commit 14845b2c459021e3dbf2ead52d707d4a7db40cbb
Author: Philip Reames <listmail@philipreames.com>
Date:   Tue Feb 25 08:56:53 2020 -0800

    Revert "[LICM] Support hosting of dynamic allocas out of loops"
    
    This reverts commit 8d22100f66c4170510c6ff028c60672acfe1cff9.
    
    There was a functional regression reported (https://bugs.llvm.org/show_bug.cgi?id=44996).  I'm not actually sure the patch is wrong, but I don't have time to investigate currently, and this line of work isn't something I'm likely to get back to quickly.

diff --git a/llvm/lib/Transforms/Scalar/LICM.cpp b/llvm/lib/Transforms/Scalar/LICM.cpp
index 15d40a50e60..d429fc24dc3 100644
--- a/llvm/lib/Transforms/Scalar/LICM.cpp
+++ b/llvm/lib/Transforms/Scalar/LICM.cpp
@@ -742,41 +742,6 @@ public:
 };
 } // namespace
 
-
-/// Return true if we know how to rewrite all uses of the given alloca after
-/// hoisting it out of the loop.  The main concerns are a) potential captures
-/// and b) invariant.start markers which don't capture, but are no longer
-/// valid w/o a corresponding invariant.end.
-static bool canRewriteUsesOfAlloca(AllocaInst &AI) {
-  // TODO: This looks a lot like capture tracking, but we need to remove any
-  // invariant starts if we extend the lifetime of the alloca by hoisting it.
-  // We should probably refactor capture tracking into a form which allows us
-  // to reuse the relevant bits and remove the duplicated logic here.
-
-  SmallVector<Use *, 16> Worklist;
-  for (Use &U : AI.uses())
-    Worklist.push_back(&U);
-  
-  unsigned NumUsesExplored = 0;
-  while (!Worklist.empty()) {
-    Use *U = Worklist.pop_back_val();
-    Instruction *I = cast<Instruction>(U->getUser());
-    NumUsesExplored++;
-    if (NumUsesExplored > DefaultMaxUsesToExplore)
-      return false;
-    // Non capturing, terminating uses
-    if (isa<LoadInst>(I) ||
-        (isa<StoreInst>(I) && U->getOperandNo() == 1))
-      continue;
-    // Non capturing, non-terminating
-    if (!isa<BitCastInst>(I) && !isa<GetElementPtrInst>(I))
-      return false;
-    for (Use &U : I->uses())
-      Worklist.push_back(&U);
-  }
-  return true;
-}
-
 /// Walk the specified region of the CFG (defined by all blocks dominated by
 /// the specified block, and that are in the current loop) in depth first
 /// order w.r.t the DominatorTree.  This allows us to visit definitions before
@@ -897,16 +862,6 @@ bool llvm::hoistRegion(DomTreeNode *N, AliasAnalysis *AA, LoopInfo *LI,
         continue;
       }
 
-      if (isa<AllocaInst>(&I) &&
-          SafetyInfo->isGuaranteedToExecute(I, DT, CurLoop) &&
-          canRewriteUsesOfAlloca(cast<AllocaInst>(I))) {
-        hoist(I, DT, CurLoop, CFH.getOrCreateHoistedBlock(BB), SafetyInfo,
-              MSSAU, SE, ORE);
-        HoistedInstructions.push_back(&I);
-        Changed = true;
-        continue;
-      }
-
       if (PHINode *PN = dyn_cast<PHINode>(&I)) {
         if (CFH.canHoistPHI(PN)) {
           // Redirect incoming blocks first to ensure that we create hoisted
diff --git a/llvm/test/Transforms/LICM/hoist-alloca.ll b/llvm/test/Transforms/LICM/hoist-alloca.ll
deleted file mode 100644
index 8e4debac283..00000000000
--- a/llvm/test/Transforms/LICM/hoist-alloca.ll
+++ /dev/null
@@ -1,168 +0,0 @@
-; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
-; RUN: opt -S -licm < %s | FileCheck %s
-
-@G = external global i64
-
-define void @test(i64 %n) {
-; CHECK-LABEL: @test(
-; CHECK-NEXT:  entry:
-; CHECK-NEXT:    [[A:%.*]] = alloca i64
-; CHECK-NEXT:    [[VAL:%.*]] = load i64, i64* [[A]]
-; CHECK-NEXT:    store i64 [[VAL]], i64* @G
-; CHECK-NEXT:    br label [[FOR_BODY:%.*]]
-; CHECK:       for.body:
-; CHECK-NEXT:    [[IV:%.*]] = phi i64 [ [[IV_NEXT:%.*]], [[FOR_BODY]] ], [ 0, [[ENTRY:%.*]] ]
-; CHECK-NEXT:    [[IV_NEXT]] = add nuw nsw i64 [[IV]], 1
-; CHECK-NEXT:    [[EXITCOND:%.*]] = icmp ult i64 [[IV]], [[N:%.*]]
-; CHECK-NEXT:    br i1 [[EXITCOND]], label [[FOR_BODY]], label [[EXIT:%.*]]
-; CHECK:       exit:
-; CHECK-NEXT:    ret void
-;
-entry:
-  br label %for.body
-
-for.body:
-  %iv = phi i64 [ %iv.next, %for.body ], [ 0, %entry ]
-  %a = alloca i64
-  %val = load i64, i64* %a
-  store i64 %val, i64* @G
-  %iv.next = add nuw nsw i64 %iv, 1
-  %exitcond = icmp ult i64 %iv, %n
-  br i1 %exitcond, label %for.body, label %exit
-exit:
-  ret void
-}
-
-define void @test2(i64 %n) {
-; CHECK-LABEL: @test2(
-; CHECK-NEXT:  entry:
-; CHECK-NEXT:    [[A:%.*]] = alloca i64
-; CHECK-NEXT:    br label [[FOR_BODY:%.*]]
-; CHECK:       for.body:
-; CHECK-NEXT:    [[IV:%.*]] = phi i64 [ [[IV_NEXT:%.*]], [[FOR_BODY]] ], [ 0, [[ENTRY:%.*]] ]
-; CHECK-NEXT:    [[IV_NEXT]] = add nuw nsw i64 [[IV]], 1
-; CHECK-NEXT:    [[EXITCOND:%.*]] = icmp ult i64 [[IV]], [[N:%.*]]
-; CHECK-NEXT:    br i1 [[EXITCOND]], label [[FOR_BODY]], label [[EXIT:%.*]]
-; CHECK:       exit:
-; CHECK-NEXT:    [[IV_LCSSA:%.*]] = phi i64 [ [[IV]], [[FOR_BODY]] ]
-; CHECK-NEXT:    store i64 [[IV_LCSSA]], i64* [[A]], align 4
-; CHECK-NEXT:    ret void
-;
-entry:
-  br label %for.body
-
-for.body:
-  %iv = phi i64 [ %iv.next, %for.body ], [ 0, %entry ]
-  %a = alloca i64
-  store i64 %iv, i64* %a
-  %iv.next = add nuw nsw i64 %iv, 1
-  %exitcond = icmp ult i64 %iv, %n
-  br i1 %exitcond, label %for.body, label %exit
-exit:
-  ret void
-}
-
-
-define void @test3(i64 %n) {
-; CHECK-LABEL: @test3(
-; CHECK-NEXT:  entry:
-; CHECK-NEXT:    [[A:%.*]] = alloca i64
-; CHECK-NEXT:    [[A_I8:%.*]] = bitcast i64* [[A]] to i8*
-; CHECK-NEXT:    [[A_OFFSET:%.*]] = getelementptr i8, i8* [[A_I8]], i64 4
-; CHECK-NEXT:    store i8 0, i8* [[A_OFFSET]]
-; CHECK-NEXT:    br label [[FOR_BODY:%.*]]
-; CHECK:       for.body:
-; CHECK-NEXT:    [[IV:%.*]] = phi i64 [ [[IV_NEXT:%.*]], [[FOR_BODY]] ], [ 0, [[ENTRY:%.*]] ]
-; CHECK-NEXT:    [[IV_NEXT]] = add nuw nsw i64 [[IV]], 1
-; CHECK-NEXT:    [[EXITCOND:%.*]] = icmp ult i64 [[IV]], [[N:%.*]]
-; CHECK-NEXT:    br i1 [[EXITCOND]], label [[FOR_BODY]], label [[EXIT:%.*]]
-; CHECK:       exit:
-; CHECK-NEXT:    ret void
-;
-entry:
-  br label %for.body
-
-for.body:
-  %iv = phi i64 [ %iv.next, %for.body ], [ 0, %entry ]
-  %a = alloca i64
-  %a.i8 = bitcast i64* %a to i8*
-  %a.offset = getelementptr i8, i8* %a.i8, i64 4
-  store i8 0, i8* %a.offset
-  %iv.next = add nuw nsw i64 %iv, 1
-  %exitcond = icmp ult i64 %iv, %n
-  br i1 %exitcond, label %for.body, label %exit
-exit:
-  ret void
-}
-
-; This example is subtle.  Because the dynamic alloca isn't reclaimed until
-; end of function scope, the captured value can legally point to a dynamic
-; alloca stack region from a previous iteration.
-define void @test4(i64 %n) {
-; CHECK-LABEL: @test4(
-; CHECK-NEXT:  entry:
-; CHECK-NEXT:    br label [[FOR_BODY:%.*]]
-; CHECK:       for.body:
-; CHECK-NEXT:    [[IV:%.*]] = phi i64 [ [[IV_NEXT:%.*]], [[FOR_BODY]] ], [ 0, [[ENTRY:%.*]] ]
-; CHECK-NEXT:    [[A:%.*]] = alloca i64
-; CHECK-NEXT:    store i64 [[IV]], i64* [[A]]
-; CHECK-NEXT:    call void @capture(i64* [[A]])
-; CHECK-NEXT:    [[IV_NEXT]] = add nuw nsw i64 [[IV]], 1
-; CHECK-NEXT:    [[EXITCOND:%.*]] = icmp ult i64 [[IV]], [[N:%.*]]
-; CHECK-NEXT:    br i1 [[EXITCOND]], label [[FOR_BODY]], label [[EXIT:%.*]]
-; CHECK:       exit:
-; CHECK-NEXT:    ret void
-;
-entry:
-  br label %for.body
-
-for.body:
-  %iv = phi i64 [ %iv.next, %for.body ], [ 0, %entry ]
-  %a = alloca i64
-  store i64 %iv, i64* %a
-  %a.i8 = bitcast i64* %a to i8*
-  call void @capture(i64* %a)
-  %iv.next = add nuw nsw i64 %iv, 1
-  %exitcond = icmp ult i64 %iv, %n
-  br i1 %exitcond, label %for.body, label %exit
-exit:
-  ret void
-}
-declare void @capture(i64* %a)
-
-
-; TODO: not yet handled
-define void @test5(i64 %n) {
-; CHECK-LABEL: @test5(
-; CHECK-NEXT:  entry:
-; CHECK-NEXT:    br label [[FOR_BODY:%.*]]
-; CHECK:       for.body:
-; CHECK-NEXT:    [[IV:%.*]] = phi i64 [ [[IV_NEXT:%.*]], [[FOR_BODY]] ], [ 0, [[ENTRY:%.*]] ]
-; CHECK-NEXT:    [[A:%.*]] = alloca i64
-; CHECK-NEXT:    store i64 [[IV]], i64* [[A]]
-; CHECK-NEXT:    [[A_I8:%.*]] = bitcast i64* [[A]] to i8*
-; CHECK-NEXT:    [[TMP0:%.*]] = call {}* @llvm.invariant.start.p0i8(i64 8, i8* [[A_I8]])
-; CHECK-NEXT:    [[IV_NEXT]] = add nuw nsw i64 [[IV]], 1
-; CHECK-NEXT:    [[EXITCOND:%.*]] = icmp ult i64 [[IV]], [[N:%.*]]
-; CHECK-NEXT:    br i1 [[EXITCOND]], label [[FOR_BODY]], label [[EXIT:%.*]]
-; CHECK:       exit:
-; CHECK-NEXT:    ret void
-;
-entry:
-  br label %for.body
-
-for.body:
-  %iv = phi i64 [ %iv.next, %for.body ], [ 0, %entry ]
-  %a = alloca i64
-  store i64 %iv, i64* %a
-  %a.i8 = bitcast i64* %a to i8*
-  call {}* @llvm.invariant.start.p0i8(i64 8, i8* %a.i8)
-  %iv.next = add nuw nsw i64 %iv, 1
-  %exitcond = icmp ult i64 %iv, %n
-  br i1 %exitcond, label %for.body, label %exit
-exit:
-  ret void
-}
-
-declare {}* @llvm.invariant.start.p0i8(i64, i8* nocapture) nounwind readonly
-