[XRay] Custom event logging intrinsic

This patch introduces an LLVM intrinsic and a target opcode for custom event
logging in XRay. Initially, its use case will be to allow users of XRay to log
some type of string ("poor man's printf"). The target opcode compiles to a noop
sled large enough to enable calling through to a runtime-determined relative
function call. At runtime, when X-Ray is enabled, the sled is replaced by
compiler-rt with a trampoline to the logic for creating the custom log entries.

Future patches will implement the compiler-rt parts and clang-side support for
emitting the IR corresponding to this intrinsic.

Reviewers: timshen, dberris

Subscribers: igorb, pelikan, rSerge, timshen, echristo, dberris, llvm-commits

Differential Revision: https://reviews.llvm.org/D27503

llvm-svn: 302405

1 files changed, 80 insertions, 0 deletions

diff --git a/llvm/lib/Target/X86/X86MCInstLower.cpp b/llvm/lib/Target/X86/X86MCInstLower.cpp
index 550e3543a71..598d88d8b9c 100644
--- a/llvm/lib/Target/X86/X86MCInstLower.cpp
+++ b/llvm/lib/Target/X86/X86MCInstLower.cpp
@@ -1040,6 +1040,83 @@ void X86AsmPrinter::LowerPATCHPOINT(const MachineInstr &MI,
            getSubtargetInfo());
 }
 
+void X86AsmPrinter::LowerPATCHABLE_EVENT_CALL(const MachineInstr &MI,
+                                              X86MCInstLower &MCIL) {
+  assert(Subtarget->is64Bit() && "XRay custom events only suports X86-64");
+
+  // We want to emit the following pattern, which follows the x86 calling
+  // convention to prepare for the trampoline call to be patched in.
+  //
+  //   <args placement according SysV64 calling convention>
+  //   .p2align 1, ...
+  // .Lxray_event_sled_N:
+  //   jmp +N                    // jump across the call instruction
+  //   callq __xray_CustomEvent  // force relocation to symbol
+  //   <args cleanup, jump to here>
+  //
+  // The relative jump needs to jump forward 24 bytes:
+  // 10 (args) + 5 (nops) + 9 (cleanup)
+  //
+  // After patching, it would look something like:
+  //
+  //   nopw (2-byte nop)
+  //   callq __xrayCustomEvent  // already lowered
+  //
+  // ---
+  // First we emit the label and the jump.
+  auto CurSled = OutContext.createTempSymbol("xray_event_sled_", true);
+  OutStreamer->AddComment("# XRay Custom Event Log");
+  OutStreamer->EmitCodeAlignment(2);
+  OutStreamer->EmitLabel(CurSled);
+
+  // Use a two-byte `jmp`. This version of JMP takes an 8-bit relative offset as
+  // an operand (computed as an offset from the jmp instruction).
+  // FIXME: Find another less hacky way do force the relative jump.
+  OutStreamer->EmitBytes("\xeb\x14");
+
+  // The default C calling convention will place two arguments into %rcx and
+  // %rdx -- so we only work with those.
+  unsigned UsedRegs[] = {X86::RDI, X86::RSI, X86::RAX};
+
+  // Because we will use %rax, we preserve that across the call.
+  EmitAndCountInstruction(MCInstBuilder(X86::PUSH64r).addReg(X86::RAX));
+
+  // Then we put the operands in the %rdi and %rsi registers.
+  for (unsigned I = 0; I < MI.getNumOperands(); ++I)
+    if (auto Op = MCIL.LowerMachineOperand(&MI, MI.getOperand(I))) {
+      if (Op->isImm())
+        EmitAndCountInstruction(MCInstBuilder(X86::MOV64ri)
+                                    .addReg(UsedRegs[I])
+                                    .addImm(Op->getImm()));
+      else if (Op->isReg()) {
+        if (Op->getReg() != UsedRegs[I])
+          EmitAndCountInstruction(MCInstBuilder(X86::MOV64rr)
+                                      .addReg(UsedRegs[I])
+                                      .addReg(Op->getReg()));
+        else
+          EmitNops(*OutStreamer, 3, Subtarget->is64Bit(), getSubtargetInfo());
+      }
+    }
+
+  // We emit a hard dependency on the __xray_CustomEvent symbol, which is the
+  // name of the trampoline to be implemented by the XRay runtime. We put this
+  // explicitly in the %rax register.
+  auto TSym = OutContext.getOrCreateSymbol("__xray_CustomEvent");
+  MachineOperand TOp = MachineOperand::CreateMCSymbol(TSym);
+  EmitAndCountInstruction(MCInstBuilder(X86::MOV64ri)
+                              .addReg(X86::RAX)
+                              .addOperand(MCIL.LowerSymbolOperand(TOp, TSym)));
+
+  // Emit the call instruction.
+  EmitAndCountInstruction(MCInstBuilder(X86::CALL64r).addReg(X86::RAX));
+
+  // Restore caller-saved and used registers.
+  OutStreamer->AddComment("xray custom event end.");
+  EmitAndCountInstruction(MCInstBuilder(X86::POP64r).addReg(X86::RAX));
+
+  recordSled(CurSled, MI, SledKind::CUSTOM_EVENT);
+}
+
 void X86AsmPrinter::LowerPATCHABLE_FUNCTION_ENTER(const MachineInstr &MI,
                                                   X86MCInstLower &MCIL) {
   // We want to emit the following pattern:
@@ -1415,6 +1492,9 @@ void X86AsmPrinter::EmitInstruction(const MachineInstr *MI) {
 
   case TargetOpcode::PATCHABLE_TAIL_CALL:
     return LowerPATCHABLE_TAIL_CALL(*MI, MCInstLowering);
+    
+  case TargetOpcode::PATCHABLE_EVENT_CALL:
+    return LowerPATCHABLE_EVENT_CALL(*MI, MCInstLowering);
 
   case X86::MORESTACK_RET:
     EmitAndCountInstruction(MCInstBuilder(getRetOpcode(*Subtarget)));