From 156f6cafc28777dc06515114b4c81fef01ac7d43 Mon Sep 17 00:00:00 2001
From: Dean Michael Berris <dberris@google.com>
Date: Tue, 18 Oct 2016 05:54:15 +0000
Subject: [XRay] Support for for tail calls for ARM no-Thumb

This patch adds simplified support for tail calls on ARM with XRay instrumentation.

Known issue: compiled with generic flags: `-O3 -g -fxray-instrument -Wall
-std=c++14  -ffunction-sections -fdata-sections` (this list doesn't include my
specific flags like --target=armv7-linux-gnueabihf etc.), the following program

    #include <cstdio>
    #include <cassert>
    #include <xray/xray_interface.h>

    [[clang::xray_always_instrument]] void __attribute__ ((noinline)) fC() {
      std::printf("In fC()\n");
    }

    [[clang::xray_always_instrument]] void __attribute__ ((noinline)) fB() {
      std::printf("In fB()\n");
      fC();
    }

    [[clang::xray_always_instrument]] void __attribute__ ((noinline)) fA() {
      std::printf("In fA()\n");
      fB();
    }

    // Avoid infinite recursion in case the logging function is instrumented (so calls logging
    //   function again).
    [[clang::xray_never_instrument]] void simplyPrint(int32_t functionId, XRayEntryType xret)
    {
      printf("XRay: functionId=%d type=%d.\n", int(functionId), int(xret));
    }

    int main(int argc, char* argv[]) {
      __xray_set_handler(simplyPrint);

      printf("Patching...\n");
      __xray_patch();
      fA();

      printf("Unpatching...\n");
      __xray_unpatch();
      fA();

      return 0;
    }

gives the following output:

    Patching...
    XRay: functionId=3 type=0.
    In fA()
    XRay: functionId=3 type=1.
    XRay: functionId=2 type=0.
    In fB()
    XRay: functionId=2 type=1.
    XRay: functionId=1 type=0.
    XRay: functionId=1 type=1.
    In fC()
    Unpatching...
    In fA()
    In fB()
    In fC()

So for function fC() the exit sled seems to be called too much before function
exit: before printing In fC().

Debugging shows that the above happens because printf from fC is also called as
a tail call. So first the exit sled of fC is executed, and only then printf is
jumped into. So it seems we can't do anything about this with the current
approach (i.e. within the simplification described in
https://reviews.llvm.org/D23988 ).

Differential Revision: https://reviews.llvm.org/D25030

llvm-svn: 284456
---
 llvm/lib/Target/ARM/ARMAsmPrinter.cpp | 3 +++
 1 file changed, 3 insertions(+)

(limited to 'llvm/lib/Target/ARM/ARMAsmPrinter.cpp')

diff --git a/llvm/lib/Target/ARM/ARMAsmPrinter.cpp b/llvm/lib/Target/ARM/ARMAsmPrinter.cpp
index bd95c33fc49..ad8a6ed0b6d 100644
--- a/llvm/lib/Target/ARM/ARMAsmPrinter.cpp
+++ b/llvm/lib/Target/ARM/ARMAsmPrinter.cpp
@@ -2050,6 +2050,9 @@ void ARMAsmPrinter::EmitInstruction(const MachineInstr *MI) {
   case ARM::PATCHABLE_FUNCTION_EXIT:
     LowerPATCHABLE_FUNCTION_EXIT(*MI);
     return;
+  case ARM::PATCHABLE_TAIL_CALL:
+    LowerPATCHABLE_TAIL_CALL(*MI);
+    return;
   }
 
   MCInst TmpInst;
-- 
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