[PATCH] D27503: [XRay] Custom event logging intrinsic

[Tim Shen via Phabricator via llvm-commits]

timshen added inline comments.


================
Comment at: lib/Target/X86/X86MCInstLower.cpp:1052
+  //   .p2align 1, ...
+  // .Lxray_event_sled_N:
+  //   jmp +N                    // jump across the call instruction
----------------
dberris wrote:
> timshen wrote:
> > I believe that before at symbol, all caller-saved registers are actually saved?
> I thought that too, except the calling convention for the intrinsic seems to be the C calling convention (default) as opposed to the SysV64 calling convention that I'd like it to be. This is where the question of whether it's possible (or whether it makes sense) for us to actually provide the calling convention for the intrinsic (or whether the front-end has to explicitly state this).
> 
> Any suggestions on how to force the calling convention here?
Why do you want SysV64 calling convention?

I was trying to confirm that no registers will be clobbered unexpectedly, causing correctness issues.


================
Comment at: lib/Target/X86/X86MCInstLower.cpp:1075
+  // FIXME: Find another less hacky way do force the relative jump.
+  OutStreamer->EmitBytes("\xeb\x14");
+
----------------
dberris wrote:
> timshen wrote:
> > Maybe jump to an "exit" symbol, and emit that symbol at the end of the sled?
> > 
> > See: https://github.com/llvm-mirror/llvm/blob/288180ac40ab050711298f42340233bd47916634/lib/Target/PowerPC/PPCAsmPrinter.cpp#L1076
> That doesn't quite work for x86 because the jump instruction emitted can vary (i.e. it's not always 2 bytes). The runtime requires us to be able to write two bytes atomically to enable/disable the jump. Since there's no way yet for us to force a specific jump instruction that will be preserved when the relaxation pass runs, for now we need to hard-code this static relative jump ourselves.
I'm not quite following - it's AsmPrinter, and there shouldn't be any lowering after that, is it right?

What is the "relaxation pass" you mentioned? Is it BranchRelaxation pass? But I think it runs before AsmPrinter?


https://reviews.llvm.org/D27503