[clang] [Clang] Add fake use emission to Clang with -fextend-lifetimes (PR #110102)

[Jeremy Morse via cfe-commits]

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+// RUN: %clang_cc1 %s -O1 -emit-llvm -fextend-lifetimes -fsanitize=null -fsanitize-trap=null -o - | FileCheck --check-prefix TRAP %s
+// RUN: %clang_cc1 %s -O1 -emit-llvm -fextend-lifetimes -o - | FileCheck --check-prefix FAKEUSE %s
+
+// With -fextend-lifetimes the compiler generated a fake.use of a
+// reference variable at the end of the function, in the cleanup block. This prompted the 
+// address sanitizer to verify that the variable has been allocated properly, even when 
+// the function returns early.
+// We check that sanitizers are disabled for code generated for the benefit of fake.use 
+// intrinsics, as well as that the fake.use is no longer generated in the cleanup block.
+// It should be generated in the block preceding the cleanup block instead.
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jmorse wrote:

Despite the filename containing "sanitizer", this test doesn't actually test sanitizer behaviour?

https://github.com/llvm/llvm-project/pull/110102