List of optimizations enabled by -O1 seems incorrect

[Andrew Haley]

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.