CODEWITHSUNDEEP
cdebugginggccgdbcompiler-optimization
Zulan
Zulan

Reputation: 22670

Which specific optimization flags are responsible for optimizing out variables

I would like to specifically select optimizations flags to prevent <optimized out> variables (parameters) in gdb, without resorting to -O0.

My background is debugging glibc, which cannot be built with -O0, as it requires some sort of function inlining. However I can see this being useful in general, e.g. to enable useful debugging without totally killing performance.

According to gcc -c -Q -O0 --help=optimizers, the following 31 optimization flags are enabled by -O1 in addition to the ones enabled at -O0 (with gcc 5.3.1):

-fbranch-count-reg
-fcombine-stack-adjustments
-fcompare-elim
-fcprop-registers
-fdefer-pop
-fforward-propagate
-fguess-branch-probability
-fif-conversion
-fif-conversion2
-finline-functions-called-once
-fipa-profile
-fipa-pure-const
-fipa-reference
-fmove-loop-invariants
-fshrink-wrap
-fsplit-wide-types
-fssa-phiopt
-ftree-bit-ccp
-ftree-ccp
-ftree-ch
-ftree-copy-prop
-ftree-copyrename
-ftree-dce
-ftree-dominator-opts
-ftree-dse
-ftree-fre
-ftree-pta
-ftree-sink
-ftree-slsr
-ftree-sra
-ftree-ter

Note: I am aware of selective optimization / volatile as a manual fix, but I am looking for a more general solution.

Upvotes: 1

Views: 301

Answers (1)

alk
alk

Reputation: 70981

Not directly answering your question, but what you are probably after is -Og:

From the GCC documentation:

-Og

Optimize debugging experience. -Og enables optimizations that do not interfere with debugging. It should be the optimization level of choice for the standard edit-compile-debug cycle, offering a reasonable level of optimization while maintaining fast compilation and a good debugging experience.

Using --help=optimizers on -Og and comparing it with what you get for --help=optimizers on -O0 then gives you the answer to your question.

Upvotes: 1

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