[compiler-rt] [win/asan] Search both higher and lower in AllocateTrampolineRegion (PR #114212)

[Reid Kleckner via llvm-commits]

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@@ -340,32 +347,66 @@ struct TrampolineMemoryRegion {
   uptr max_size;
 };
 
-UNUSED static const uptr kTrampolineScanLimitRange = 1ull << 31;  // 2 gig
+UNUSED static const uptr kTrampolineRangeLimit = 1ull << 31;  // 2 gig
 static const int kMaxTrampolineRegion = 1024;
 static TrampolineMemoryRegion TrampolineRegions[kMaxTrampolineRegion];
 
-static void *AllocateTrampolineRegion(uptr image_address, size_t granularity) {
-#if SANITIZER_WINDOWS64
-  uptr address = image_address;
-  uptr scanned = 0;
-  while (scanned < kTrampolineScanLimitRange) {
+static void *AllocateTrampolineRegion(uptr min_addr, uptr max_addr,
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rnk wrote:

I think this deserves a significant doc comment. The loop below probes the address space in a search that alternates between forward probing and backwards probing, and that makes the logic fairly hard to follow. I'd stop short of ASCII art, but a text list like this would help:
```
// This loop probes the virtual address space to find free memory in range to implement a trampoline. It alternates searching backwards and forwards, probing regions below in the order P1, P2, P3, P4, etc.
// min_addr
// ...
// P4
// P2
// func_addr
// P1
// P3
// ...
// max_addr
```

https://github.com/llvm/llvm-project/pull/114212