Which flags are enabled by -O?

Question

My application has some portability issues when compiled with the -O flag. I would like to find out which flag exactly triggers the problem, yet I am still unsuccessful.. I used all flags found in

gcc -c -Q -O --help=optimizers

but I still cannot reproduce the problem. Is it possible that the -O enables also additional flags?

Btw, the error I am getting is the following:

[Hein@Heinrich]$ ./parscons
./parscons: /usr/lib64/libstdc++.so.6: version `GLIBCXX_3.4.9' not found (required by ./parscons)
./parscons: /usr/lib64/libstdc++.so.6: version `GLIBCXX_3.4.11' not found (required by ./parscons)

score 2 · Answer 1 · answered May 06 '15 at 22:52

The list of flags enabled by gcc for -O can be found here - Optimize-Options

From the doc:

-O -O1 Optimize. Optimizing compilation takes somewhat more time, and a lot more memory for a large function. With -O, the compiler tries to reduce code size and execution time, without performing any optimizations that take a great deal of compilation time.

-O turns on the following optimization flags:
      -fauto-inc-dec 
      -fbranch-count-reg 
      -fcombine-stack-adjustments 
      -fcompare-elim 
      -fcprop-registers 
      -fdce 
      -fdefer-pop 
      -fdelayed-branch 
      -fdse 
      -fforward-propagate 
      -fguess-branch-probability 
      -fif-conversion2 
      -fif-conversion 
      -finline-functions-called-once 
      -fipa-pure-const 
      -fipa-profile 
      -fipa-reference 
      -fmerge-constants 
      -fmove-loop-invariants 
      -fshrink-wrap 
      -fsplit-wide-types 
      -ftree-bit-ccp 
      -ftree-ccp 
      -fssa-phiopt 
      -ftree-ch 
      -ftree-copy-prop 
      -ftree-copyrename 
      -ftree-dce 
      -ftree-dominator-opts 
      -ftree-dse 
      -ftree-forwprop 
      -ftree-fre 
      -ftree-phiprop 
      -ftree-sink 
      -ftree-slsr 
      -ftree-sra 
      -ftree-pta 
      -ftree-ter 
      -funit-at-a-time
-O also turns on -fomit-frame-pointer on machines where doing so does not interfere with debugging.

Hi thanks for your reply, I had already found your list and tried compiling my application with all those flags, but it still does not reproduce my portability issue. — Heinrich Heine, May 06 '15 at 22:56
I am compiling my application on one computer and running it on a second (which does not have the same libstdc++.so6 version) — Heinrich Heine, May 07 '15 at 05:59

Which flags are enabled by -O?

1 Answers1