[Bug c/88481] -O1 causes optimizer to drop 'then' clause in conditional
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88481 Heinrich Seebauer changed: What|Removed |Added Status|WAITING |RESOLVED Resolution|--- |INVALID --- Comment #10 from Heinrich Seebauer --- Alexander, you are right, it is only strange behaviour. Using -O1 drops the 'then' clause, but the results are the same, if one takes a broader view. The program deals with controlling schedules using actual time. Each test run has different input values (current UTC time). I set up a situation where I was able to control system time, and the outcomes were consistent, either using -O0 oe -O1. What confused me was the fact that -O2/-Os did NOT drop the then clause. So -O2 ist not just optimizing the -O1 outcome, but different from the beginning of the optimizer pass. I apologize for not getting this point quickly. Thank you very much for your support and your work for the commuinty, kind regards Heinrich
[Bug c/88481] -O1 causes optimizer to drop 'then' clause in conditional
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88481 --- Comment #9 from Alexander Monakov --- Thanks. I still don't see what's wrong. Are you testing only by single-stepping in gdb, or does your program overall behave differently with/without if-conversion? In other words, do you see if-conversion causing wrong code, or only worse debug experience?
[Bug c/88481] -O1 causes optimizer to drop 'then' clause in conditional
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88481 --- Comment #8 from Heinrich Seebauer --- Sorry, my comment was lost completely after uploading the attachment. Try again... So I hope, I got this right. -fdbg-cnt=if_conversion:xx,if_after_combine:yy xx yy result --- 20 20 success 20 21 fail 0 21 fail 21 20 fail tested with -fdbg-cnt=if_conversion:20,if_after_combine:21 -fno-if-conversion and got functioning code. I did gdb stepping in the source file where I observed, that the condition was true, but the branch was not executed. Same with the tests here - they were all strepped via source code.
[Bug c/88481] -O1 causes optimizer to drop 'then' clause in conditional
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88481 --- Comment #7 from Heinrich Seebauer --- Created attachment 45232 --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=45232=edit zipped asm files for three cases
[Bug c/88481] -O1 causes optimizer to drop 'then' clause in conditional
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88481 --- Comment #6 from Alexander Monakov --- And just to be sure, can you confirm that -fno-if-conversion changes program behavior (the testcase is not executable so I cannot check), and the issue is not about debug info quality (i.e. that single-stepping in the debugger does not behave as you expect, but overall the function computes correct values)?
[Bug c/88481] -O1 causes optimizer to drop 'then' clause in conditional
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88481 --- Comment #5 from Alexander Monakov --- The code shown in the opening comment looks fine to me, so please isolate the issue further using debug counters. Add -fdbg-cnt=if_conversion:99,if_after_combine:99 to -O1. This should lead to broken code as with plain -O1. Add -fdbg-cnt=if_conversion:0,if_after_combine:0 to -O1. This should lead to correct code as with -O1 -fno-if-conversion. Then you can try 99/0 and 0/99 combinations to find which of two passes is broken. After that you can bisect by the value of debug counter to find exact threshold at which it becomes broken (e.g. 0/42 works, 0/43 breaks). Please attach the two resulting assembly dumps obtained with -S after that.
[Bug c/88481] -O1 causes optimizer to drop 'then' clause in conditional
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88481 --- Comment #4 from Heinrich Seebauer --- To isolate the cause, I suppressed several optimization options from thge -O1 group. Adding -fno-if-conversion to the options got the code correct, other -O1 options didn't change anything. I think, this is some strong evidence that -fif-conversion causes the bug. Proposed workaround: use option "-fno-if-conversion" in combination with -O1.
[Bug c/88481] -O1 causes optimizer to drop 'then' clause in conditional
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88481 --- Comment #3 from Heinrich Seebauer --- Thanks, Alexander, for your advice, I will try to isolate this symptom further. Added the attachment ...
[Bug c/88481] -O1 causes optimizer to drop 'then' clause in conditional
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88481 --- Comment #2 from Heinrich Seebauer --- Created attachment 45227 --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=45227=edit precompiled source of fsch.c
[Bug c/88481] -O1 causes optimizer to drop 'then' clause in conditional
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88481 Alexander Monakov changed: What|Removed |Added Status|UNCONFIRMED |WAITING Last reconfirmed||2018-12-13 CC||amonakov at gcc dot gnu.org Ever confirmed|0 |1 --- Comment #1 from Alexander Monakov --- Without -O, optimization passes are not enabled, even if individual -f options are passed on the command line. If you want to manually find optimizations that affect the outcome, you need to start with -O1 and add -fno options to selectively disable optimizations. The attachment is missing, so changing bug status to "waiting".
