[common/sparse_large_vector] decommit unused pages + fix first page in zeroed region (#4471)

- [x] I have read and followed the [Contribution Guidelines](https://git.eden-emu.dev/eden-emu/eden/src/branch/master/CONTRIBUTING.md#code-contributions).
- [x] I have read and followed the [AI Policy](https://git.eden-emu.dev/eden-emu/eden/src/branch/master/docs/policies/AI.md)
- [x] I have read and followed the [Coding Guidelines](https://git.eden-emu.dev/eden-emu/eden/src/branch/master/docs/policies/Coding.md) to the best of my ability.

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Fixes a bug where the first page of a zeroed out region would not be properly zeroed, and now frees unused memory.

Reviewed-on: https://git.eden-emu.dev/eden-emu/eden/pulls/4471
Reviewed-by: lizzie <lizzie@eden-emu.dev>
Reviewed-by: MaranBr <maranbr@eden-emu.dev>

1 files changed, 15 insertions, 0 deletions

diff --git a/src/common/sparse_large_vector.cpp b/src/common/sparse_large_vector.cpp
index ce6455a235..745575a880 100644
--- a/src/common/sparse_large_vector.cpp
+++ b/src/common/sparse_large_vector.cpp
@@ -96,6 +96,21 @@ bool CommitVectorPage(uintptr_t addr, bool write) noexcept {
 #ifndef MAP_NOCORE
 #define MAP_NOCORE 0
 #endif
+#ifndef MADV_FREE
+#define MADV_FREE MADV_DONTNEED
+#endif
+
+void DecommitVectorPage(uintptr_t base) noexcept {
+#if defined(_WIN32)
+    VirtualFree(reinterpret_cast<LPVOID>(base), HostPageSize, MEM_DECOMMIT);
+#elif defined(__linux__)
+    // Linux's MADV_DONTNEED zeros out pages for us
+    madvise(reinterpret_cast<void*>(base), HostPageSize, MADV_DONTNEED);
+#else
+    madvise(reinterpret_cast<void*>(base), HostPageSize, MADV_FREE);
+    std::memset(reinterpret_cast<void*>(base), 0, HostPageSize);
+#endif
+}
 
 void* AllocateMemoryPages(std::size_t size) noexcept {
     if (auto page = HostPageSize; size % page != 0) {