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Fixes and improvements for removal of redundant zero inits #37786

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Jun 12, 2020
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Fixes and improvements for removal of redundant zero inits #37786

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7 changes: 7 additions & 0 deletions src/coreclr/src/jit/compiler.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -4197,6 +4197,13 @@ bool Compiler::fgVarNeedsExplicitZeroInit(unsigned varNum, bool bbInALoop, bool
{
LclVarDsc* varDsc = lvaGetDesc(varNum);

if (lvaIsFieldOfDependentlyPromotedStruct(varDsc))
{
// Fields of dependently promoted structs may only be initialized in the prolog when the whole
// struct is initialized in the prolog.
return fgVarNeedsExplicitZeroInit(varDsc->lvParentLcl, bbInALoop, bbIsReturn);
}

if (bbInALoop && !bbIsReturn)
{
return true;
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121 changes: 81 additions & 40 deletions src/coreclr/src/jit/optimizer.cpp
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Expand Up @@ -9224,6 +9224,8 @@ void Compiler::optRemoveRedundantZeroInits()

CompAllocator allocator(getAllocator(CMK_ZeroInit));
LclVarRefCounts refCounts(allocator);
BitVecTraits bitVecTraits(lvaCount, this);
BitVec zeroInitLocals = BitVecOps::MakeEmpty(&bitVecTraits);
bool hasGCSafePoint = false;
bool canThrow = false;

Expand Down Expand Up @@ -9271,55 +9273,94 @@ void Compiler::optRemoveRedundantZeroInits()
case GT_ASG:
{
GenTreeOp* treeOp = tree->AsOp();
if (treeOp->gtOp1->OperIs(GT_LCL_VAR, GT_LCL_FLD))
if (!treeOp->gtOp1->OperIs(GT_LCL_VAR, GT_LCL_FLD))
{
unsigned lclNum = treeOp->gtOp1->AsLclVarCommon()->GetLclNum();
LclVarDsc* const lclDsc = lvaGetDesc(lclNum);
unsigned* pRefCount = refCounts.LookupPointer(lclNum);

// pRefCount can't be null because the local node on the lhs of the assignment
// must have already been seen.
assert(pRefCount != nullptr);
if (*pRefCount == 1)
break;
}

unsigned lclNum = treeOp->gtOp1->AsLclVarCommon()->GetLclNum();
LclVarDsc* const lclDsc = lvaGetDesc(lclNum);
unsigned* pRefCount = refCounts.LookupPointer(lclNum);

// pRefCount can't be null because the local node on the lhs of the assignment
// must have already been seen.
assert(pRefCount != nullptr);
if (*pRefCount != 1)
{
break;
}

unsigned parentRefCount = 0;
if (lclDsc->lvIsStructField && refCounts.Lookup(lclDsc->lvParentLcl, &parentRefCount) &&
(parentRefCount != 0))
{
break;
}

unsigned fieldRefCount = 0;
if (lclDsc->lvPromoted)
{
for (unsigned i = lclDsc->lvFieldLclStart;
(fieldRefCount == 0) && (i < lclDsc->lvFieldLclStart + lclDsc->lvFieldCnt); ++i)
{
// The local hasn't been referenced before this assignment.
bool removedExplicitZeroInit = false;
if (!lclDsc->lvTracked && treeOp->gtOp2->IsIntegralConst(0))
{
bool bbInALoop = (block->bbFlags & BBF_BACKWARD_JUMP) != 0;
bool bbIsReturn = block->bbJumpKind == BBJ_RETURN;
refCounts.Lookup(i, &fieldRefCount);
}
}

if (!fgVarNeedsExplicitZeroInit(lclNum, bbInALoop, bbIsReturn))
{
// We are guaranteed to have a zero initialization in the prolog and
// the local hasn't been redefined between the prolog and this explicit
// zero initialization so the assignment can be safely removed.
if (tree == stmt->GetRootNode())
{
fgRemoveStmt(block, stmt);
removedExplicitZeroInit = true;
*pRefCount = 0;
lclDsc->lvSuppressedZeroInit = 1;
lclDsc->setLvRefCnt(lclDsc->lvRefCnt() - 1);
}
}
}
if (fieldRefCount != 0)
{
break;
}

// The local hasn't been referenced before this assignment.
bool removedExplicitZeroInit = false;

if (treeOp->gtOp2->IsIntegralConst(0))
{
if (!lclDsc->lvTracked)
{
bool bbInALoop = (block->bbFlags & BBF_BACKWARD_JUMP) != 0;
bool bbIsReturn = block->bbJumpKind == BBJ_RETURN;

if (!removedExplicitZeroInit && treeOp->gtOp1->OperIs(GT_LCL_VAR) &&
(!canThrow || !lclDsc->lvLiveInOutOfHndlr))
if (BitVecOps::IsMember(&bitVecTraits, zeroInitLocals, lclNum) ||
(lclDsc->lvIsStructField &&
BitVecOps::IsMember(&bitVecTraits, zeroInitLocals, lclDsc->lvParentLcl)) ||
!fgVarNeedsExplicitZeroInit(lclNum, bbInALoop, bbIsReturn))
{
// If compMethodRequiresPInvokeFrame() returns true, lower may later
// insert a call to CORINFO_HELP_INIT_PINVOKE_FRAME which is a gc-safe point.
if (!lclDsc->HasGCPtr() ||
(!GetInterruptible() && !hasGCSafePoint && !compMethodRequiresPInvokeFrame()))
// We are guaranteed to have a zero initialization in the prolog or a
// dominating explicit zero initialization and the local hasn't been redefined
// between the prolog and this explicit zero initialization so the assignment
// can be safely removed.
if (tree == stmt->GetRootNode())
{
// The local hasn't been used and won't be reported to the gc between
// the prolog and this explicit intialization. Therefore, it doesn't
// require zero initialization in the prolog.
lclDsc->lvHasExplicitInit = 1;
fgRemoveStmt(block, stmt);
removedExplicitZeroInit = true;
lclDsc->lvSuppressedZeroInit = 1;
lclDsc->setLvRefCnt(lclDsc->lvRefCnt() - 1);
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We generally don't incrementally update ref counts anymore. Is this important?

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I added that in #37280. The idea is to detect cases where we make the struct dependently promoted because of a block assignment and then delete that block assignment. In that case we can make the field tracked and eliminate dead field assignments.

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I wonder if there is any guarantee that the count is > 0 at this point? I suppose this is running not long after the first call to lvaMarkLocalVars so the ref counts are probably still fairly accurate.

And the code to determine which locals to track is just a heuristic, so it's ok if it is based on incorrect ref count info.

}
}
}

if (treeOp->gtOp1->OperIs(GT_LCL_VAR))
{
BitVecOps::AddElemD(&bitVecTraits, zeroInitLocals, lclNum);
}
*pRefCount = 0;
}

if (!removedExplicitZeroInit && treeOp->gtOp1->OperIs(GT_LCL_VAR) &&
(!canThrow || !lclDsc->lvLiveInOutOfHndlr))
{
// If compMethodRequiresPInvokeFrame() returns true, lower may later
// insert a call to CORINFO_HELP_INIT_PINVOKE_FRAME which is a gc-safe point.
if (!lclDsc->HasGCPtr() ||
(!GetInterruptible() && !hasGCSafePoint && !compMethodRequiresPInvokeFrame()))
{
// The local hasn't been used and won't be reported to the gc between
// the prolog and this explicit intialization. Therefore, it doesn't
// require zero initialization in the prolog.
lclDsc->lvHasExplicitInit = 1;
}
}
break;
}
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71 changes: 71 additions & 0 deletions src/coreclr/tests/src/JIT/Regression/JitBlue/GitHub_37666/GitHub_37666.cs
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Original file line number Diff line number Diff line change
@@ -0,0 +1,71 @@
// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
// See the LICENSE file in the project root for more information.

using System;
using System.Runtime.CompilerServices;

public struct TwoBools
{
public bool b1;
public bool b2;

public TwoBools(bool b1, bool b2)
{
this.b1 = b1;
this.b2 = b2;
}
}

class Test
{
public static int Main()
{
int result = 100;

RunTest(Test1, "Test1", ref result);
RunTest(Test2, "Test2", ref result);

return result;
}

delegate TwoBools TestZeroInit();

[MethodImpl(MethodImplOptions.NoInlining)]
static void RunTest(TestZeroInit test, string testName, ref int result)
{
if (test().b2)
{
Console.WriteLine(testName + " failed");
result = -1;
}
else
{
Console.WriteLine(testName + " passed");
}
}

[MethodImpl(MethodImplOptions.NoInlining)]
static TwoBools Test1()
{
TwoBools result = CreateTwoBools();
result.b2 = false;
return result;
}

[MethodImpl(MethodImplOptions.NoInlining)]
static TwoBools Test2()
{
TwoBools result = default(TwoBools);
result.b2 = true;
result = default(TwoBools);
return result;
}

[MethodImpl(MethodImplOptions.NoInlining)]
static TwoBools CreateTwoBools()
{
TwoBools result = new TwoBools(true, true);
return result;
}
}
13 changes: 13 additions & 0 deletions src/coreclr/tests/src/JIT/Regression/JitBlue/GitHub_37666/GitHub_37666.csproj
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Original file line number Diff line number Diff line change
@@ -0,0 +1,13 @@
<Project Sdk="Microsoft.NET.Sdk">
<PropertyGroup>
<OutputType>Exe</OutputType>
<CLRTestPriority>0</CLRTestPriority>
</PropertyGroup>
<PropertyGroup>
<DebugType>None</DebugType>
<Optimize>True</Optimize>
</PropertyGroup>
<ItemGroup>
<Compile Include="$(MSBuildProjectName).cs" />
</ItemGroup>
</Project>