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Improve linker performance by avoiding IndexOf #90721

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Merged
lewing merged 2 commits into from
Aug 18, 2023
Merged

Improve linker performance by avoiding IndexOf #90721

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Improve linker performance by avoiding IndexOf
Youssef1313 Aug 17, 2023
7de45ed
Fix typos
Youssef1313 Aug 17, 2023
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43 changes: 35 additions & 8 deletions src/tools/illink/src/linker/Linker.Steps/UnreachableBlocksOptimizer.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -92,21 +92,39 @@ static bool IsMethodSupported (MethodDefinition method)
return true;
}

static bool HasJumpIntoTargetRange (Collection<Instruction> instructions, int firstInstr, int lastInstr, Func<Instruction, int>? mapping = null)
static bool HasJumpIntoTargetRange (Collection<Instruction> instructions, int firstInstr, int lastInstr, Func<Instruction, int?>? mapping = null)
{
foreach (var instr in instructions) {
switch (instr.OpCode.FlowControl) {
case FlowControl.Branch:
case FlowControl.Cond_Branch:
if (instr.Operand is Instruction target) {
int index = mapping == null ? instructions.IndexOf (target) : mapping (target);
if (index >= firstInstr && index <= lastInstr)
return true;
if (mapping != null && mapping (target) is int index) {
if (index >= firstInstr && index <= lastInstr) {
return true;
}
}
else {
for (int i = firstInstr; i <= lastInstr; i++) {
if (instructions[i] == target) {
return true;
}
}
Comment on lines +108 to +112

@Youssef1313 Youssef1313 Aug 17, 2023

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This loop will have maximum of three iterations. This is a lot better than IndexOf which will search through the whole instructions collection which can have thousands of elements.

@vitek-karas vitek-karas Aug 18, 2023

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Should we even bother with the lookup then?
Can we measure this - if the lookup doesn't bring interesting perf wins I would vote for keeping it simple and just do the linear search over the known possible range (that should be really fast).

@Youssef1313 Youssef1313 Aug 18, 2023

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@vitek-karas I'm not sure about the logic and wanted to avoid potentially changing the meaning of the code, e.g, I have no idea if the dictionary lookup could yield a different result than the loop. So, I wanted to make a change that is clear it doesn't change the code semantics, and at the same time is more performant.

Regarding measuring this, I currently don't have the dotnet/runtime cloned locally to try something (the changes in this PR are done through GitHub UI actually). But I think the performance trace is quite clear this lookup is problematic, and it also the change should hopefully be making sense as a performance improvement. Yet, I understand that measuring would still be a good idea to at least know how much improvement this is. But unfortunately, I'm not able to do it right now. If necessary, would someone from the team be able to do it?

}
} else {
foreach (var rtarget in (Instruction[]) instr.Operand) {
int index = mapping == null ? instructions.IndexOf (rtarget) : mapping (rtarget);
if (index >= firstInstr && index <= lastInstr)
return true;
if (mapping != null && mapping (rtarget) is int index) {
if (index >= firstInstr && index <= lastInstr) {
return true;
}
}
else {
for (int i = firstInstr; i <= lastInstr; i++) {
if (instructions[i] == rtarget) {
return true;
}
}
}
}
}

Expand Down Expand Up @@ -1175,6 +1193,15 @@ int GetInstructionIndex (Instruction instruction)
return idx;
}

int? TryGetInstructionIndex (Instruction instruction)
{
Debug.Assert (mapping != null);
if (mapping.TryGetValue (instruction, out int idx))
return idx;

return null;
}

bool GetOperandsConstantValues (int index, out object? left, out object? right)
{
Debug.Assert (FoldedInstructions != null);
Expand Down Expand Up @@ -1213,7 +1240,7 @@ static bool IsPairedStlocLdloc (Instruction first, Instruction second)
bool IsJumpTargetRange (int firstInstr, int lastInstr)
{
Debug.Assert (FoldedInstructions != null);
return HasJumpIntoTargetRange (FoldedInstructions, firstInstr, lastInstr, GetInstructionIndex);
return HasJumpIntoTargetRange (FoldedInstructions, firstInstr, lastInstr, TryGetInstructionIndex);
}
}

Expand Down