[XRay] Align entry and return sleds to 2 byte boundaries

[dberris]

Summary

This should ensure that we can atomically write two bytes (on top of the
retq and the one past it) and have those two bytes not straddle cache
lines.

We also move the label past the alignment instruction so that we can refer
to the actual first instruction, as opposed to potential padding before the
aligned instruction.

Update the tests to allow us to reflect the new order of assembly.

[rSerge]

lib/Target/X86/X86MCInstLower.cpp

1079

Did you mean .p2align directive instead of .palign ?

1086

Why aligning at 4 bytes? Wouldn't 2 suffice?

[dberris]

Use 2-byte alignment for both entry and exit sleds.

lib/Target/X86/X86MCInstLower.cpp

1079

Thanks, yes, typo. :)

1086

That's right, I clearly wasn't thinking this through very well. :)

[dberris]

• Emit label after alignment directive
• Adjust test to new order

lib/Target/X86/X86MCInstLower.cpp

void X86AsmPrinter::recordSled(MCSymbol *Sled, const MachineInstr &MI,
   Sleds.emplace_back(
       XRayFunctionEntry{Sled, CurrentFnSym, Kind, AlwaysInstrument, Fn});
 }
 
 void X86AsmPrinter::LowerPATCHABLE_FUNCTION_ENTER(const MachineInstr &MI,
                                                   X86MCInstLower &MCIL) {
   // We want to emit the following pattern:
   //
+  //   .p2align 1, ...
   // .Lxray_sled_N:
-  //   .palign 2, ...
   //   jmp .tmpN
   //   # 9 bytes worth of noops
   // .tmpN
   //
   // We need the 9 bytes because at runtime, we'd be patching over the full 11
   // bytes with the following pattern:
   //
   //   mov %r10, <function id, 32-bit>   // 6 bytes
   //   call <relative offset, 32-bits>   // 5 bytes
   //
   auto CurSled = OutContext.createTempSymbol("xray_sled_", true);
+  OutStreamer->EmitCodeAlignment(2);
   OutStreamer->EmitLabel(CurSled);
-  OutStreamer->EmitCodeAlignment(4);
   auto Target = OutContext.createTempSymbol();
 
   // 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\x09");
   EmitNops(*OutStreamer, 9, Subtarget->is64Bit(), getSubtargetInfo());
   OutStreamer->EmitLabel(Target);
   recordSled(CurSled, MI, SledKind::FUNCTION_ENTER);
 }
 
 void X86AsmPrinter::LowerPATCHABLE_RET(const MachineInstr &MI,
                                        X86MCInstLower &MCIL) {
   // Since PATCHABLE_RET takes the opcode of the return statement as an
   // argument, we use that to emit the correct form of the RET that we want.
   // i.e. when we see this:
   //
   //   PATCHABLE_RET X86::RET ...
   //
   // We should emit the RET followed by sleds.
   //
+  //   .p2align 1, ...
   // .Lxray_sled_N:
   //   ret  # or equivalent instruction
   //   # 10 bytes worth of noops
   //
   // This just makes sure that the alignment for the next instruction is 2.
   auto CurSled = OutContext.createTempSymbol("xray_sled_", true);
+  OutStreamer->EmitCodeAlignment(2);
   OutStreamer->EmitLabel(CurSled);
   unsigned OpCode = MI.getOperand(0).getImm();
   MCInst Ret;
   Ret.setOpcode(OpCode);
   for (auto &MO : make_range(MI.operands_begin() + 1, MI.operands_end()))
     if (auto MaybeOperand = MCIL.LowerMachineOperand(&MI, MO))
       Ret.addOperand(MaybeOperand.getValue());
   OutStreamer->EmitInstruction(Ret, getSubtargetInfo());
   EmitNops(*OutStreamer, 10, Subtarget->is64Bit(), getSubtargetInfo());
   recordSled(CurSled, MI, SledKind::FUNCTION_EXIT);

test/CodeGen/X86/xray-attribute-instrumentation.ll

 ; RUN: llc -filetype=asm -o - -mtriple=x86_64-unknown-linux-gnu < %s | FileCheck %s
 
 define i32 @foo() nounwind noinline uwtable "function-instrument"="xray-always" {
+; CHECK:       .p2align 1, 0x90
 ; CHECK-LABEL: Lxray_sled_0:
-; CHECK-NEXT:  .p2align 2, 0x90
 ; CHECK-NEXT:  .ascii "\353\t"
 ; CHECK-NEXT:  nopw 512(%rax,%rax)
 ; CHECK-LABEL: Ltmp0:
   ret i32 0
+; CHECK:       .p2align 1, 0x90
 ; CHECK-LABEL: Lxray_sled_1:
 ; CHECK-NEXT:  retq
 ; CHECK-NEXT:  nopw %cs:512(%rax,%rax)
 }