From 586eec8826ef20d1a6b9c1de3b9fd17e261d0f6f Mon Sep 17 00:00:00 2001
From: Jake Bailey <5341706+jakebailey@users.noreply.github.com>
Date: Sat, 17 Oct 2026 09:07:26 -0700
Subject: [PATCH] Store merged symbols in a paged symbol link store
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getMergedSymbol runs on nearly every symbol access, and almost every
call misses because few symbols are merge sources. Each miss still
paid for a pointer-keyed map hash and probe.

Indexing by dense symbol ID makes the common miss a range check and
a page load.

--checkers 4, n=8, default GC settings:

                  │  before    │              after               │
                  │   sec/op   │   sec/op    vs base              │
Tsc/bluesky         2.799 ± 2%   2.767 ± 1%       ~ (p=0.266 n=8)
Tsc/sentry          10.86 ± 2%   10.78 ± 2%       ~ (p=0.314 n=8)
Tsc/t3code-server   13.89 ± 6%   13.50 ± 4%  -2.85% (p=0.050 n=8)
geomean             7.502        7.384       -1.58%

                  │   before     │               after                │
                  │    rss-B     │    rss-B      vs base              │
Tsc/bluesky         1.203Gi ± 2%   1.183Gi ± 2%       ~ (p=0.291 n=8)
Tsc/sentry          3.535Gi ± 6%   3.549Gi ± 6%       ~ (p=1.000 n=8)
Tsc/t3code-server   2.877Gi ± 3%   2.880Gi ± 2%       ~ (p=0.721 n=8)
geomean             2.304Gi        2.296Gi       -0.37%

                  │   before     │               after                │
                  │    heap-B    │    heap-B     vs base              │
Tsc/bluesky         934.8Mi ± 0%   934.5Mi ± 0%  -0.04% (p=0.038 n=8)
Tsc/sentry          2.603Gi ± 0%   2.608Gi ± 0%  +0.16% (p=0.000 n=8)
Tsc/t3code-server   2.312Gi ± 0%   2.301Gi ± 0%  -0.45% (p=0.000 n=8)
geomean             1.765Gi        1.763Gi       -0.11%
---
 tsc/internal/checker/checker.go | 10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/tsc/internal/checker/checker.go b/tsc/internal/checker/checker.go
index 10ddb3c0c88b4..2a1985ac572c1 100644
--- a/tsc/internal/checker/checker.go
+++ b/tsc/internal/checker/checker.go
@@ -692,7 +692,7 @@ type Checker struct {
 	structuredTypeExtraArena                    core.Arena[structuredTypeExtra]
 	freeTypeSets                                *temporaryTypeSet
 	freePropertyCounts                          *temporaryPropertyCounts
-	mergedSymbols                               map[*ast.Symbol]*ast.Symbol
+	mergedSymbols                               symbolLinkStore[*ast.Symbol]
 	mergedExportsChecked                        collections.Set[*ast.Symbol]
 	factory                                     ast.NodeFactory
 	nodeLinks                                   core.LinkStore[*ast.Node, NodeLinks]
@@ -1003,7 +1003,6 @@ func NewChecker(program Program, tracer *Tracer) (*Checker, *sync.Mutex) {
 	c.unionOfUnionTypes = make(map[UnionOfUnionKey]*Type)
 	c.intersectionTypes = make(map[CacheHashKey]*Type)
 	c.propertiesTypes = make(map[PropertiesTypesKey]*Type)
-	c.mergedSymbols = make(map[*ast.Symbol]*ast.Symbol)
 	c.patternForType = make(map[*Type]*ast.Node)
 	c.contextFreeTypes = make(map[*ast.Node]*Type)
 	c.anyType = c.newIntrinsicType(TypeFlagsAny, "any")
@@ -14624,9 +14623,8 @@ func (c *Checker) cloneSymbol(symbol *ast.Symbol) *ast.Symbol {
 
 func (c *Checker) getMergedSymbol(symbol *ast.Symbol) *ast.Symbol {
 	if symbol != nil {
-		merged := c.mergedSymbols[symbol]
-		if merged != nil {
-			return merged
+		if merged := c.mergedSymbols.TryGet(symbol); merged != nil && *merged != nil {
+			return *merged
 		}
 	}
 	return symbol
@@ -14640,7 +14638,7 @@ func (c *Checker) getParentOfSymbol(symbol *ast.Symbol) *ast.Symbol {
 }
 
 func (c *Checker) recordMergedSymbol(target *ast.Symbol, source *ast.Symbol) {
-	c.mergedSymbols[source] = target
+	*c.mergedSymbols.Get(source) = target
 }
 
 func (c *Checker) getResolvedTarget(symbol *ast.Symbol) *ast.Symbol {