Why Does GCC's -O3 Flag Sometimes Make My Code Slower Than -O2?

Unexpected Performance Impact of GCC Optimization Flag -O3

When optimizing code using GCC, it is not uncommon for users to encounter unexpected performance differences between different optimization levels. In this instance, we're examining a specific case where the -O3 flag appears to make the code run slower than the -O2 flag.

To better understand the issue, let's delve into the details of the optimization techniques employed by GCC under each flag:

Optimization Level -O3:

• GCC -O3 optimizes the code for maximum performance, often resulting in the most efficient executable code.
• However, this level of optimization can also lead to changes in the instruction set used, potentially impacting execution speed due to architectural constraints.

Optimization Level -O2:

• GCC -O2 aims to strike a balance between code efficiency and predictability.
• It employs optimizations that typically improve performance while maintaining consistency in the generated code.

Explanation of Observed Performance Difference:

In the case of the code provided, the -O3 optimization flag causes GCC to utilize a conditional move instruction (cmov) within the primary loop. This instruction, while efficient in certain situations, can lengthen the loop-carried dependency chain by two clock cycles.

The loop in question iterates over an array and performs a conditional summation based on the value at each index. With -O2, GCC uses a branch instruction instead of cmov, which effectively reduces the dependency chain length to a single clock cycle. This shorter chain allows for faster execution, particularly in scenarios where data is sorted and predictability is high.

Software Profiling and Optimizations:

To confirm these observations, the code was compiled using both -O3 and -O2 flags and analyzed using software profiling tools. The results indicated that the branchy version (compiled with -O2) indeed executed faster than the branchless version (compiled with -O3).

Despite -O3 being theoretically more aggressive in optimization, the choice of using the cmov instruction can result in performance degradation in certain cases. This highlights the importance of selecting the right optimization flag based on the specific code characteristics, data patterns, and target architecture.

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