[llvm] Implement reserveAllocationSpace for SectionMemoryManager (PR #71968)

[via llvm-commits]

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@@ -18,6 +18,66 @@
 
 namespace llvm {
 
+static uintptr_t requiredPageSize(uintptr_t Size, uint32_t Alignment) {
+  static const size_t PageSize = sys::Process::getPageSize();
+  // Use the same calculation as allocateSection because we need to be able to
+  // satisfy it.
+  uintptr_t RequiredSize = Alignment * ((Size + Alignment - 1) / Alignment + 1);
+  // Round up to the nearest page size. Blocks must be page-aligned.
+  return PageSize * ((RequiredSize + PageSize - 1) / PageSize);
+}
+
+void SectionMemoryManager::reserveAllocationSpace(
+    uintptr_t CodeSize, uint32_t CodeAlign, uintptr_t RODataSize,
+    uint32_t RODataAlign, uintptr_t RWDataSize, uint32_t RWDataAlign) {
+  if (CodeSize == 0 && RODataSize == 0 && RWDataSize == 0)
+    return;
+
+  // Get space required for each section.
+  CodeAlign = CodeAlign ? CodeAlign : 16;
+  RODataAlign = RODataAlign ? RODataAlign : 16;
+  RWDataAlign = RWDataAlign ? RWDataAlign : 16;
+  uintptr_t RequiredCodeSize = requiredPageSize(CodeSize, CodeAlign);
+  uintptr_t RequiredRODataSize = requiredPageSize(RODataSize, RODataAlign);
+  uintptr_t RequiredRWDataSize = requiredPageSize(RWDataSize, RWDataAlign);
+  uintptr_t RequiredSize =
+      RequiredCodeSize + RequiredRODataSize + RequiredRWDataSize;
+
+  std::error_code ec;
+  sys::MemoryBlock MB = sys::Memory::allocateMappedMemory(
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lhames wrote:

I think it's essential to `SectionMemoryManager` that the actual allocation is done by the mapper. We may have to add a `reserve` to the mapper interface to support this.

As a fallback (since this is opt-in anyway, and will be deprecated in the near future): Could we use a Mapper request for RW- memory? 

https://github.com/llvm/llvm-project/pull/71968