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Miloslav Ciz 2023-07-12 14:38:55 +02:00
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c_pitfalls.md
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@ -68,7 +68,7 @@ TODO: more examples
## Compiler Optimizations
C compilers perform automatic optimizations and other transformations of the code, especially when you tell it to optimize aggressively (`-O3`) which is a standard practice to make programs run faster. However this makes compilers perform a lot of [magic](magic.md) and may lead to unexpected and unintuitive undesired behavior such as bugs or even the "unoptimization of code". { I've seen a code I've written have bigger size when I set the `-Os` flag (optimize for smaller size). ~drummyfish }
C compilers perform automatic optimizations and other transformations of the code, especially when you tell them to optimize aggressively (`-O3`) which is a standard practice to make programs run faster. However this makes compilers perform a lot of [magic](magic.md) and may lead to unexpected and unintuitive undesired behavior such as bugs or even the "unoptimization of code". { I've seen a code I've written have bigger size when I set the `-Os` flag (optimize for smaller size). ~drummyfish }
Aggressive optimization may firstly lead to tiny bugs in your code manifesting in very weird ways, it may happen that a line of code somewhere which may somehow trigger some tricky [undefined behavior](undefined_behavior.md) may cause your program to crash in some completely different place. Compilers exploit undefined behavior to make all kinds of big brain reasoning and when they see code that MAY lead to undefined behavior a lot of chain reasoning may lead to very weird compiled results. Remember that undefined behavior, such as overflow when adding signed integers, doesn't mean the result is undefined, it means that ANYTHING CAN HAPPEN, the program may just start printing nonsensical stuff on its own or your computer may explode. So it may happen that the line with undefined behavior will behave as you expect but somewhere later on the program will just shit itself. For these reasons if you encounter a very weird bug, try to disable optimizations and see if it goes away -- if it does, you may be dealing with this kind of stuff. Also check your program with tools like [cppcheck](cppcheck.md).