JIT: Treat store in JTRUE block as the ElseOperation in if-conversion by BoyBaykiller · Pull Request #124738 · dotnet/runtime

BoyBaykiller · 2026-02-23T04:54:28Z

If-conversion phase now produces the same IR for these two cases:

bool First(int tMinLeft, int tMinRight)
{
    bool leftCloser = false;
    if (tMinLeft < tMinRight)
    {
        leftCloser = true;
    }
    return leftCloser;
}

bool Second(int tMinLeft, int tMinRight)
{
    bool leftCloser;
    if (tMinLeft < tMinRight)
    {
        leftCloser = true;
    }
    else
    {
        leftCloser = false;
    }
    return leftCloser;
}

The last unconditional store (leftCloser = false) is substituted into the SELECT which enables further optimization. Specifically it fixes #124713.

dotnet-policy-service · 2026-02-23T04:55:28Z

Tagging subscribers to this area: @JulieLeeMSFT, @jakobbotsch
See info in area-owners.md if you want to be subscribed.

BoyBaykiller · 2026-02-23T04:55:48Z

@dotnet-policy-service agree

BoyBaykiller · 2026-04-16T04:08:20Z

@a74nh @jakobbotsch PTAL

BoyBaykiller · 2026-04-16T04:10:53Z

+                    GenTreeLclVar* prevStore = tree->AsLclVar();
+                    if (prevStore->GetLclNum() == targetLclNum)
+                    {
+                        if (prevStore->Data()->IsInvariant())


If we ever get a HIR version of IsInvariantInRange then we could use it here instead of IsInvariant. But for now this is the best I can do

…xecuted, so not inside Else block

Copilot

Pull request overview

This PR updates the CoreCLR JIT if-conversion phase to treat an unconditional prior store in the JTRUE block as the effective “else” value when forming a SELECT, so equivalent C# patterns yield equivalent IR and enable downstream optimizations (per #124713).

Changes:

Replaces explicit m_doElseConversion state with a HasElseBlock() helper and uses m_elseOperation.block != nullptr to drive behavior.
Adds logic to locate a prior invariant STORE_LCL_VAR in the JTRUE block and use it as an ElseOperation when there is no explicit else block.
Adjusts debug dumping/logging and CFG cleanup to handle both explicit else blocks and synthesized else operations.

Copilot · 2026-04-24T23:59:09Z

+    {
+        // There is no Else block, but we can still find an Else operation. Search for
+        // most recent STORE to the local in JTRUE block and see if it's legal to fwd sub
+        // it's definition into the SELECT and remove the STORE.


In this comment, "it's" should be the possessive "its" (no apostrophe).

Suggested change

// it's definition into the SELECT and remove the STORE.

// its definition into the SELECT and remove the STORE.

Copilot · 2026-04-24T23:59:10Z

@@ -535,28 +590,36 @@ bool OptIfConversionDsc::optIfConvert(int* pReachabilityBudget)

    // JTRUE block now contains SELECT. Change it's kind and make it flow


In this comment, "Change it's kind" should use the possessive "its" (no apostrophe).

Suggested change

// JTRUE block now contains SELECT. Change it's kind and make it flow

// JTRUE block now contains SELECT. Change its kind and make it flow

Copilot · 2026-04-24T23:59:10Z

+        // There is no Else block, but we can still find an Else operation. Search for
+        // most recent STORE to the local in JTRUE block and see if it's legal to fwd sub
+        // it's definition into the SELECT and remove the STORE.
+
+        assert(m_mainOper == GT_STORE_LCL_VAR);
+
+        GenTreeLclVar* thenStore    = m_thenOperation.node->AsLclVar();
+        unsigned       targetLclNum = thenStore->GetLclNum();
+
+        bool usedInThenBlock = m_compiler->gtHasRef(thenStore->Data(), targetLclNum);
+        if (!usedInThenBlock)
+        {
+            Statement* last = m_startBlock->lastStmt();
+            Statement* stmt = last;
+            do
+            {
+                GenTree* tree = stmt->GetRootNode();
+                if (tree->OperIs(GT_STORE_LCL_VAR))
+                {
+                    GenTreeLclVar* prevStore = tree->AsLclVar();
+                    if (prevStore->GetLclNum() == targetLclNum)
+                    {
+                        if (prevStore->Data()->IsInvariant())
+                        {
+                            m_elseOperation.block = m_startBlock;
+                            m_elseOperation.stmt  = stmt;
+                            m_elseOperation.node  = tree;
+                        }
+
+                        break;
+                    }
+                }
+
+                if (m_compiler->gtHasRef(tree, targetLclNum))
+                {
+                    break;
+                }
+
+                stmt = stmt->GetPrevStmt();
+            } while (stmt != last);
+        }


The new path that synthesizes an ElseOperation from a previous STORE in the JTRUE block (when there is no explicit else block) changes if-conversion behavior but isn't covered by a JIT disasm-check regression test. Consider adding a small test case for the "bool x=false; if(cond){x=true;}" pattern to ensure it if-converts into a SELECT and enables the expected setcc/codegen improvement, so this doesn’t regress silently.

jakobbotsch · 2026-04-27T10:12:22Z

+            Statement* last = m_startBlock->lastStmt();
+            Statement* stmt = last;
+            do
+            {
+                GenTree* tree = stmt->GetRootNode();
+                if (tree->OperIs(GT_STORE_LCL_VAR))
+                {
+                    GenTreeLclVar* prevStore = tree->AsLclVar();
+                    if (prevStore->GetLclNum() == targetLclNum)
+                    {
+                        if (prevStore->Data()->IsInvariant())
+                        {
+                            m_elseOperation.block = m_startBlock;
+                            m_elseOperation.stmt  = stmt;
+                            m_elseOperation.node  = tree;
+                        }
+
+                        break;
+                    }
+                }
+
+                if (m_compiler->gtHasRef(tree, targetLclNum))
+                {
+                    break;
+                }
+
+                stmt = stmt->GetPrevStmt();
+            } while (stmt != last);


An unbounded IR search like this is concerning. I would not be surprised if this can introduce JIT hangs in pathological cases.
This change to walk the IR seems like an additional change that is not needed for this PR. Can it be done in a separate PR and follow-up so that it can be evaluated in isolation?

This sits at the heart of this PR. How should I find the most recent store into targetLclNum in m_startBlock without an IR walk?

The pattern you are targeting looks like:

x = foo; if (cond) x = bar;

I would not expect to need to look backwards more than one statement from the JTRUE for this pattern.

I would be ok with just bounding this walk to some constant, like try to find the statement within the previous 8 statements.

Ok let me try.

Given https://github.com/dotnet/runtime/pull/124738/changes#r3148881220 I would suggest not bothering. Instead I would propose to just validate that the conditional has no side effects (filters out both (2) and (3)) and then use gtTreeHasLocalRead to handle (1). If you want the search I suggest a follow-up PR.

jakobbotsch · 2026-04-27T16:47:20Z

+                    }
+                }
+
+                if (m_compiler->gtHasRef(tree, targetLclNum))


gtHasRef is not a sufficient check here for a few reasons:

It does not check for promoted cases, i.e. if targetLclNum is a field and the parent local has a use, then the value will still depend on the value of targetLclNum. See fgCanMoveFirstStatementIntoPred for how to handle this.

The local can have uses in EH successors that control gets transferred to by any intervening exception throw.

If it is address exposed then not all uses are visible, and you really cannot reason about it in any meaningful fashion when intervening statements have global side effects.

See fgCanMoveFirstStatementIntoPred for how to handle this.

Actually we have gtTreeHasLocalRead for a version of gtHasRef that handles promotion.

* update the 'is this lcl used?' check

BoyBaykiller · 2026-04-27T22:25:37Z

The amount of diffs got halved.
Old arm64 -25,036
New arm64 -16,324

I am not sure that's more because we now bail if condition has side effects or that we only check the immediate predecessor. Like you said I can investigate and possibly improve this in a future PR. @jakobbotsch

jakobbotsch · 2026-04-27T23:28:27Z

+        bool unusedInThen = !m_compiler->gtTreeHasLocalRead(thenStore->Data(), targetLclNum);
+        bool unusedInCond =
+            ((m_cond->gtFlags & GTF_SIDE_EFFECT) == 0) && !m_compiler->gtTreeHasLocalRead(m_cond, targetLclNum);


If targetLclNum is address exposed then these checks are not sufficient. GTF_SIDE_EFFECT needs to be GTF_GLOB_EFFECT and unusedInThen needs to also check GTF_GLOB_REF.
Or you can just skip the opt if targetLclNum is address exposed. It is unlikely to be a common scenario in the diffs.

github-actions Bot added the area-CodeGen-coreclr CLR JIT compiler in src/coreclr/src/jit and related components such as SuperPMI label Feb 23, 2026

dotnet-policy-service Bot added the community-contribution Indicates that the PR has been added by a community member label Feb 23, 2026

BoyBaykiller mentioned this pull request Feb 24, 2026

JIT: Make cmov codegen the same for these equivalent IR sequences #124785

Open

This was referenced Feb 24, 2026

[android] Android.Device_Emulator.JIT.Test failing on emulators with CoreCLR #112633

Open

Unable to pull image from mcr.microsoft.com #117164

Open

[Android][CoreCLR] System.Security.Cryptography.Tests killed by lowmemorykiller #118603

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BoyBaykiller marked this pull request as ready for review February 24, 2026 19:40

BoyBaykiller marked this pull request as draft February 24, 2026 20:01

BoyBaykiller changed the title ~~JIT: Treat unconditional store as the else case in if-conversion~~ JIT: Treat store in JTRUE block as the ElseOperation in if-conversion Mar 10, 2026

BoyBaykiller mentioned this pull request Mar 17, 2026

JIT: Simplify if-conversion #125347

Merged

BoyBaykiller force-pushed the if-conv-unconditional-store-to-else-124713 branch from 208cc34 to cba3e37 Compare March 19, 2026 00:48

BoyBaykiller mentioned this pull request Mar 22, 2026

JIT: Optimize some bitwise ops in context of enum flags #125899

Open

Copilot AI mentioned this pull request Mar 22, 2026

Fix intermittent NRE in ETW listener tests by using GUID overload of EnableProvider #125908

Merged

BoyBaykiller closed this Apr 15, 2026

BoyBaykiller force-pushed the if-conv-unconditional-store-to-else-124713 branch from 8997012 to 2ae217c Compare April 15, 2026 18:29

* treat store in JTRUE block as ElseOperation

2a9eccc

BoyBaykiller reopened this Apr 16, 2026

BoyBaykiller marked this pull request as ready for review April 16, 2026 04:07

BoyBaykiller commented Apr 16, 2026

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build-analysis Bot mentioned this pull request Apr 16, 2026

System.Net.NameResolution.Tests DNS failures: Name or service not known #126641

Open

BoyBaykiller and others added 2 commits April 24, 2026 17:25

Merge branch 'main' into if-conv-unconditional-store-to-else-124713

e16b809

* have ElseCost be 0 when ElseOperation was already unconditionally e…

6f8a69e

…xecuted, so not inside Else block

build-analysis Bot mentioned this pull request Apr 24, 2026

slow macOS - "##[error]The job running on agent Azure Pipelines 9 ran longer than the maximum time of 60 minutes." dotnet/dnceng#1883

Open

3 tasks

EgorBo requested a review from Copilot April 24, 2026 23:49

Copilot started reviewing on behalf of EgorBo April 24, 2026 23:50 View session

Copilot AI reviewed Apr 24, 2026

View reviewed changes

* fix typo

6009c10

build-analysis Bot mentioned this pull request Apr 25, 2026

XHarness package install failure on iOS due to devicectl NSPOSIXErrorDomain error 49 #123796

Open

jakobbotsch reviewed Apr 27, 2026

View reviewed changes

* only check immediate predecessor stmt of JTRUE

083b153

* update the 'is this lcl used?' check

jakobbotsch reviewed Apr 27, 2026

View reviewed changes

	// it's definition into the SELECT and remove the STORE.
	// its definition into the SELECT and remove the STORE.

		@@ -535,28 +590,36 @@ bool OptIfConversionDsc::optIfConvert(int* pReachabilityBudget)

		// JTRUE block now contains SELECT. Change it's kind and make it flow

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