Can DragonEgg run an optimization pass available in gcc and produce llvm IR?

Question

Is it possible to run a gcc optimization pass on any code and in the end obtain llvm IR? I want to use dead store elimination available in gcc. I am aware that llvm also has this pass but it is not as advanced as in gcc.

Answer 1

I don't have experience with it and I haven't tried, but I think the following command-line should work in DragonEgg version 3.0 and above:

-fplugin=./dragonegg.so -O0 -fdse -fplugin-arg-dragonegg-enable-gcc-optzns -fplugin-arg-dragonegg-llvm-ir-optimize=2 -fplugin-arg-dragonegg-emit-ir -S

• -fplugin=./dragonegg.so enables DragonEgg
• -O0 turns off all GCC optimization passes
• -fdse turns on GCC dead store elimination
• -fplugin-arg-dragonegg-enable-gcc-optzns enables GCC optimizations in DragonEgg
• -fplugin-arg-dragonegg-llvm-ir-optimize=2 forces LLVM to optimize to -O2 instead of following the GCC optimization level
• -fplugin-arg-dragonegg-emit-ir generates LLVM IR instead of asm, which according to the question is what you want here
• -S because you kinda need it when you use -fplugin-arg-dragonegg-emit-ir

And this should work on any GCC tree pass.

If you need more fine-grained control than this, you can tweak DragonEgg: the correct place to edit is the plugin_init function in Backend.cpp - you can see precisely how it sets up the passes to run there.