[dynarmic] BACKPORT: remove RNG from regalloc, use incremental LRU

- Backported from Azahar's dynarmic <526227eebe>

> From: PabloMK7 <hackyglitch2@gmail.com>
> Date: Sun, 4 Jan 2026 13:57:44 +0100
> Subject: [PATCH] backend: Do not use random generator for reg allocation (#7)

Signed-off-by: lizzie <lizzie@eden-emu.dev>

src/core/hle/service/filesystem/fsp/fsp_srv.cpp

@@ -286,6 +286,7 @@ Result FSP_SRV::OpenSaveDataFileSystem(OutInterface<IFileSystem> out_interface,
     case FileSys::SaveDataSpaceId::Temporary:
     case FileSys::SaveDataSpaceId::ProperSystem:
     case FileSys::SaveDataSpaceId::SafeMode:
+    default:
         ASSERT(false);
     }
 

src/dynarmic/src/dynarmic/backend/arm64/reg_alloc.cpp

@@ -433,9 +433,10 @@ int RegAlloc::AllocateRegister(const std::array<HostLocInfo, 32>& regs, const st
     std::vector<int> candidates;
     std::copy_if(order.begin(), order.end(), std::back_inserter(candidates), [&](int i) { return regs[i].MaybeAllocatable(); });
 
-    // TODO: LRU
-    std::uniform_int_distribution<size_t> dis{0, candidates.size() - 1};
-    return candidates[dis(rand_gen)];
+    // TODO: The candidate was chosen randomly before, and a LRU was
+    // suggested as an improvement. However, using an incrementing index
+    // seems to be close enough. Determine if an LRU is still needed.
+    return candidates[alloc_candidate_index++ % candidates.size()];
 }
 
 void RegAlloc::SpillGpr(int index) {

src/dynarmic/src/dynarmic/backend/arm64/reg_alloc.h

@@ -160,8 +160,12 @@ class RegAlloc final {
 public:
     using ArgumentInfo = std::array<Argument, IR::max_arg_count>;
 
-    explicit RegAlloc(oaknut::CodeGenerator& code, FpsrManager& fpsr_manager, std::vector<int> gpr_order, std::vector<int> fpr_order)
-            : code{code}, fpsr_manager{fpsr_manager}, gpr_order{gpr_order}, fpr_order{fpr_order}, rand_gen{std::random_device{}()} {}
+    explicit RegAlloc(oaknut::CodeGenerator& code, FpsrManager& fpsr_manager, std::vector<int> gpr_order, std::vector<int> fpr_order) noexcept
+        : code{code}
+        , fpsr_manager{fpsr_manager}
+        , gpr_order{gpr_order}
+        , fpr_order{fpr_order}
+    {}
 
     ArgumentInfo GetArgumentInfo(IR::Inst* inst);
     bool WasValueDefined(IR::Inst* inst) const;
@@ -331,7 +335,7 @@ private:
     HostLocInfo flags;
     std::array<HostLocInfo, SpillCount> spills;
 
-    mutable std::mt19937 rand_gen;
+    mutable size_t alloc_candidate_index = 0;
     ankerl::unordered_dense::set<const IR::Inst*> defined_insts;
 };