Document native GPS patch trampoline

red4ext/EdgeWeightGPS/src/Main.cpp

@@ -15,6 +15,9 @@
 #include <sstream>
 #include <string>
 
+// The public SDK headers used by this MinGW build provide the exported plugin
+// types, but not every callback table layout consumed here. Keep these minimal
+// ABI mirrors in field order and only use the members this plugin touches.
 namespace RED4ext::v1
 {
 struct Logger
@@ -82,9 +85,15 @@ struct Sdk
 
 namespace
 {
+// Executable RVAs in the tested Cyberpunk 2077 build. The two expansion sites
+// are ordinary road-neighbor relax callsites; both feed the native relax/update
+// helper that commits a candidate route node when the new score wins.
 constexpr uintptr_t kGpsRelaxUpdateRva = 0x040BA58;
 constexpr uintptr_t kGpsExpandListARelaxPatchRva = 0x040B49A;
 constexpr uintptr_t kGpsExpandListBRelaxPatchRva = 0x040B6C3;
+// Size of the overwritten instruction window at each expansion site. The
+// trampoline must return after these whole instructions, never into the middle
+// of native code.
 constexpr size_t kRelaxPatchSize = 23;
 constexpr size_t kPatchCaveSize = 80;
 constexpr float kFixedEdgePenalty = 80.0f;
@@ -104,6 +113,9 @@ RED4ext::v1::PluginHandle gHandle = nullptr;
 const RED4ext::v1::Sdk* gSdk = nullptr;
 
 std::array<PatchSite, 2> gPatchSites{{
+    // The two sites share the same relax/update tail but arrive with the
+    // current node cost in different XMM registers. Each expected instruction
+    // is replayed first so the cave preserves vanilla's edge accumulation.
     {
         "expand-list-a",
         kGpsExpandListARelaxPatchRva,
@@ -238,6 +250,13 @@ bool InstallPatchSite(PatchSite& aSite)
         cave[offset++] = byte;
     }
 
+    // Register contract for the native relax/update call:
+    // - xmm6 holds this edge's accumulated g-score.
+    // - xmm0 is the candidate f-score passed through [rsp+0x20].
+    // - xmm3 mirrors xmm6 for the helper call.
+    // The cave replays vanilla's first add into xmm6, adds our fixed segment
+    // penalty, rebuilds the helper arguments, then jumps back after the
+    // overwritten instruction window.
     const auto addPenaltyOffset = offset;
     EmitBytes(cave, offset, {0xF3, 0x0F, 0x58, 0x35, 0x00, 0x00, 0x00, 0x00}); // addss xmm6, [rip+disp32]
     EmitBytes(cave, offset, {0xF3, 0x0F, 0x58, 0xC6});                         // addss xmm0, xmm6
@@ -246,6 +265,8 @@ bool InstallPatchSite(PatchSite& aSite)
     EmitAbsoluteRaxCall(cave, offset, GameImageBase() + kGpsRelaxUpdateRva);
     EmitAbsoluteRaxJump(cave, offset, GameImageBase() + aSite.targetRva + kRelaxPatchSize);
 
+    // Keep the embedded float naturally aligned. The RIP-relative addss above
+    // is patched after the final cave layout is known.
     while ((offset % alignof(float)) != 0)
     {
         cave[offset++] = 0x90;
@@ -340,6 +361,8 @@ bool InstallPatchSites()
 
     if (!ok)
     {
+        // Route scoring should be all-or-nothing: one patched expansion site
+        // and one vanilla site would produce inconsistent route costs.
         RemovePatchSites();
     }