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Summary: Based on two patches by Michael Mueller. This is a target attribute that causes a function marked with it to be emitted as "hotpatchable". This particular mechanism was originally devised by Microsoft for patching their binaries (which they are constantly updating to stay ahead of crackers, script kiddies, and other ne'er-do-wells on the Internet), but is now commonly abused by Windows programs to hook API functions. This mechanism is target-specific. For x86, a two-byte no-op instruction is emitted at the function's entry point; the entry point must be immediately preceded by 64 (32-bit) or 128 (64-bit) bytes of padding. This padding is where the patch code is written. The two byte no-op is then overwritten with a short jump into this code. The no-op is usually a `movl %edi, %edi` instruction; this is used as a magic value indicating that this is a hotpatchable function. Reviewers: majnemer, sanjoy, rnk Subscribers: dberris, llvm-commits Differential Revision: https://reviews.llvm.org/D19908 llvm-svn: 278048
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