[llvm-dev] XRay: Demo on x86_64/Linux almost done; some questions.

[Dean Michael Berris via llvm-dev]

Hi,

The following three patches, when applied cleanly to LLVM+Clang+compiler-rt
will now allow for building and running binaries with a very simple version
of XRay included:

http://reviews.llvm.org/D21612 (compiler-rt)
http://reviews.llvm.org/D20352 (clang)
http://reviews.llvm.org/D19904 (llvm)

To use this on x86_64-unknown-linux-gnu you'd need to set the flags
'-fxray-instrument' and if you'd like to instrument all functions, set
'-fxray-instruction-threshold=1'. Apply those flags to all your object
files, and link statically for best effect.

Now, some questions before I continue:

- What is the preferred way of controlling the behaviour of the runtime
library for something like XRay? Do the sanitizers check/use environment
variables or commandline-flags to control behaviour at runtime/init?
- We would like to be able to trigger the patching/unpatching routines at
runtime in a portable manner. In Linux and other UNIX-like environments
signals (SIGUSR1 and SIGUSR2) might be good candidates for signalling the
tracing infrastructure to start/stop at user-controlled times (useful for
long-running servers). Is there a preference for this, or are there
alternatives in this space that might make better sense?
- Currently we are using 'printf' for the logging, but could use a simple
thread-local in-memory log that flushes to disk when full. Any other
preferred ways of doing this?
- Documentation for how to use/run XRay may need to live in a central
location, but since the changes to the LLVM pieces are currently in three
different places, are there suggestions for where the docs should live?

Cheers

PS. An example log of how to use XRay with clang+llvm+compiler-rt with the
above patches:

[16-06-28 17:21:02] dberris at dberris: ~/xray/llvm-build% cat test.cc
#include <cstdio>
#include <cassert>

[[clang::xray_always_instrument]] void foo() { std::printf("Hello,
XRay!\n"); }

[[clang::xray_never_instrument]] void bar() { std::printf("Not
instrumented\n"); }

extern void baz();  // defined in other.cc

int main(int argc, char* argv[]) {
  printf("main has started.\n");
  bar();
  foo();
  baz();
}
[16-06-28 17:30:13] dberris at dberris: ~/xray/llvm-build% cat other.cc
#include <cstdio>

[[clang::xray_always_instrument]] void baz() {
  std::printf("Welcome!\n");
}

[16-06-28 17:30:16] dberris at dberris: ~/xray/llvm-build% ./bin/clang -c
test.cc -x c++ -std=c++11 -fxray-instrument
[16-06-28 17:30:34] dberris at dberris: ~/xray/llvm-build% ./bin/clang -c
other.cc -x c++ -std=c++11 -fxray-instrument
[16-06-28 17:30:46] dberris at dberris: ~/xray/llvm-build% ./bin/clang -o
test.bin test.o other.o -fxray-instrument
[16-06-28 17:30:55] dberris at dberris: ~/xray/llvm-build% ./test.bin
__xray_instr_map at 0x400f5b..0x400fdb
400cf0  E       *       @function(400cf0)
400d1c  X       *       @function(400cf0)
400da0  E       *       @function(400da0)
400dcc  X       *       @function(400da0)
main has started.
Not instrumented
4133: [9699491610577808] E1
Hello, XRay!
4133: [9699491610592074] X1
4133: [9699491610599840] E2
Welcome!
4133: [9699491610611254] X2
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