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JIT: Normalize switch so it gets collapse into range check #128524

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JIT: Normalize switch so it gets collapse into range check #128524

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f24cf07
* normalize switch whenever jump targets form a continous range, so i…
BoyBaykiller May 23, 2026
32ec5eb
* add val!=0 guard when creating GT_SUB node, as nothing optimizes it…
BoyBaykiller May 23, 2026
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28 changes: 15 additions & 13 deletions src/coreclr/jit/switchrecognition.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -354,7 +354,7 @@ bool Compiler::optSwitchDetectAndConvert(BasicBlock* firstBlock, bool testingFor
// testsCount - Number of conditional blocks in the chain
// testValues - Array of constants that are tested against the variable
// falseLikelihood - Likelihood of control flow reaching the false block
// nodeToTest - Variable node that is tested against the constants
// switchValue - Variable node that is tested against the constants
// testingForConversion - Test if its likely a switch conversion will happen.
// Used to prevent a pessimization when optimizing for conditional chaining.
// Done in this function to prevent maintaining the check in two places.
Expand All @@ -367,12 +367,12 @@ bool Compiler::optSwitchConvert(BasicBlock* firstBlock,
int testsCount,
ssize_t* testValues,
weight_t falseLikelihood,
GenTree* nodeToTest,
GenTree* switchValue,
bool testingForConversion,
BitVec* ccmpVec)
{
assert(firstBlock->KindIs(BBJ_COND));
assert(!varTypeIsSmall(nodeToTest));
assert(!varTypeIsSmall(switchValue));

if (testingForConversion && (testsCount < CONVERT_SWITCH_TO_CCMP_MIN_TEST))
{
Expand Down Expand Up @@ -424,8 +424,18 @@ bool Compiler::optSwitchConvert(BasicBlock* firstBlock,
return false;
}

// if MaxValue is less than SWITCH_MAX_DISTANCE then don't bother with SUB(val, minValue)
if (maxValue <= SWITCH_MAX_DISTANCE)
// Normalize switch whenever it's a requirement to fit the table or as an optimization to collapse into range check
bool doesntFitTable = maxValue > SWITCH_MAX_DISTANCE;
bool continousRange = testsCount == (maxValue - minValue + 1);
if (doesntFitTable || continousRange)
{
if (minValue != 0)
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@BoyBaykiller BoyBaykiller May 23, 2026 •
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Unfortunately, this identity check is needed as nothing after "recognize switch" phase optimizes out the SUB(x, 0)

{
switchValue = gtNewOperNode(GT_SUB, switchValue->TypeGet(), switchValue,
gtNewIconNode(minValue, switchValue->TypeGet()));
}
}
else
{
minValue = 0;
}
Expand Down Expand Up @@ -478,14 +488,6 @@ bool Compiler::optSwitchConvert(BasicBlock* firstBlock,
firstBlock->bbCodeOffsEnd = lastBlock->bbCodeOffsEnd;
firstBlock->lastStmt()->GetRootNode()->ChangeOper(GT_SWITCH);

// The root node is now SUB(nodeToTest, minValue) if minValue != 0
GenTree* switchValue = nodeToTest;
if (minValue != 0)
{
switchValue =
gtNewOperNode(GT_SUB, nodeToTest->TypeGet(), switchValue, gtNewIconNode(minValue, nodeToTest->TypeGet()));
}

firstBlock->lastStmt()->GetRootNode()->AsOp()->gtOp1 = switchValue;
gtSetStmtInfo(firstBlock->lastStmt());
fgSetStmtSeq(firstBlock->lastStmt());
Expand Down
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