A co-worker of mine is having some odd floating-point issues that are related to the compilation options passed to GCC v4.3.3. Compiling his program without any optimizations works fine (p_lower should be exactly 1.0...): gfortran main.f90 -o main ./main p_lower: 1.00000000000000000 Even raising the optimization level to -O1 causes problems: gfortran main.f90 -o main -O1 ./main p_lower: 0.96875000000000000 The problem is associated with a different FP rounding causing a <= to go from one corner case to another. For the purposes of this message, the magnitude of the difference is irrelevant, as is the fact that the problem can be fixed by coding it a different way. I decided to try and figure out which compiler option(s) were affecting things. I looked at http://gcc.gnu.org/onlinedocs/gcc-4.3.3/gcc/Optimize-Options.html for the list of compiler options associated with -O1. However, even using all of the individual flags that are supposedly in -O1 doesn't cause the error to occur: gfortran main.f90 -o main \ -fauto-inc-dec \ -fcprop-registers \ -fdce \ -fdefer-pop \ -fdse \ -fguess-branch-probability \ -fif-conversion2 \ -fif-conversion \ -finline-small-functions \ -fipa-pure-const \ -fipa-reference \ -fmerge-constants \ -fsplit-wide-types \ -ftree-ccp \ -ftree-ch \ -ftree-copyrename \ -ftree-dce \ -ftree-dominator-opts \ -ftree-dse \ -ftree-fre \ -ftree-sra \ -ftree-ter \ -funit-at-a-time ./main p_lower: 1.00000000000000000 In fact, we can add all optimizations for -O3 and the error doesn't manifest itself: gfortran main.f90 -o main \ -fauto-inc-dec \ -fcprop-registers \ -fdce \ -fdefer-pop \ -fdse \ -fguess-branch-probability \ -fif-conversion2 \ -fif-conversion \ -finline-small-functions \ -fipa-pure-const \ -fipa-reference \ -fmerge-constants \ -fsplit-wide-types \ -ftree-ccp \ -ftree-ch \ -ftree-copyrename \ -ftree-dce \ -ftree-dominator-opts \ -ftree-dse \ -ftree-fre \ -ftree-sra \ -ftree-ter \ -funit-at-a-time \ -fthread-jumps \ -falign-functions \ -falign-jumps \ -falign-loops \ -falign-labels \ -fcaller-saves \ -fcrossjumping \ -fcse-follow-jumps \ -fcse-skip-blocks \ -fdelete-null-pointer-checks \ -fexpensive-optimizations \ -fgcse \ -fgcse-lm \ -foptimize-sibling-calls \ -fpeephole2 \ -fregmove \ -freorder-blocks \ -freorder-functions \ -frerun-cse-after-loop \ -fsched-interblock \ -fsched-spec \ -fschedule-insns \ -fschedule-insns2 \ -fstrict-aliasing \ -fstrict-overflow \ -ftree-pre \ -ftree-vrp \ -finline-functions \ -funswitch-loops \ -fpredictive-commoning \ -fgcse-after-reload \ -ftree-vectorize ./main p_lower: 1.00000000000000000 So, my question is: what might be going on to make the "packaged" optimizations (-O1, -O2, -O3) cause anomalous results, but passing the individual optimization flags doesn't affect anything? Do I not understand how the optimization flags work with GCC? Thank you in advance, Michael PS: I omitted -fdelayed-branch because this is running on an x86 machine.
