Hi, On Wed, 21 Nov 2018, Richard Biener wrote:
> then this leads to wrong debug info showing a value for 'a' that never > existed. var-tracking was specifically created so that the base-vars of SSA names aren't necessary for debug info anymore ... > when you build this with -O -g -fno-var-tracking-assignments (VTA seems > to hide the issue, probably due to not enough instructions in the end) ... which makes this an non-test. Sure, if you cripple debug info generation you get ... crippled debug info. > and the fact that var-tracking looks at REG_DECL for > debug info location generation. With VTA we get > > # DEBUG BEGIN_STMT > # DEBUG D#3 => a_1(D) + 1 > # DEBUG a => D#3 > # DEBUG BEGIN_STMT > a_11 = d_7(D) + 4; > # DEBUG D#2 => D#3 + b_3(D) > # DEBUG a => D#2 > # DEBUG BEGIN_STMT > a_12 = a_11 + c_5(D); > # DEBUG D#1 => D#2 + c_5(D) > # DEBUG a => D#1 > # DEBUG BEGIN_STMT > a_13 = a_12 + b_3(D); > # DEBUG a => D#1 + d_7(D) > # DEBUG BEGIN_STMT > a_9 = a_13 + a_1(D); > # DEBUG a => a_9 This seems to be exactly correct and reflecting the original code, i.e. VTA works as designed even with "invented" base variables. > So your patch has to be much more careful to never change the LHS of > stmts that are adjusted (which I think reassoc already does). I can't do much more in reassoc than what I implemented, it's constrained to cases where we have a known LHS. If you think that's still too much then the problem needs to remain unfixed in reassoc, and it would need TER changes. An easy way would be to disable TERin also of SSA names having no base var (essentially on the ground that it's the same base var then), but that probably disables TER too much, though I haven't tried. In addition: I'd always happily trade less correct debug info (where the impact of the incorrectness is fairly trivial) with no stack accesses in inner loops; i.e. a tradeoff. Ciao, Michael.
