src/hotspot/share/adlc/main.cpp parses in the -D inputs and sets AD.set_preproc_def(flag, def)
And then when the AD file is parsed, it checks ArchDesc::get_preproc_def to see if it should include the relevant code section or not.
You can for example find #ifdef _LP64 all over the AD files.

The flags are set in make/hotspot/gensrc/GensrcAdlc.gmk

ADLC_CFLAGS += -DASSERT
...
ifeq ($(call isTargetOs, linux), true)
  ADLCFLAGS += -DLINUX=1 -D_GNU_SOURCE=1

We never pass ASSERT or PRODUCT to ADLCFLAGS, hence we cannot use #ifdef with those flags (or rather they are always undefined).

We should fix this, so that the flags can be used consistently with all other C2 code. For example, I would like to be able to make some rules in AD files available only in debug (ASSERT) mode.