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Fix #3891: don't reduce nesting when there is no else block #3898

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Merged
siegfriedpammer merged 1 commit into from
Jul 20, 2026
Merged

Fix #3891: don't reduce nesting when there is no else block #3898

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12 changes: 7 additions & 5 deletions ICSharpCode.Decompiler/IL/Transforms/ReduceNestingTransform.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -254,7 +254,9 @@ private bool ReduceNesting(Block block, IfInstruction ifInst, ILInstruction exit
ifInst = elseIfInst;
}

if (!ShouldReduceNesting(ifInst.FalseInst, maxStatements, maxDepth))
// A chain with no trailing else has no block to reduce: ifInst.FalseInst is a bare Nop.
// Guarding here keeps the else-block cast in ExtractElseBlock safe (#3891).
if (ifInst.FalseInst is not Block falseBlock || !ShouldReduceNesting(falseBlock, maxStatements, maxDepth))
return false;

// extract the else block and insert exit points all the way up the else-if tree
Expand Down Expand Up @@ -582,16 +584,16 @@ private void ComputeStats(ILInstruction inst, ref int numStatements, ref int max

/// <summary>
/// Heuristic to determine whether it is worth duplicating exits into the preceeding sibling blocks (then/else-if/case)
/// in order to reduce the nesting of inst by 1
/// in order to reduce the nesting of block by 1
/// </summary>
/// <param name="inst">The instruction heading the nested candidate block</param>
/// <param name="block">The nested candidate block (an else or default block)</param>
/// <param name="maxStatements">The number of statements in the largest sibling block</param>
/// <param name="maxDepth">The relative depth of the most nested statement in the sibling blocks</param>
/// <returns></returns>
private bool ShouldReduceNesting(ILInstruction inst, int maxStatements, int maxDepth)
private bool ShouldReduceNesting(Block block, int maxStatements, int maxDepth)
{
int maxStatements2 = 0, maxDepth2 = 0;
UpdateStats(inst, ref maxStatements2, ref maxDepth2);
UpdateStats(block, ref maxStatements2, ref maxDepth2);
// if the max depth is 2, always reduce nesting (total depth 3 or more)
// if the max depth is 1, reduce nesting if this block is the largest
// otherwise reduce nesting only if this block is twice as large as any other
Expand Down
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