[compiler-rt] [win/asan] AllocateMemoryForTrampoline within 2 GB of the module's base address (PR #108822)

[Reid Kleckner via llvm-commits]

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@@ -385,7 +386,29 @@ void TestOnlyReleaseTrampolineRegions() {
   }
 }
 
-static uptr AllocateMemoryForTrampoline(uptr image_address, size_t size) {
+static uptr AllocateMemoryForTrampoline(uptr func_address, size_t size) {
+  uptr image_address = func_address;
+
+#if SANITIZER_WINDOWS64
+  // Since we may copy code to the trampoline which could reference data
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rnk wrote:

This is a long comment. For clarity, I would lead by describing what we're trying to do, which is to:
Allocate trampoline memory after the DLL, but within 2GiB of the start of the DLL, so that any address in the DLL can be referenced with PC-relative operands.

It would be more complicated, but potentially less subtle, to generalize this to be an interval comparison of abs([image_base, image_end) - [next_address, next_address+size)) < [2GiB, 2GiB)
(not sure if that's the right pseudocode...)

https://github.com/llvm/llvm-project/pull/108822