Using the ppprofiler pass with LLVM tools – Optimizing IR-1
Recall the ppprofiler pass that we developed as a plugin out of the LLVM tree in the Developing the ppprofiler pass as a plugin section. Here, we’ll learn how to use this pass with LLVM tools, such as opt and clang, as they can load plugins.
Let’s look at opt first.
Run the pass plugin in opt
To play around with the new plugin, you need a file containing LLVM IR. The easiest way to do this is to translate a C program, such as a basic “Hello World” style program:
include
int main(int argc, char *argv[]) {
puts(“Hello”);
return 0;
}
Compile this file, hello.c, with clang:
$ clang -S -emit-llvm -O1 hello.c
You will get a very simple IR file called hello.ll that contains the following code:
$ cat hello.ll
@.str = private unnamed_addr constant [6 x i8] c”Hello\00″,
align 1
define dso_local i32 @main(
i32 noundef %0, ptr nocapture noundef readnone %1) {
%3 = tail call i32 @puts(
ptr noundef nonnull dereferenceable(1) @.str)
ret i32 0
}
This is enough to test the pass.
To run the pass, you have to provide a couple of arguments. First, you need to tell opt to load the shared library via the –load-pass-plugin option. To run a single pass, you must specify the–-passes option. Using the hello.ll file as input, you can run the following:
$ opt –load-pass-plugin=./PPProfile.so \
–passes=”ppprofiler” –stats hello.ll -o hello_inst.bc
If statistic generation is enabled, you will see the following output:
===——————————————————–===
… Statistics Collected …
===——————————————————–===
1 ppprofiler – Number of instrumented functions.
Otherwise, you will be informed that statistic collection is not enabled:
Statistics are disabled. Build with asserts or with
-DLLVM_FORCE_ENABLE_STATS
The bitcode file, hello_inst.bc, is the result. You can turn this file into readable IR with the llvm-dis tool. As expected, you will see the calls to the __ppp_enter() and __ppp_exit() functions and a new constant for the name of the function:
$ llvm-dis hello_inst.bc -o –
@.str = private unnamed_addr constant [6 x i8] c”Hello\00″,
align 1
@0 = private unnamed_addr constant [5 x i8] c”main\00″,
align 1
define dso_local i32 @main(i32 noundef %0,
ptr nocapture noundef readnone %1) {
call void @__ppp_enter(ptr @0)
%3 = tail call i32 @puts(
ptr noundef nonnull dereferenceable(1) @.str)
call void @__ppp_exit(ptr @0)
ret i32 0
}
This already looks good! It would be even better if we could turn this IR into an executable and run it. For this, you need to provide implementations for the called functions.