[clang] [lld] [llvm] [Windows] Add support for emitting PGO/LTO magic strings in the Windows PE debug directory (PR #114260)

[Mikołaj Piróg via cfe-commits]

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@@ -0,0 +1,13 @@
+// This test checks if Window PE file compiled with -flto option contains a magic 
+// string "LTCG" to indicate LTO compilation.
+
+// REQUIRES: system-windows
+
+// RUN: %clang --target=x86_64-pc-windows-msvc -flto -fuse-ld=lld %s -o %t.exe
+// RUN: dumpbin /HEADERS %t.exe | FileCheck %s
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mikolaj-pirog wrote:

I was using `llvm-objdump` while I was working on that, but I think that using `dumpbin` is better compared to that. It can be somewhat tricky to emit this info in such a way that `dumpbin` catches it: first this info has to be emitted in the debug dir and in such a way that `dumpbin` recognizes the string (I believe it's length has to be divisible by 4 -- if not it has to prefixed with null bytes). Using the `dumpbin` to test assures that it correctly sees the magic strings. Checking for simple existence of string in the Windows PE file, using `llvm-objdump` for example, is not enough, since the string can have wrong length, and not be recognized by `dumpbin`. I was also aiming to achieve running on other OS, but that means dropping the `dumpbin`, which I think is wrong and the "PGU" test still has to be run on Windows (since the binary has to be run)

https://github.com/llvm/llvm-project/pull/114260