[llvm] bdcf042 - [LoopIdiom] Don't transform loop into memmove when load from body has more than one use

[Dawid Jurczak via llvm-commits]

Author: Dawid Jurczak
Date: 2021-08-25T14:22:40+02:00
New Revision: bdcf04246c401aec9bdddf32fabc99fa4834a477

URL: https://github.com/llvm/llvm-project/commit/bdcf04246c401aec9bdddf32fabc99fa4834a477
DIFF: https://github.com/llvm/llvm-project/commit/bdcf04246c401aec9bdddf32fabc99fa4834a477.diff

LOG: [LoopIdiom] Don't transform loop into memmove when load from body has more than one use

This change fixes issue found by Markus: https://reviews.llvm.org/rG11338e998df1
Before this patch following code was transformed to memmove:

for (int i = 15; i >= 1; i--) {
  p[i] = p[i-1];
  sum += p[i-1];
}

However load from p[i-1] is used not only by store to p[i] but also by sum computation.
Therefore we cannot emit memmove in loop header.

Differential Revision: https://reviews.llvm.org/D107964

Added: 
    

Modified: 
    llvm/lib/Transforms/Scalar/LoopIdiomRecognize.cpp
    llvm/test/Transforms/LoopIdiom/basic.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Transforms/Scalar/LoopIdiomRecognize.cpp b/llvm/lib/Transforms/Scalar/LoopIdiomRecognize.cpp
index f57ee475fcc3e..45dee978e69b4 100644
--- a/llvm/lib/Transforms/Scalar/LoopIdiomRecognize.cpp
+++ b/llvm/lib/Transforms/Scalar/LoopIdiomRecognize.cpp
@@ -1318,6 +1318,11 @@ bool LoopIdiomRecognize::processLoopStoreOfLoopLoad(
       mayLoopAccessLocation(StoreBasePtr, ModRefInfo::ModRef, CurLoop, BECount,
                             StoreSizeSCEV, *AA, IgnoredInsts);
   if (UseMemMove) {
+    // For memmove case it's not enough to guarantee that loop doesn't access
+    // TheStore and TheLoad. Additionally we need to make sure that TheStore is
+    // the only user of TheLoad.
+    if (!TheLoad->hasOneUse())
+      return Changed;
     IgnoredInsts.insert(TheLoad);
     if (mayLoopAccessLocation(StoreBasePtr, ModRefInfo::ModRef, CurLoop,
                               BECount, StoreSizeSCEV, *AA, IgnoredInsts)) {

diff  --git a/llvm/test/Transforms/LoopIdiom/basic.ll b/llvm/test/Transforms/LoopIdiom/basic.ll
index ea6d607c8ef12..c8c6d187a0ccf 100644
--- a/llvm/test/Transforms/LoopIdiom/basic.ll
+++ b/llvm/test/Transforms/LoopIdiom/basic.ll
@@ -1300,14 +1300,10 @@ for.end:                                          ; preds = %for.body, %entry
   ret void
 }
 
-;; FIXME: Do not form memmove when load has more than one use.
+;; Do not form memmove when load has more than one use.
 define i32 @do_not_form_memmove5(i32* %p) {
 ; CHECK-LABEL: @do_not_form_memmove5(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    [[P2:%.*]] = bitcast i32* [[P:%.*]] to i8*
-; CHECK-NEXT:    [[SCEVGEP:%.*]] = getelementptr i32, i32* [[P:%.*]], i64 1
-; CHECK-NEXT:    [[SCEVGEP1:%.*]] = bitcast i32* [[SCEVGEP]] to i8*
-; CHECK-NEXT:    call void @llvm.memmove.p0i8.p0i8.i64(i8* align 4 [[SCEVGEP1]], i8* align 4 [[P2]], i64 60, i1 false)
 ; CHECK-NEXT:    br label [[FOR_BODY:%.*]]
 ; CHECK:       for.cond.cleanup:
 ; CHECK-NEXT:    [[ADD_LCSSA:%.*]] = phi i32 [ [[ADD:%.*]], [[FOR_BODY]] ]
@@ -1321,6 +1317,7 @@ define i32 @do_not_form_memmove5(i32* %p) {
 ; CHECK-NEXT:    [[TMP1:%.*]] = load i32, i32* [[ARRAYIDX]], align 4
 ; CHECK-NEXT:    [[IDXPROM:%.*]] = zext i32 [[INDEX]] to i64
 ; CHECK-NEXT:    [[ARRAYIDX2:%.*]] = getelementptr inbounds i32, i32* [[P:%.*]], i64 [[IDXPROM]]
+; CHECK-NEXT:    store i32 [[TMP1]], i32* [[ARRAYIDX2]], align 4
 ; CHECK-NEXT:    [[ADD]] = add nsw i32 [[TMP1]], [[SUM:%.*]]
 ; CHECK-NEXT:    [[CMP:%.*]] = icmp sgt i32 [[INDEX]], 1
 ; CHECK-NEXT:    br i1 [[CMP]], label [[FOR_BODY]], label [[FOR_COND_CLEANUP]]