[XRay] Allow logging the first argument of a function call.

[pelikan]

Summary

Functions with the "xray-log-args" attribute will have a special XRay sled kind
emitted, for compiler-rt to copy any call arguments to your logging handler.

For practical and performance reasons, only the first argument is supported, and
only up to 64 bits.

[dberris]

include/llvm/CodeGen/AsmPrinter.h

203–204

As mentioned elsewhere, please use 3.

[pelikan]

• change the number back

[pelikan]

• fix the number in the test too

[dberris]

LGTM

[pelikan]

• fix test on x86_64-darwin-unknown (I had no idea this was enabled)

include/llvm/CodeGen/AsmPrinter.h

public:
   // XRay instrumentation implementation.
   //===------------------------------------------------------------------===//
 public:
   // This describes the kind of sled we're storing in the XRay table.
   enum class SledKind : uint8_t {
     FUNCTION_ENTER = 0,
     FUNCTION_EXIT = 1,
     TAIL_CALL = 2,
+    // 3 unused and available (custom logging?)
+    LOG_ARGS_ENTER = 4,
   };
 
   // The table will contain these structs that point to the sled, the function
   // containing the sled, and what kind of sled (and whether they should always
   // be instrumented).
   struct XRayFunctionEntry {
     const MCSymbol *Sled;
     const MCSymbol *Function;

lib/CodeGen/AsmPrinter/AsmPrinter.cpp

void AsmPrinter::emitXRayTable() {
   OutStreamer->SwitchSection(PrevSection);
   Sleds.clear();
 }
 
 void AsmPrinter::recordSled(MCSymbol *Sled, const MachineInstr &MI,
   SledKind Kind) {
   auto Fn = MI.getParent()->getParent()->getFunction();
   auto Attr = Fn->getFnAttribute("function-instrument");
+  bool LogArgs = Fn->hasFnAttribute("xray-log-args");
   bool AlwaysInstrument =
     Attr.isStringAttribute() && Attr.getValueAsString() == "xray-always";
+  if (Kind == SledKind::FUNCTION_ENTER && LogArgs)
+    Kind = SledKind::LOG_ARGS_ENTER;
   Sleds.emplace_back(
     XRayFunctionEntry{ Sled, CurrentFnSym, Kind, AlwaysInstrument, Fn });
 }
 
 uint16_t AsmPrinter::getDwarfVersion() const {
   return OutStreamer->getContext().getDwarfVersion();
 }
 
 void AsmPrinter::setDwarfVersion(uint16_t Version) {
   OutStreamer->getContext().setDwarfVersion(Version);
 }

test/CodeGen/X86/xray-log-args.ll

+; When logging arguments is specified, emit the entry sled accordingly.
+
+; RUN: llc -filetype=asm -o - -mtriple=x86_64-unknown-linux-gnu < %s | FileCheck %s
+; RUN: llc -filetype=asm -o - -mtriple=x86_64-darwin-unknown < %s | FileCheck %s
+
+define i32 @callee(i32 %arg) nounwind noinline uwtable "function-instrument"="xray-always" "xray-log-args"="1" {
+  ret i32 %arg
+}
+; CHECK-LABEL: Lxray_synthetic_0:
+; CHECK:  .quad .Lxray_sled_0
+; CHECK:  .quad callee
+; CHECK:  .byte 4
+; CHECK:  .byte 1
+; CHECK:  .zero 14
+; CHECK:  .quad .Lxray_sled_1
+; CHECK:  .quad callee
+; CHECK:  .byte 1
+; CHECK:  .byte 1
+; CHECK:  .zero 14
+
+define i32 @caller(i32 %arg) nounwind noinline uwtable "function-instrument"="xray-always" "xray-log-args"="1" {
+  %retval = tail call i32 @callee(i32 %arg)
+  ret i32 %retval
+}
+; CHECK-LABEL: Lxray_synthetic_1:
+; CHECK:  .quad .Lxray_sled_2
+; CHECK:  .quad caller
+; CHECK:  .byte 4
+; CHECK:  .byte 1
+; CHECK:  .zero 14
+; CHECK:  .quad .Lxray_sled_3
+; CHECK:  .quad caller
+; CHECK:  .byte 2
+; CHECK:  .byte 1
+; CHECK:  .zero 14