[llvm] 0b774ac - [SLP] Make sure instructions are ordered when computing spill cost.

[Dávid Bolvanský via llvm-commits]

Author: Florian Hahn
Date: 2020-08-11T11:18:12+02:00
New Revision: 0b774acf1189f589140ad49e9146e1406be91ca1

URL: https://github.com/llvm/llvm-project/commit/0b774acf1189f589140ad49e9146e1406be91ca1
DIFF: https://github.com/llvm/llvm-project/commit/0b774acf1189f589140ad49e9146e1406be91ca1.diff

LOG: [SLP] Make sure instructions are ordered when computing spill cost.

The entries in VectorizableTree are not necessarily ordered by their
position in basic blocks. Collect them and order them by dominance so
later instructions are guaranteed to be visited first. For instructions
in different basic blocks, we only scan to the beginning of the block,
so their order does not matter, as long as all instructions in a basic
block are grouped together. Using dominance ensures a deterministic order.

The modified test case contains an example where we compute a wrong
spill cost (2) without this patch, even though there is no call between
any instruction in the bundle.

This seems to have limited practical impact, .e.g on X86 with a recent
Intel Xeon CPU with -O3 -march=native -flto on MultiSource,SPEC2000,SPEC2006
there are no binary changes.

Reviewed By: ABataev

Differential Revision: https://reviews.llvm.org/D82444

Added: 
    

Modified: 
    llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
    llvm/test/Transforms/SLPVectorizer/AArch64/spillcost-order.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp b/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
index 8410f7201c48..3502ae90d695 100644
--- a/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
+++ b/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
@@ -3768,11 +3768,24 @@ int BoUpSLP::getSpillCost() const {
   SmallPtrSet<Instruction*, 4> LiveValues;
   Instruction *PrevInst = nullptr;
 
+  // The entries in VectorizableTree are not necessarily ordered by their
+  // position in basic blocks. Collect them and order them by dominance so later
+  // instructions are guaranteed to be visited first. For instructions in
+  // 
diff erent basic blocks, we only scan to the beginning of the block, so
+  // their order does not matter, as long as all instructions in a basic block
+  // are grouped together. Using dominance ensures a deterministic order.
+  SmallVector<Instruction *, 16> OrderedScalars;
   for (const auto &TEPtr : VectorizableTree) {
     Instruction *Inst = dyn_cast<Instruction>(TEPtr->Scalars[0]);
     if (!Inst)
       continue;
+    OrderedScalars.push_back(Inst);
+  }
+  llvm::stable_sort(OrderedScalars, [this](Instruction *A, Instruction *B) {
+    return DT->dominates(B, A);
+  });
 
+  for (Instruction *Inst : OrderedScalars) {
     if (!PrevInst) {
       PrevInst = Inst;
       continue;

diff  --git a/llvm/test/Transforms/SLPVectorizer/AArch64/spillcost-order.ll b/llvm/test/Transforms/SLPVectorizer/AArch64/spillcost-order.ll
index 9286b7ce9a69..8e0ca4b29384 100644
--- a/llvm/test/Transforms/SLPVectorizer/AArch64/spillcost-order.ll
+++ b/llvm/test/Transforms/SLPVectorizer/AArch64/spillcost-order.ll
@@ -13,22 +13,19 @@ define void @test(i64* %ptr, i64* noalias %res) {
 ; CHECK-NEXT:    br label [[FOR_BODY:%.*]]
 ; CHECK:       for.body:
 ; CHECK-NEXT:    [[CALL_I_I:%.*]] = call i32* @get_ptr()
-; CHECK-NEXT:    [[L_0_0:%.*]] = load i32, i32* [[CALL_I_I]], align 2
 ; CHECK-NEXT:    [[GEP_1:%.*]] = getelementptr i32, i32* [[CALL_I_I]], i32 2
-; CHECK-NEXT:    [[L_1_0:%.*]] = load i32, i32* [[GEP_1]], align 2
-; CHECK-NEXT:    [[EXT_0_0:%.*]] = zext i32 [[L_0_0]] to i64
-; CHECK-NEXT:    [[EXT_1_0:%.*]] = zext i32 [[L_1_0]] to i64
-; CHECK-NEXT:    [[SUB_1:%.*]] = sub nsw i64 [[EXT_0_0]], [[EXT_1_0]]
 ; CHECK-NEXT:    [[GEP_2:%.*]] = getelementptr i32, i32* [[CALL_I_I]], i32 1
-; CHECK-NEXT:    [[L_0_1:%.*]] = load i32, i32* [[GEP_2]], align 2
+; CHECK-NEXT:    [[TMP0:%.*]] = bitcast i32* [[CALL_I_I]] to <2 x i32>*
+; CHECK-NEXT:    [[TMP1:%.*]] = load <2 x i32>, <2 x i32>* [[TMP0]], align 2
 ; CHECK-NEXT:    [[GEP_3:%.*]] = getelementptr i32, i32* [[CALL_I_I]], i32 3
-; CHECK-NEXT:    [[L_1_1:%.*]] = load i32, i32* [[GEP_3]], align 2
-; CHECK-NEXT:    [[EXT_0_1:%.*]] = zext i32 [[L_0_1]] to i64
-; CHECK-NEXT:    [[EXT_1_1:%.*]] = zext i32 [[L_1_1]] to i64
-; CHECK-NEXT:    [[SUB_2:%.*]] = sub nsw i64 [[EXT_0_1]], [[EXT_1_1]]
-; CHECK-NEXT:    store i64 [[SUB_1]], i64* [[RES:%.*]], align 8
-; CHECK-NEXT:    [[RES_1:%.*]] = getelementptr i64, i64* [[RES]], i64 1
-; CHECK-NEXT:    store i64 [[SUB_2]], i64* [[RES_1]], align 8
+; CHECK-NEXT:    [[TMP2:%.*]] = bitcast i32* [[GEP_1]] to <2 x i32>*
+; CHECK-NEXT:    [[TMP3:%.*]] = load <2 x i32>, <2 x i32>* [[TMP2]], align 2
+; CHECK-NEXT:    [[TMP4:%.*]] = zext <2 x i32> [[TMP1]] to <2 x i64>
+; CHECK-NEXT:    [[TMP5:%.*]] = zext <2 x i32> [[TMP3]] to <2 x i64>
+; CHECK-NEXT:    [[TMP6:%.*]] = sub nsw <2 x i64> [[TMP4]], [[TMP5]]
+; CHECK-NEXT:    [[RES_1:%.*]] = getelementptr i64, i64* [[RES:%.*]], i64 1
+; CHECK-NEXT:    [[TMP7:%.*]] = bitcast i64* [[RES]] to <2 x i64>*
+; CHECK-NEXT:    store <2 x i64> [[TMP6]], <2 x i64>* [[TMP7]], align 8
 ; CHECK-NEXT:    [[C:%.*]] = call i1 @cond()
 ; CHECK-NEXT:    br i1 [[C]], label [[FOR_BODY]], label [[EXIT:%.*]]
 ; CHECK:       exit: