On Tue, Mar 1, 2022 at 3:37 PM Erick Ochoa via Gcc <gcc@gcc.gnu.org> wrote: > > Hi, > > I am working on an analysis that is able to determine some static > information about a specific variable. At the moment, I would like to avoid > much of the transformation by taking advantage of other GCC's passes. So, I > can imagine something like, create an ASSERT_EXPR and let other passes > change the CFG, and remove dead code, fold constants, etc. > > At the moment I am only prototyping it, and so I have created a > SIMPLE_IPA_PASS during LTO that finds the variables of interest and then > creates an ASSERT_EXPR which contains the static information of interest. > > This looks to be working somewhat correctly. When I print the functions' > gimple everything looks ok. The pass doesn't fail even if I use the > TODO_verify_all flag at the end of the SIMPLE_IPA_PASS. However, the > transformation triggers a segfault during fixup_cfg and I am unsure what > can be the case. > > My questions are: > 1. Is it possible to insert new ASSERT_EXPR during LTO and have other tree > passes transform the function based on this?
ASSERT_EXPR is something only used internally by the VRP pass, it cannot be used elsewhere and is not expected by most of the compiler. > 2. How complex can the ASSERT_EXPR be? I can see that it must be a COND > expr, by which it can be EQ_EXPR, NE_EXPR, LT_EXPR... and others. But what > if I need to express something like a bitmask to guarantee that certain > bits are zero? Could I have a complicated ASSERT_EXPR where COND is > ASSERT_EXPR (a & 0xff == 0)? > Or should I break it down? > temp = a & 0xff; > ASSERT_EXPR (temp == 0); > 3. Any other advice? > > Thanks!
