#pragma GCC optimize("unroll-loops")
#pragma GCC optimize("-falign-jumps")
#pragma GCC optimize("-falign-loops")
#pragma GCC optimize("-falign-labels")
#pragma GCC optimize("-fdevirtualize")
#pragma GCC optimize("-fcaller-saves")
#pragma GCC optimize("-fcrossjumping")
#pragma GCC optimize("-fthread-jumps")
#pragma GCC optimize("-funroll-loops")
#pragma GCC optimize("-fwhole-program")
#pragma GCC optimize("-freorder-blocks")
#pragma GCC optimize("-fschedule-insns")
#pragma GCC optimize("inline-functions")
#pragma GCC optimize("-ftree-tail-merge")
#pragma GCC optimize("-fschedule-insns2")
#pragma GCC optimize("-fstrict-aliasing")
#pragma GCC optimize("-fstrict-overflow")
#pragma GCC optimize("-falign-functions")
#pragma GCC optimize("-fcse-skip-blocks")
#pragma GCC optimize("-fcse-follow-jumps")
#pragma GCC optimize("-fsched-interblock")
#pragma GCC optimize("-fpartial-inlining")
#pragma GCC optimize("no-stack-protector")
#pragma GCC optimize("-freorder-functions")
#pragma GCC optimize("-findirect-inlining")
#pragma GCC optimize("-fhoist-adjacent-loads")
#pragma GCC optimize("-frerun-cse-after-loop")
#pragma GCC optimize("inline-small-functions")
#pragma GCC optimize("-finline-small-functions")
#pragma GCC optimize("-ftree-switch-conversion")
#pragma GCC optimize("-foptimize-sibling-calls")
#pragma GCC optimize("-fexpensive-optimizations")
#pragma GCC optimize("-funsafe-loop-optimizations")
#pragma GCC optimize("inline-functions-called-once")
#pragma GCC optimize("-fdelete-null-pointer-checks")
In this article, we will dive deeper into the world of G++ optimizer flags, specifically focusing on the flags listed above. We'll discuss their use cases, performance analysis, targets, and potential issues to help you make informed decisions about which flags to use in your own code.
First up, we have the "unroll-loops" flag. This flag is used to unroll loops in your code, which can improve performance by reducing the number of times the loop needs to be executed. This can be especially useful for loops that have a small number of iterations or that are executed frequently. However, be aware that unrolling loops can also increase the size of your code, so use this flag with caution.
Next, we have the "-falign-jumps", "-falign-loops", and "-falign-labels" flags. These flags are used to improve performance by aligning jump targets, loop headers, and labels in memory. Aligning these targets can lead to faster branching and looping operations, as the processor can predict the outcome of certain branches and loops more accurately.
The "-falign-jumps" flag aligns jump targets to a power of 2, which can help improve performance on processors that have a small instruction fetch buffer. The "-falign-loops" flag aligns the beginning of loops to a power of 2, which can improve performance on processors that have a small instruction cache. The "-falign-labels" flag aligns labels to a power of 2, which can improve performance on processors that have a small instruction cache.
The "-fdevirtualize" flag is used to perform devirtualization, which is the process of removing virtual function calls and replacing them with direct function calls. This can lead to a performance improvement by reducing the overhead of virtual function calls. However, it is important to note that this flag only works on C++ code that has been written in a certain way, and it may not work on all C++ code.
The "-fcaller-saves" flag is used to improve performance by reducing the number of registers that need to be saved and restored when calling functions. This can lead to a performance improvement by reducing the overhead of function calls. However, it is important to note that this flag can increase the register pressure in the caller, which may lead to a performance decrease.
The "-fcrossjumping" flag is used to improve performance by reducing the number of jumps that need to be taken. This can lead to a performance improvement by reducing the overhead of jumps. However, it is important to note that this flag may increase code size and decrease readability.
The "-fthread-jumps" flag is used to improve performance in multi-threaded code by reducing the number of jumps that need to be taken. This can lead to a performance improvement by reducing the overhead of jumps in multi-threaded code. However, it is important to note that this flag may increase code size and decrease readability.
The "-funroll-loops" flag is used to improve performance by unrolling loops. This can lead to a performance improvement by reducing the overhead of loops. However, it is important to note that this flag may increase code size and decrease readability.
The "-fwhole-program" flag is used to improve performance by performing interprocedural optimization across multiple compilation units. This can lead to a performance improvement by taking into account the whole program when performing optimization. However, it is important to note that this flag may increase compilation time.
The "-freorder-blocks" flag is used to improve performance by reordering basic blocks. This can lead to a performance improvement by reducing the number of branches that need to be taken. However, it is important to note that this flag may increase code size and decrease readability.
The "-fschedule-insns" flag is used to improve performance by scheduling instructions. This can lead to a performance improvement by reducing the number of stalls caused by data dependencies. However, it is important to note that this flag may increase code size and decrease readability.
In conclusion, these optimizer flags offer a variety of options for improving performance in different situations. However, it's important to note that using these flags may also lead to increased code size and decreased readability. It is always important to test the performance of your code with and without these flags, to see which combination works best for your specific use case.