List of optimizations enabled by -O1 seems incorrect
Thu Aug 13 18:25:00 GMT 2009
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?
I tested with GCC 4.3.4 and 4.4.1.
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