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

[Christopher Ferris via llvm-commits]

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@@ -117,40 +117,24 @@ template <typename Config> class SizeClassAllocator64 {
     SmallerBlockReleasePageDelta =
         PagesInGroup * (1 + MinSizeClass / 16U) / 100;
 
-    // Reserve the space required for the Primary.
-    CHECK(ReservedMemory.create(/*Addr=*/0U, PrimarySize,
-                                "scudo:primary_reserve"));
-    PrimaryBase = ReservedMemory.getBase();
-    DCHECK_NE(PrimaryBase, 0U);
-
-    u32 Seed;
-    const u64 Time = getMonotonicTimeFast();
-    if (!getRandom(reinterpret_cast<void *>(&Seed), sizeof(Seed)))
-      Seed = static_cast<u32>(Time ^ (PrimaryBase >> 12));
-
-    for (uptr I = 0; I < NumClasses; I++) {
-      RegionInfo *Region = getRegionInfo(I);
-
-      // The actual start of a region is offset by a random number of pages
-      // when PrimaryEnableRandomOffset is set.
-      Region->RegionBeg = (PrimaryBase + (I << RegionSizeLog)) +
-                          (Config::getEnableRandomOffset()
-                               ? ((getRandomModN(&Seed, 16) + 1) * PageSize)
-                               : 0);
-      Region->RandState = getRandomU32(&Seed);
-      // Releasing small blocks is expensive, set a higher threshold to avoid
-      // frequent page releases.
-      if (isSmallBlock(getSizeByClassId(I)))
-        Region->TryReleaseThreshold = PageSize * SmallerBlockReleasePageDelta;
-      else
-        Region->TryReleaseThreshold = PageSize;
-      Region->ReleaseInfo.LastReleaseAtNs = Time;
-
-      Region->MemMapInfo.MemMap = ReservedMemory.dispatch(
-          PrimaryBase + (I << RegionSizeLog), RegionSize);
-      CHECK(Region->MemMapInfo.MemMap.isAllocated());
+    if (Config::getPreserveAllRegions()) {
+      ReservedMemoryT ReservedMemory = {};
+      // Reserve the space required for the Primary.
+      CHECK(ReservedMemory.create(/*Addr=*/0U, RegionSize * NumClasses,
+                                  "scudo:primary_reserve"));
+      const uptr PrimaryBase = ReservedMemory.getBase();
+      const bool EnableRandomOffset =
+          Config::getPreserveAllRegions() && Config::getEnableRandomOffset();
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cferris1000 wrote:

Isn't the getPreservedAllRegions check redundant to the check above?

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