[DSE,MemorySSA] Account for ScanLimit == 0 on entry.

Currently the code does not account for the fact that getDomMemoryDef
can be called with ScanLimit == 0, if we reached the limit while
processing an earlier access. Also tighten the check a bit more and bump
the scan limit now that it is handled properly.

In some cases, this brings a 2x speedup in terms of compile-time.

diff --git a/llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp b/llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp
index 261fb38..faa936e 100644 (file)
--- a/llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp
+++ b/llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp
@@ -107,7 +107,7 @@ static cl::opt<bool>
                     cl::desc("Use the new MemorySSA-backed DSE."));
 
 static cl::opt<unsigned>
-    MemorySSAScanLimit("dse-memoryssa-scanlimit", cl::init(100), cl::Hidden,
+    MemorySSAScanLimit("dse-memoryssa-scanlimit", cl::init(150), cl::Hidden,
                        cl::desc("The number of memory instructions to scan for "
                                 "dead store elimination (default = 100)"));
 
@@ -1743,7 +1743,12 @@ struct DSEState {
   Optional<MemoryAccess *>
   getDomMemoryDef(MemoryDef *KillingDef, MemoryAccess *Current,
                   MemoryLocation DefLoc, bool DefVisibleToCallerBeforeRet,
-                  bool DefVisibleToCallerAfterRet, int &ScanLimit) const {
+                  bool DefVisibleToCallerAfterRet, unsigned &ScanLimit) const {
+    if (ScanLimit == 0) {
+      LLVM_DEBUG(dbgs() << "\n    ...  hit scan limit\n");
+      return None;
+    }
+
     MemoryAccess *DomAccess;
     bool StepAgain;
     LLVM_DEBUG(dbgs() << "  trying to get dominating access for " << *Current
@@ -1803,10 +1808,12 @@ struct DSEState {
       MemoryAccess *UseAccess = WorkList[I];
 
       LLVM_DEBUG(dbgs() << "   " << *UseAccess);
-      if (--ScanLimit == 0) {
+      // Bail out if the number of accesses to check exceeds the scan limit.
+      if (ScanLimit < (WorkList.size() - I)) {
         LLVM_DEBUG(dbgs() << "\n    ...  hit scan limit\n");
         return None;
       }
+      --ScanLimit;
 
       if (isa<MemoryPhi>(UseAccess)) {
         LLVM_DEBUG(dbgs() << "\n    ... adding PHI uses\n");
@@ -2154,7 +2161,7 @@ bool eliminateDeadStoresMemorySSA(Function &F, AliasAnalysis &AA,
     LLVM_DEBUG(dbgs() << "Trying to eliminate MemoryDefs killed by "
                       << *KillingDef << " (" << *SI << ")\n");
 
-    int ScanLimit = MemorySSAScanLimit;
+    unsigned ScanLimit = MemorySSAScanLimit;
     // Worklist of MemoryAccesses that may be killed by KillingDef.
     SetVector<MemoryAccess *> ToCheck;
     ToCheck.insert(KillingDef->getDefiningAccess());

diff --git a/llvm/test/Transforms/DeadStoreElimination/MSSA/memoryssa-scan-limit.ll b/llvm/test/Transforms/DeadStoreElimination/MSSA/memoryssa-scan-limit.ll
index c2b2eba..5ee6fe1 100644 (file)
--- a/llvm/test/Transforms/DeadStoreElimination/MSSA/memoryssa-scan-limit.ll
+++ b/llvm/test/Transforms/DeadStoreElimination/MSSA/memoryssa-scan-limit.ll
@@ -1,14 +1,13 @@
 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
 ; RUN: opt < %s -basic-aa -dse -enable-dse-memoryssa -S | FileCheck --check-prefix=NO-LIMIT %s
 ; RUN: opt < %s -basic-aa -dse -enable-dse-memoryssa -dse-memoryssa-scanlimit=0 -S | FileCheck --check-prefix=LIMIT-0 %s
+; RUN: opt < %s -basic-aa -dse -enable-dse-memoryssa -dse-memoryssa-scanlimit=2 -S | FileCheck --check-prefix=LIMIT-2 %s
 ; RUN: opt < %s -basic-aa -dse -enable-dse-memoryssa -dse-memoryssa-scanlimit=3 -S | FileCheck --check-prefix=LIMIT-3 %s
-; RUN: opt < %s -basic-aa -dse -enable-dse-memoryssa -dse-memoryssa-scanlimit=4 -S | FileCheck --check-prefix=LIMIT-4 %s
 
 target datalayout = "e-m:e-p:32:32-i64:64-v128:64:128-a:0:32-n32-S64"
 
 
 define void @test2(i32* noalias %P, i32* noalias %Q, i32* noalias %R) {
-;
 ; NO-LIMIT-LABEL: @test2(
 ; NO-LIMIT-NEXT:    br i1 true, label [[BB1:%.*]], label [[BB2:%.*]]
 ; NO-LIMIT:       bb1:
@@ -16,48 +15,49 @@ define void @test2(i32* noalias %P, i32* noalias %Q, i32* noalias %R) {
 ; NO-LIMIT:       bb2:
 ; NO-LIMIT-NEXT:    br label [[BB3]]
 ; NO-LIMIT:       bb3:
-; NO-LIMIT-NEXT:    store i32 0, i32* [[Q:%.*]]
-; NO-LIMIT-NEXT:    store i32 0, i32* [[R:%.*]]
-; NO-LIMIT-NEXT:    store i32 0, i32* [[P:%.*]]
+; NO-LIMIT-NEXT:    store i32 0, i32* [[Q:%.*]], align 4
+; NO-LIMIT-NEXT:    store i32 0, i32* [[R:%.*]], align 4
+; NO-LIMIT-NEXT:    store i32 0, i32* [[P:%.*]], align 4
 ; NO-LIMIT-NEXT:    ret void
 ;
 ; LIMIT-0-LABEL: @test2(
+; LIMIT-0-NEXT:    store i32 1, i32* [[P:%.*]], align 4
 ; LIMIT-0-NEXT:    br i1 true, label [[BB1:%.*]], label [[BB2:%.*]]
 ; LIMIT-0:       bb1:
 ; LIMIT-0-NEXT:    br label [[BB3:%.*]]
 ; LIMIT-0:       bb2:
 ; LIMIT-0-NEXT:    br label [[BB3]]
 ; LIMIT-0:       bb3:
-; LIMIT-0-NEXT:    store i32 0, i32* [[Q:%.*]]
-; LIMIT-0-NEXT:    store i32 0, i32* [[R:%.*]]
-; LIMIT-0-NEXT:    store i32 0, i32* [[P:%.*]]
+; LIMIT-0-NEXT:    store i32 0, i32* [[Q:%.*]], align 4
+; LIMIT-0-NEXT:    store i32 0, i32* [[R:%.*]], align 4
+; LIMIT-0-NEXT:    store i32 0, i32* [[P]], align 4
 ; LIMIT-0-NEXT:    ret void
 ;
+; LIMIT-2-LABEL: @test2(
+; LIMIT-2-NEXT:    store i32 1, i32* [[P:%.*]], align 4
+; LIMIT-2-NEXT:    br i1 true, label [[BB1:%.*]], label [[BB2:%.*]]
+; LIMIT-2:       bb1:
+; LIMIT-2-NEXT:    br label [[BB3:%.*]]
+; LIMIT-2:       bb2:
+; LIMIT-2-NEXT:    br label [[BB3]]
+; LIMIT-2:       bb3:
+; LIMIT-2-NEXT:    store i32 0, i32* [[Q:%.*]], align 4
+; LIMIT-2-NEXT:    store i32 0, i32* [[R:%.*]], align 4
+; LIMIT-2-NEXT:    store i32 0, i32* [[P]], align 4
+; LIMIT-2-NEXT:    ret void
+;
 ; LIMIT-3-LABEL: @test2(
-; LIMIT-3-NEXT:    store i32 1, i32* [[P:%.*]]
 ; LIMIT-3-NEXT:    br i1 true, label [[BB1:%.*]], label [[BB2:%.*]]
 ; LIMIT-3:       bb1:
 ; LIMIT-3-NEXT:    br label [[BB3:%.*]]
 ; LIMIT-3:       bb2:
 ; LIMIT-3-NEXT:    br label [[BB3]]
 ; LIMIT-3:       bb3:
-; LIMIT-3-NEXT:    store i32 0, i32* [[Q:%.*]]
-; LIMIT-3-NEXT:    store i32 0, i32* [[R:%.*]]
-; LIMIT-3-NEXT:    store i32 0, i32* [[P]]
+; LIMIT-3-NEXT:    store i32 0, i32* [[Q:%.*]], align 4
+; LIMIT-3-NEXT:    store i32 0, i32* [[R:%.*]], align 4
+; LIMIT-3-NEXT:    store i32 0, i32* [[P:%.*]], align 4
 ; LIMIT-3-NEXT:    ret void
 ;
-; LIMIT-4-LABEL: @test2(
-; LIMIT-4-NEXT:    br i1 true, label [[BB1:%.*]], label [[BB2:%.*]]
-; LIMIT-4:       bb1:
-; LIMIT-4-NEXT:    br label [[BB3:%.*]]
-; LIMIT-4:       bb2:
-; LIMIT-4-NEXT:    br label [[BB3]]
-; LIMIT-4:       bb3:
-; LIMIT-4-NEXT:    store i32 0, i32* [[Q:%.*]]
-; LIMIT-4-NEXT:    store i32 0, i32* [[R:%.*]]
-; LIMIT-4-NEXT:    store i32 0, i32* [[P:%.*]]
-; LIMIT-4-NEXT:    ret void
-;
   store i32 1, i32* %P
   br i1 true, label %bb1, label %bb2
 bb1: