[llvm-dev] RFC: New function attribute "patchable-prologue"="<kind>"

[Mehdi Amini via llvm-dev]

> On Apr 6, 2016, at 12:10 AM, Sanjoy Das via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> [Proposed langref entry]
> 
> The "patchable-prologue" attribute on a function is a general
> mechanism to control the form of a function's prologue in ways that
> make it easy to patch at runtime.
> 
> Currently only one value is supported:
> 
> # "hotpatch-compact"
> 
> If a function is marked with "patchable-prologue"="hotpatch-compact"
> then:
> 
> 1. The first instruction of the function is at least two bytes long.

IIRC the motivation is to insert a branch instruction in a thread-safe way?
Isn’t the “two bytes” something that is target specific?

— 
Mehdi

> 2. The first two bytes of the first instruction does not span a cache
>    line boundary.
> 3. The instruction denoted by (1) is preferably not a no-op, i.e.,
>    we'd prefer to "re-use" an instruction already present.  For
>    instance, we can emit a two byte form of a "push CSR" instruction
>    that we'd have needed anyway.
> 
> "hotpatch-compact" is useful for runtimes that want to thread-safely
> overwrite the first instruction of a function with a short branch.
> 
> [End proposed langref entry]
> 
> 
> We can consider adding more "schemes" in the future, for instance
> "hotpatch-ms" for the Microsoft hotpatching scheme.  I was initially
> thinking of proposing "num-patchable-bytes"="<count>" as the
> fundamental building block, but the more I thought about it the less I
> liked it, since a specific patching scheme has several tightly
> integrated constraints of which the number of bytes is just one.
> 
> What do you think?
> 
> -- Sanjoy
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