[PATCH] Patchpoint - support symbolic targets.

[Lang Hames]

Hi All,

At the moment llvm.patchpoint call targets must be integer constants (e.g.
0xDEADBEEF). This patch adds support for symbolic targets like @foo, which
addresses a couple of FIXMEs.

Making this work just involved teaching FastISel and SelectionDAG to
construct the appropriate MI/SDNodes, and teaching the Targets how to lower
these to MCInsts. Target support for x86 is included in this patch. Support
for other targets should be easy to derive from that.

With this patch applied, you can write patchpoints of the following form:

tail call i64 (i64, i32, i8*, i32, ...)*
  @llvm.experimental.patchpoint.void(i64 9, i32 15,
    i8* bitcast (i64 (i64, i64)* @foo to i8*),      ; <- Call target
    i32 2, i64 %p1, i64 %p2)

and this will generate:

movabsq $_foo, %r11
callq   *%r11
// <nop-padding>

For integer targets this would have been something like:

movabsq $0xDEADBEEF, %r11
callq   *%r11
// <nop-padding>

The advantage of symbolic targets, beyond improved readability, is that you
can cache the IR or compiled objects and re-use them in contexts where the
target address may have changed. For example, objects that use symbolic
patchpoints can be cached in Orc/MCJIT object-caches and re-used across JIT
invocations.

Immediate-address targets are still fully supported.

I don't really see any downside to this patch, but I thought I'd throw it
out for general discussion before I go and widen what patchpoint is
supposed to support.

Does anyone see any problem with this idea?

For the curious, my motivation is that I'd like (eventually) to support
patchpoints for JIT re-entry in Orc as an alternative to indirect calls,
and I want that transformation to be straightforward. E.g.

call @not_yet_compiled, ...
to
call llvm.patchpoint ..., @not_yet_compiled, ...

Cheers,
Lang.
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