From 6561a98a61bb54c9d6c868f788da3eaa6f48d2c3 Mon Sep 17 00:00:00 2001
From: mlugg <mlugg@mlugg.co.uk>
Date: Sun, 20 Apr 2025 02:01:46 +0100
Subject: incremental: correctly handle dead exporters

Resolves: #23604
---
 src/Zcu/PerThread.zig | 32 +++++++++++++++++++++++++++++---
 1 file changed, 29 insertions(+), 3 deletions(-)

(limited to 'src')

diff --git a/src/Zcu/PerThread.zig b/src/Zcu/PerThread.zig
index 095cd6c1cb..9084e8b0ca 100644
--- a/src/Zcu/PerThread.zig
+++ b/src/Zcu/PerThread.zig
@@ -2837,6 +2837,11 @@ pub fn processExports(pt: Zcu.PerThread) !void {
     const zcu = pt.zcu;
     const gpa = zcu.gpa;
 
+    if (zcu.single_exports.count() == 0 and zcu.multi_exports.count() == 0) {
+        // We can avoid a call to `resolveReferences` in this case.
+        return;
+    }
+
     // First, construct a mapping of every exported value and Nav to the indices of all its different exports.
     var nav_exports: std.AutoArrayHashMapUnmanaged(InternPool.Nav.Index, std.ArrayListUnmanaged(Zcu.Export.Index)) = .empty;
     var uav_exports: std.AutoArrayHashMapUnmanaged(InternPool.Index, std.ArrayListUnmanaged(Zcu.Export.Index)) = .empty;
@@ -2857,8 +2862,18 @@ pub fn processExports(pt: Zcu.PerThread) !void {
     // So, this ensureTotalCapacity serves as a reasonable (albeit very approximate) optimization.
     try nav_exports.ensureTotalCapacity(gpa, zcu.single_exports.count() + zcu.multi_exports.count());
 
-    for (zcu.single_exports.values()) |export_idx| {
+    const unit_references = try zcu.resolveReferences();
+
+    for (zcu.single_exports.keys(), zcu.single_exports.values()) |exporter, export_idx| {
         const exp = export_idx.ptr(zcu);
+        if (!unit_references.contains(exporter)) {
+            // This export might already have been sent to the linker on a previous update, in which case we need to delete it.
+            // The linker export API should be modified to eliminate this call. #23616
+            if (zcu.comp.bin_file) |lf| {
+                lf.deleteExport(exp.exported, exp.opts.name);
+            }
+            continue;
+        }
         const value_ptr, const found_existing = switch (exp.exported) {
             .nav => |nav| gop: {
                 const gop = try nav_exports.getOrPut(gpa, nav);
@@ -2873,8 +2888,19 @@ pub fn processExports(pt: Zcu.PerThread) !void {
         try value_ptr.append(gpa, export_idx);
     }
 
-    for (zcu.multi_exports.values()) |info| {
-        for (zcu.all_exports.items[info.index..][0..info.len], info.index..) |exp, export_idx| {
+    for (zcu.multi_exports.keys(), zcu.multi_exports.values()) |exporter, info| {
+        const exports = zcu.all_exports.items[info.index..][0..info.len];
+        if (!unit_references.contains(exporter)) {
+            // This export might already have been sent to the linker on a previous update, in which case we need to delete it.
+            // The linker export API should be modified to eliminate this loop. #23616
+            if (zcu.comp.bin_file) |lf| {
+                for (exports) |exp| {
+                    lf.deleteExport(exp.exported, exp.opts.name);
+                }
+            }
+            continue;
+        }
+        for (exports, info.index..) |exp, export_idx| {
             const value_ptr, const found_existing = switch (exp.exported) {
                 .nav => |nav| gop: {
                     const gop = try nav_exports.getOrPut(gpa, nav);
-- 
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