How to disable compiler optimizations in gcc?
CGccCompiler OptimizationCompiler OptionsC Problem Overview
I am trying to learn assembly language. I have searched and found how to disassemble a .c file but I think it produces some optimized version of the program. Is there any way so that I can see the exact assembly code which corresponds to my C file.
C Solutions
Solution 1 - C
The gcc option -O enables different levels of optimization. Use -O0 to disable them and use -S to output assembly. -O3 is the highest level of optimization.
Starting with gcc 4.8 the optimization level -Og is available. It enables optimizations that do not interfere with debugging and is the recommended default for the standard edit-compile-debug cycle.
To change the dialect of the assembly to either intel or att use -masm=intel or -masm=att.
You can also enable certain optimizations manually with -fname.
Have a look at the gcc manual for much more.
Solution 2 - C
For gcc you want to omit any -O1 -O2 or -O3 options passed to the compiler or if you already have them you can append the -O0 option to turn it off again. It might also help you to add -g for debug so that you can see the c source and disassembled machine code in your debugger.
See also: http://sourceware.org/gdb/onlinedocs/gdb/Optimized-Code.html
Solution 3 - C
Use the command-line option -O0 (-[capital o][zero]) to disable optimization, and -S to get assembly file. Look here to see more gcc command-line options.
Solution 4 - C
To test without copy elision and see you copy/move constructors/operators in action add "-fno-elide-constructors".
Even with no optimizations (-O0 ), GCC and Clang will still do copy elision, which has the effect of skipping copy/move constructors in some cases. See this question for the details about copy elision.
However, in Clang 3.4 it does trigger a bug (an invalid temporary object without calling constructor), which is fixed in 3.5.
Solution 5 - C
You can also control optimisations internally with #pragma GCC push_options
#pragma GCC push_options
/* #pragma GCC optimize ("unroll-loops") */
.... code here .....
#pragma GCC pop_options
Solution 6 - C
Long time ago, but still needed.
info - https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html
list - gcc -Q --help=optimizers test.c | grep enabled
disable as many as you like with:
gcc **-fno-web** -Q --help=optimizers test.c | grep enabled
Solution 7 - C
You can disable optimizations if you pass -O0 with the gcc command-line.
E.g. to turn a .C file into a .S file call:
gcc -O0 -S test.c