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How to disable GCC optimization

+5 votes
120 views

GCC is stripping the object file when nothing from the file is being used anywhere in the executable (even -o0 does nor help). We have to disable this feature of linker g++. could any one please help us to resolve this issue??

posted Sep 11, 2013 by Anderson

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1 Answer

+1 vote

Use "-O0" with gcc command to disable optimization. But Optimization can be also disabled selectively.
Here using "-O1" enable all optimizations. And some of those optimizations can be disabled using a -f switch.

To view the -f flags enabled by -O1 use this handy tip:gcc -fverbose-asm -H -x c <(echo '') -O1 -S -o O1.s

answer Sep 11, 2013 by Satyabrata Mahapatra
Please go through this following Link:
http://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html
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I have a C code like this:

int foo(void)
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I am looking for detailed description of optimization flags that are provided by GCC compiler ?

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My current compiler settings are

arm-none-eabi-gcc -Wall -mfpu=fpv4-sp-d16 -mfloat-abi=softfp -fshort-double-mcpu=cortex-m4 -mthumb -Qn -Os -finstrument-functions -mlong-calls -c temp.c -o temp.o
so on for temp1, temp2... Etc

I am compiling multiple C files. and linker settings are

ld -r temp.o -o one.o
so on for temp1, temp2... Etc

I am linking multiple .O files. Also I am using linker script i.e link.txt (invoked externally as below)

ld -cref -Map map.txt -S -T link.txt -temp.o -lm -lc -lgcc
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Now, At output I get .abs file of 140KB.

My Questions are
1. How to optimize (reduce size of .abs) by using compiler or linker specific options?
2. There are Number of NOT used global variables and Blank Function Calls in my C files (NO Dead Code!!!!). Is it possible to perform some link time optimization.
3. Please Elaborate as I am new to this, I have referred sites, gcc help and tried some "lto" options but NO reduction is size.
4. I also tried using -fdata-sections -ffunction-sections
But It has INCREASED my firmware.abs file size from KB to MB (yes it is MB!!!!) I also wondered WHY?
I am using codesourcery arm toolchain evaluation. It has gcc version 4.5.2


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