[llvm-dev] llvm.read_register for %RIP on x86_64

[Renato Golin via llvm-dev]

On 26 January 2017 at 00:08, Kostya Serebryany via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> I want implement an instrumentation that gets the current PC.
> On x86_64 I can do it using inline asm (something like "lea (%%rip),%0"),
> but I wonder if there is some more LLVM-ish way to do it, e.g. an intrinsic?

Hi Kostya,

I'd also want something that GCC understands, as this code could end
up there, too.

The read_register intrinsic can be lowered by Clang from a number of
different builtins, so we could easily "support" some already-existing
GCC builtin for reading the PC, if you need to get it from C code.

Right now, the read_register is locked at the stack pointer as a
design decision. We do not know, nor we discussed the implications of
that intrinsic for any other register on purpose. If you want to read
the PC via a builtin, then we'll have to have that conversation one
way or another.

I strongly recommend you to use read_register, since support is
already there (you only need to add "PC" to the list and everything
works), and it's documented and the semantics are clear.

A way to convince people that reading the PC in certain cases is not
just ok, but meaningful, is to create a piece of inline asm and show
your case. It will certainly help the discussion to understand the
constraints and limit support for the cases we know are safe.

These are the original threads:

http://lists.llvm.org/pipermail/llvm-dev/2014-March/071472.html

http://lists.llvm.org/pipermail/llvm-dev/2014-March/071530.html

cheers,
--renato