Compiler and Library

I modified LLVM to generate executables that write the stack in opposite direction. It took me several weeks till I had a customized llvm-gcc which could generate reverse-stack executables that ran correctly. My approach was to modify the code generator, patch stack manipulating instructions and change stack offset computations.

To generate reverse-stack executables modifying a compiler is not enough. You also need a reverse-stack library to link against. It may seem that when we have a compiler, we can compile a library for reverse stack growth, but life is not always easy! There is good amount of assembly code in all libraries that should be modified for reverse stack. I picked dietlibc and modified assembly files manually. Since my intent was to run SPEC CPU benchmarks, I had to modify some of the C sources as well to make the library compatible with the standard libc. Without these modifications, runspec would fail during output verification.

At the end, results exceeded my expectations. Less than one percent performance reduction on average, comparing to a normal executable!