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compiler: use Tarjan's SCC algorithm to detect loops for defer

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The compiler needs to know whether a defer is in a loop to determine whether to allocate stack or heap memory.
Previously, this performed a DFS of the CFG every time a defer was found.
This resulted in time complexity jointly proportional to the number of defers and the number of blocks in the function.

Now, the compiler will instead use Tarjan's strongly connected components algorithm to find cycles in linear time.
The search is performed lazily, so this has minimal performance impact on functions without defers.

In order to implement Tarjan's SCC algorithm, additional state needed to be attached to the blocks.
I chose to merge all of the per-block state into a single slice to simplify memory management.
This commit is contained in:
Nia Waldvogel
2025-11-08 14:00:22 -05:00
committed by Ron Evans
parent bf613175e7
commit 852dde671e
6 changed files with 400 additions and 46 deletions
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compiler/interface.go
+1 -1
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@@ -737,7 +737,7 @@ func (b *builder) createTypeAssert(expr *ssa.TypeAssert) llvm.Value {
prevBlock := b.GetInsertBlock()
okBlock := b.insertBasicBlock("typeassert.ok")
nextBlock := b.insertBasicBlock("typeassert.next")
b.blockExits[b.currentBlock] = nextBlock // adjust outgoing block for phi nodes
b.currentBlockInfo.exit = nextBlock // adjust outgoing block for phi nodes
b.CreateCondBr(commaOk, okBlock, nextBlock)
// Retrieve the value from the interface if the type assert was