How to add a NOP instruction in each basic block for obj code that gcc generates

[Ian Lance Taylor]

Jeff <jeffhuang9999@gmail.com> writes:

> It would be cleaner if I know how to modify the gcc source code and
> let it insert a nop to each basic block.  This shouldn't be a hard job
> for an experienced gcc developer, should this?

It's doable but it's probably harder than you seem to think it is.
gcc is not in the practice of inserting instructions which do nothing
but must not be removed.  One simple approach would be to
automatically insert the internal equivalent of
    asm volatile ("nop");
into each basic block.  That would be relatively simple, but it would
significantly change the generated code, because those volatile asms
would be preventing all sorts of optimizations.  If you don't care
whether the generated code changes, then do that.  The place to do it
would be expand_gimple_basic_block.

If you don't want to change the generated code other than inserting
the nops, and you can restrict yourself to a processor which does not
need to track addresses to avoid out-of-range branches, then you could
approximate what you want by emitting a nop in final_scan_insn when
you see a CODE_LABEL, after the label.  You'll need to also emit a nop
in the first basic block in the function.  That probably won't be the
precise set of basic blocks as the compiler sees them, but basic block
is a somewhat nebulous concept and that may be good enough for your
purposes, whatever they are.

Ian