how to tweak x86 code generation to instrument certain opcodes with CC trap?

[Yasser Shalabi]

Hello,

I am new to the GCC code. I want to make a simple modification to the
back end. I want to add a debug exception (int3) to be generated
before any instance of certain x86 instructions.

I tried to modify gcc/config/i386/i386.md by adding a "int3" to the
define_insn for instructions of interest. But that just caused
configure to fail (cannot run generated C programs).

Any pointers on how to approach this? Also, suggestions for
alternative approaches are also welcome.

Thanks!