Nikos Chantziaras wrote: > I'm trying to discover which GCC optimization is responsive for a > runtime error reported by Valgrind: > > "Conditional jump or move depends on uninitialised value(s)" > > This pops up when compiling with -O1 and above. The code in question is > C++ and is of this form: > > if (!foo && !bar) > > The uninitialized variable is 'bar'. However, it shouldn't be evaluated > at all due to short-circuit if rules. Of course the optimizer is free > to evaluate it anyway if there are no side-effects when doing so. > > So just out of interest, I'm trying to find out which optimization is > responsible for this. I looked up the GCC info pages and I compiled > with -O1 and then disabled the optimizations enabled by -O1 one by one, > in hope to hit the one that triggers the error, until all of them were > disabled in the end: > > -O1 -fno-auto-inc-dec -fno-cprop-registers -fno-dce -fno-defer-pop > -fno-delayed-branch -fno-dse -fno-guess-branch-probability > -fno-if-conversion2 -fno-if-conversion -fno-inline-small-functions > -fno-ipa-pure-const -fno-ipa-reference -fno-merge-constants > -fno-split-wide-types -fno-tree-ccp -fno-tree-ch -fno-tree-copyrename > -fno-tree-dce -fno-tree-dominator-opts -fno-tree-dse -fno-tree-fre > -fno-tree-sra -fno-tree-ter -fno-unit-at-a-time > > However, the error still appears which makes me conclude that the above > list of -O1 optimizations is not complete. Which ones am I missing? Not all optimizations are switchable. If you want to know the complete set of switches, compile with -fverbose-asm and look at the .s file. If you really want to know when a particular optimization was done, you'll have to look at the dumps. Andrew.
