[compiler-rt] [scudo] Support no-preserve-all-regions mode (PR #85149)

[via llvm-commits]

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@@ -601,6 +593,35 @@ template <typename Config> class SizeClassAllocator64 {
     return BlockSize > PageSize;
   }
 
+  void initRegion(RegionInfo *Region, uptr ClassId, MemMapT MemMap,
+                  bool EnableRandomOffset) REQUIRES(Region->MMLock) {
+    DCHECK(!Region->MemMapInfo.MemMap.isAllocated());
+    DCHECK(MemMap.isAllocated());
+
+    const uptr PageSize = getPageSizeCached();
+    const uptr RegionBase = MemMap.getBase();
+
+    Region->MemMapInfo.MemMap = MemMap;
+
+    u32 Seed;
+    const u64 Time = getMonotonicTimeFast();
+    if (!getRandom(reinterpret_cast<void *>(&Seed), sizeof(Seed)))
+      Seed = static_cast<u32>(Time ^ (RegionBase >> 12));
+
+    Region->RegionBeg = RegionBase;
+    if (EnableRandomOffset)
+      Region->RegionBeg += (getRandomModN(&Seed, 16) + 1) * PageSize;
+
+    // Releasing small blocks is expensive, set a higher threshold to avoid
+    // frequent page releases.
+    if (isSmallBlock(getSizeByClassId(ClassId)))
+      Region->TryReleaseThreshold = PageSize * SmallerBlockReleasePageDelta;
+    else
+      Region->TryReleaseThreshold = PageSize;
+
+    Region->ReleaseInfo.LastReleaseAtNs = Time;
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ChiaHungDuan wrote:

Given that the seed has been moved to init() and we think it should be no significant difference if we set `LastReleaseAtNs` to current time or zero, I leave it as 0.

https://github.com/llvm/llvm-project/pull/85149