: %cmp = icmp sgt i32 %N, 0 @@ -88,6 +102,14 @@ 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: A[i+1][j+1] from iteration (i,j) doesn't conflict with +; any A[i'][j'] from unrolled j iterations since the forward dependency pattern +; doesn't create overlapping accesses between unrolled iterations. define void @sub_sub_more(ptr noalias nocapture %A, i32 %N, ptr noalias nocapture readonly %B) { entry: %cmp = icmp sgt i32 %N, 0 @@ -126,12 +148,21 @@ 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) { @@ -147,13 +178,13 @@ 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 @@ -175,7 +206,16 @@ for.end: ; CHECK-LABEL: sub_sub_outer_scalar ; CHECK: %k = phi ; CHECK-NOT: %k.1 = phi - +; +; sub_sub_outer_scalar should NOT be unroll-and-jammed due to a loop-carried dependency. +; 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 in the j dimension. The test attempts to unroll-and-jam the j loop +; with the k loop being jammed. When this happens, iterations j, j+1, j+2, j+3 +; would be unrolled and their k loops jammed together, but j+1's write to A[j][k] +; would conflict with j's read from A[j][k], violating sequential semantics. define void @sub_sub_outer_scalar(ptr %A) { entry: br label %for.i @@ -185,7 +225,7 @@ 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: