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How to disable optimization for particular assignment statements?

+5 votes
94 views

I have a C code like this:

int foo(void)
{ 
 int phase;
 . . .
 phase = 1;
 phase = 2;
 phase = 3;
 . . .
}

In case of -O0 gcc generates machine instructions for every assignment 'phase = ...'. But in case of -O2 gcc does not generate instructions for some assignments. Of course, this is correct. However, is there any way to tell gcc that 'phase' object is inspected by another thread, so it should not remove such statements?

posted Nov 21, 2013 by Amit Parthsarthi

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Check with these two and see Assembly code
  gcc -c -g -Wa,-a,-ad test.c > test1.lst // Deafault optimization
  gcc -O2 -c -g -Wa,-a,-ad test.c > test2.lst //O2  optimization


and u can check with changing the type of variable also ..like changing to volatile.

2 Answers

+2 votes

Define the variable as volatile.

answer Nov 21, 2013 by Naveena Garg
+1 vote

Basic rule of thumb: if code is not thread-safe, adding volatile will never make it thread-safe. That rule of thumb applies here. Adding volatile will not work, because it will not make the changes to the value visible to other processors.

http://gcc.gnu.org/onlinedocs/gcc/_005f_005fatomic-Builtins.html

Yes, if you have decided that this is what you really want to do, using the atomic (or sync) builtins is the only viable approach (my thoughts)

answer Nov 22, 2013 by anonymous
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